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[Articles Home]  [Add Article]  

Why 1/4 Wavelengths?

Scott R. Archer (KB9WIS) on April 19, 2004
View comments about this article!

I'm a little new to the HF bands, so please pardon my ignorance. From my previous and little VHF/UHF experience, I recall that the higher the wavelength of the antenna (especially if collinear), the more "gain" the antenna has.

My question is, why when building wire (such as dipoles, inverted V) antennas do we only use 1/4 wavelengths of antenna wire on each side of the feedline? Why not use a higher wavelength on the higher bands such as 10-20 meters? Wouldn't a higher wavelength (such as 1/2, 3/4, 1 wavelength, or even 10 wavelengths) offer slightly higher gain without creating a mismatch, or would one just be wasting their time and antenna wire? Are there any special rules, such as you may only use odd wavelengths, (versus even) when building a center-fed dipole? Is it possible to make an HF dipole "colinear"?

Many hams make or purchase multi-band antennas with different wire resonators for each band. Couldn't one use a single wire that is 1/2 wavelength on 80 meters and 4 wavelengths on 10 meters? Wouldn't 4 wavelengths offer more gain than a 1/2-wavelength dipole anyways? If a multi-band antenna (with separate resonators) that has a 132' wire on 80 meters, why use only 14' of wire for 10 meters, if there would be more gain if we made the antenna 28 or even 56 feet long (or at an even higher wavelength)? Since the 80-meter resonator (or wire) is already 132', the antenna would not take up any additional room.

Perhaps HF works differently, and larger wavelengths do not offer any gain. Perhaps the antennas lowest angle of radiation is achieved only if a 1/4-wavelength antenna is used?

Any input to alleviate my confusion would be desired.

Thanks,

KB9WIS

Member Comments:
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Why 1/4 Wavelengths?  
by G5FSD on April 19, 2004 Mail this to a friend!
Try a google search like "reactance" + "inductive" + "dipole" + "resonance"!

A 1/4 is a standard 'unit' of antenna design, because it's the smallest length where a simple wire can be tuned to 'resonance' without extra fiddling around with coils or capacitance. At this resonant length, the inductive and capacitive reactances are equal and opposite, cancelling each other, leaving only a resistive component of the impedance.. which happens to be at an impendance around 50 Ohms (a 1/4 wave over an infinite groundplane) - a good match for coaxial cables.
 
Why 1/4 Wavelengths?  
by G5FSD on April 19, 2004 Mail this to a friend!
Check this page out
http://www.eece.ksu.edu/~wkuhn/useful_stuff/AntennaDesigns.PDF

This introduces the delightful concept of imaginary numbers. Imaginary? Yes - mathematicians will delight in telling you that there IS a square root of -1 after all, and they know it as 'i'. A complex number is made up of a real, tangible, quantity together with an imaginary component (I'm NOT making this up!!) e.g. a + bi see http://www.1728.com/compnum2.htm for more details.

In engineering this turns out to be very useful, as something 90 degrees out of phase can be treated as a complex number. This applies to impedance - where sum impedance at a given frequency can be represented as x Ohms + j Ohms ... the imaginary 'j' bit is the reactive component, either + or - depending on whether it's is inductive or capacitive. More details? Google can help - I found http://whatis.techtarget.com/definition/0,,sid9_gci212333,00.html in no time!
 
Why 1/4 Wavelengths?  
by KC7APQ on April 19, 2004 Mail this to a friend!
The basics on how antennas work is simply explained in a book called "Now Your Talking", you can get a copy a Radio Shack.
 
RE: Why 1/4 Wavelengths?  
by N6AYJ on April 19, 2004 Mail this to a friend!
The reason an antenna radiates is because, as you increase the frequency of an RF signal, the electric field of the antenna current is able to change direction faster than the magnetic part. This is because the magnetic lines of force resist the change in current direction of the magnetic part of the field as the magnetic field collapses with change in the direction of the antenna current, but the electric field does not resist the collapse of the electric part of the field. When the frequency gets high enough, the magnetic part of the field can't follow the electric part as the direction of its current flow changes, because it takes too long for the magnetic field to collpase; then the magnetic part of the wave gets cut off from the electric part, and the magnetic part gets launched off into space from the antenna. Therefore, anything we can do to make the electric part of the wave change direction faster and more efficiently will cut off the magnetic part of the wave more effectively and will therefore increase the amount of radiation from the antenna.
Cutting the antenna to resonance simply means that, when one cycle of RF current hits the end of the antenna and reflects back to the feed point, there will not be another cycle of RF racing towards it to interfere with, or cancel, its travel back to the feed point, and the electric part of the wave will therefore be able to reflect back to the center feed point with minimal losses. Resonance also means that, when the antenna current returns to the feed point after being reflected from the ends, it will be timed perfectly so that when it again reflects back from the feed point toward the end of the antenna, another cycle of RF will be just ready to go with it, thereby reinforcing the reflection rather than tending to cancel it (i.e., standing waves, like pushing on a swing at just the right time, at the top of its travel, rather than pushing on it a little before the top, or a little after). This renders efficiency of the RF current reflections in the antenna optimal because they reinforce each other, and makes the antenna "cut off" the magnetic part of the wave in the most effective way possible. Any other length of antenna (besides multiples of a half-wavelength) will not reflect current as well off the center and ends of the antenna because there will be destructive interference between the current reflected back from the ends and the incoming current. Note that each half of the antenna is 1/4 wavelength, or 1/2 wave overall. A vertical is only 1/4 wave long because the ground forms the other half of the antenna.
 
Why 1/4 Wavelengths?  
by W4BQF on April 19, 2004 Mail this to a friend!
Your asking some good questions! You CAN use a 'colinear' antenna on HF. And NO, the rules for antennas are frequency independant; the theory at HF is exactly the same as at UHF or microwaves. The only 'problem' becomes the physical size of HF antennas compared to UHF antennas, just making the physical construction a little more combersome to work with. If you use, say, a 80m dipole on 80m, then the major lobe of that antenna is broadside to the antenna wire, and when you use that same 80m dipole antenna on higher frequencies, the radiation pattern changes and eventually, at a high enought frequency, say 15m or 10m, most of the radiation is toward the end's of the wire.
Your antenna concepts apply to HF also.

Tom - W4BQF
 
RE: Why 1/4 Wavelengths?  
by KC8Y on April 19, 2004 Mail this to a friend!
I totally agree with the remarks, by G5FSD. I graduated with an Electrical Engineering degree & have been in this hobby over +30-yrs...
Math-computations seem to be the basic rule-of-thumb for all ideas...
 
long wire gain  
by AA6E on April 19, 2004 Mail this to a friend!
To answer the main question (IMO), "yes". Longer wire antennas (resonant or not) can give more gain. The problem is that the gain is in narrow lobes, which point in fixed directions. This might be useful sometimes, but unless you have a rotatable long wire (!), it's probably not what you want.

The only way you get "gain" from an antenna in one direction is by reducing the gain in other directions. I.e., directivity and gain go together. This can be good for a rotating yagi, but not for your fixed wire.

[The gain situation is easiest to understand if you think about transmitting. If you put 100 watts into an antenna, you are only going to radiate 100 watts - at most. The choice you have when you design your antenna is _where_ you are going to send those watts. If you direct all the power into 10% of the available sky, you have a gain of 10 dB in that direction, and your "effective radiated power" is 1000 watts in that direction. That's over-simplified, but the idea is right.]

The radiation patterns for long wires are given in the ARRL Antenna Book and other places.

73, Martin
 
RE: Why 1/4 Wavelengths?  
by N3TUZ on April 19, 2004 Mail this to a friend!
Thanks to N6AYJ for an explanation in plain, non-technical, English!!
 
Why 1/4 Wavelengths?  
by ZS1AN on April 19, 2004 Mail this to a friend!
Hi Scott

You can use co-linear antennas on HF just as on VHF and UHF, except that the size is much larger and less convenient.

You can use a simple centre-fed piece of wire up to about 1.25 wavelengths long to get additional gain over a dipole. The reason why this is not very common is because the only length within this range that presents a purely resistive (no reactance) low-impedance feedpoint impedance is 1/2 wavelength, i.e. the common dipole.

If the length of the antenna (including both legs) is an odd multiple of 1/2 wavelength, then the feedpoint impedance will be resistive and low. If it is an even number of half wavelengths, it will be resisitive and high. Any other length will include some reactance. Since coax cable is designed for low, purely resistive loads, only the 1/2 wavelength dipole provides a good match to coax cable.

If the total length (both legs) is 1.25 wavelengths, then the antenna is an "extended double zepp" and has a gain of approximately 3 dBd (i.e. 3 dB better than a dipole).

If you extend a simple doublet (centre-fed wire antenna) beyond this length its pattern breaks up into multiple lobes instead of the two broadside lobes found in shorter doublets like the 1/2 wave dipole, and the gain decreases. This is because the antenna now includes current concentrations in opposite directions, and the radiation from these cancels out in the broadside direction while reinforcing in other directions, depending on how long the wire is!

So it's best not to make a straight wire longer than about 1.25 wavelengths. There are two alternatives:

1. Bend the wire in the middle so that the angled lobes from the two sides add up in one direction. This is the basis of the "V beam" and Rhombus antennas, which can be excellent for a fixed-direction path if you have lots of space.

2. Include either coils or shorted stubs at the points where there would be current concentrations in the "wrong" direction, to minimise the effect of these on the radiation pattern, so it remains a simple broadside pattern. If you look at VHF colinears you will see that they generally have a coil or shorted stub in the antenna for this reason. This can be done on HF, but usually the feeling is that the trouble is not worth the added performance over an extended double zepp.

However these perfrectly good options are not common at HF due to the sizes involved and the fact that better performance can often be obtained from a smaller beam antenna.

Good luck and 73,
Andrew ZS1AN



 
RE: Why 1/4 Wavelengths?  
by N0EW on April 19, 2004 Mail this to a friend!
I would add that "imaginary" power is a measure of the reactance - either capacitive or inductive - and is in fact _real_ energy. It is however not providing any _work_ but is instead caught in a _loop_ being handed back and forth rather than being available to radiate as a useful RF signal. The loop will consist of magnetic or electrostatic energy depending upon whether there is inductive (magnetic) or capacitive (electrostatic) reactance present (if any). Further the definition of resonance is the presence of zero reactance. Reference: "Reflections II" by Maxwell w2du.
 
RE: Why 1/4 Wavelengths?  
by AA4PB on April 19, 2004 Mail this to a friend!
An antenna cannot create energy. There are only two ways to get "gain" from an antenna; reduce the losses or concentrate the energy in one direction by subtracting it from other directions. In the case of an omnidirectional antenna the energy is compressed on the vertical axis, concentrating it on the horizon. The other option is to make the antenna directional like a yagi and concentrate the energy on the horizontal axis. When you make a wire antenna longer it begins to form lobes off the ends and reduces the signal broad-side - it becomes directional. On VHF and UHF, no problem we just rotate the antenna to point it in the desired direction. With an HF antenna that becomes more difficult as the antenna is much larger.

The other issue is height above ground. Antennas tend to loose their directive patterns when located less than 1/2-wavelength above ground. For example, a 40M dipole placed 25-feet above ground is pretty omni-directional. Even if you design an HF antenna with good gain, you've got to place it high enough to maintain that theoretical directivity and gain.

Directional, gain antennas are used on HF. Its just that the average ham doesn't have the real estate to put one up and so dipoles rule on the lower bands. 20M thru 10M yagis are quite often used. A few have 40M or even 80M directional antennas.
 
RE: Why 1/4 Wavelengths?  
by K1CJS on April 19, 2004 Mail this to a friend!
"The basics on how antennas work is simply explained in a book called "Now Your Talking", you can get a copy a Radio Shack."

Not anymore, pal! That is unless you're lucky and can still find a Rat Shack that stocks it. Rat Shack has just about nothing in stock for hams anymore--including books. It may still be available 'on line' though.

Sorry for going off subject, but the 'Now You're Talking" book does have some excellent and simple explanations in it, including antennae and wavelength.
 
Why 1/4 Wavelengths?  
by W4VR on April 19, 2004 Mail this to a friend!
The problem with making an antenna too long is that you get multiple lobes with deep nulls, and if the antenna is several wavelenghs long it will radiate off the ends more than broadside to wire. If you are interested in providing radiation with uniform lobing you need to specifically design the antenna to get rid of the smaller lobes and obtain a uniform pattern. One way to do this and obtain some gain at the same time is to use a collinear array such as an extended double zepp...making each leg of the antenna a little longer than a half wave (.64 wavelengths) you get rid of the minor lobes and obtain a nice broadside pattern. You can also do this by feeding two half wave dipoles in the same plane with two equal lengths of transmission line and separate the ends that are closest to each other by approximately a quarter wavelength...I use this design on 17 meters and it works like a champ. Hope this helps.
 
Why 1/4 Wavelengths?  
by VE7AZC on April 19, 2004 Mail this to a friend!
IMHO a good deal of emphasis is commonly placed on resonant antennas, which is un-necessary. I found the paper "Another Look At Reflections" written by W2DU (the late Walt Maxwell) to be an excellent sorce of antenna information, dispelling all the myths that amateurs often use to guide their antenna building.

Cheers,

H
 
Why 1/4 Wavelengths?  
by W5DXP on April 19, 2004 Mail this to a friend!
A 1/4WL vertical is what is known as a "standing-wave" antenna as there are reflections from the open end (top) of the antenna. The impedance at any point is the ratio of total voltage to total current. For a 1/4WL vertical, at the feedpoint, the forward voltage and reflected voltage are 180 degrees out of phase so they subtract yielding a low voltage. The forward current and reflected current are in phase so they add yielding a high current. Low-voltage divided by high-current gives us a low feedpoint impedance, a good match for coax which is exactly what we want. If it were not for the reflections on that standing-wave antenna, the feedpoint impedance would be in the hundreds of ohms.
--
73, Cecil http://www.qsl.net/w5dxp
 
RE: Why 1/4 Wavelengths?  
by N0SP on April 19, 2004 Mail this to a friend!
To KB9WIS...
Sheesh, pick up a book and read it... the answers to these questions are plainly visible in graphic form right there on the pages.
Authors like:
Moxon
Devoldere
Maxwell
Orr.. many others,
and the big ARRL Antenna Book all explain the relationships between gain, height, tuning, etc. After you've done that come back and ask for fill-in information. But it would be impossible to give you a complete picture this way. Already many of these posts have become way to complicated for someone who hasn't studied the basics. It's a facinating area of radio and you'd be very entertained by the right reading. Also, if you don't want to buy sometimes expensive books, there are old military training manuals around. Back when I started I got all my basic information from an old dog-eared copy of the Philco Training Manual for field tech reps that was printed in 1956. I paid 25¢ for it at a flea market and it taught me all I ever needed to know about antennas.

Good luck
Dennis
NØSP
 
Why 1/4 Wavelengths?  
by NC2W on April 19, 2004 Mail this to a friend!
There is a good website for antenna information, actively written by a ham, W4RNL. Take it to 'google', and search for W4RNL.

The questions you have asked, can be ansered there, or, I can try to answer a few of them.

NC2W
Eric
 
Why 1/4 Wavelengths?  
by K3YD on April 19, 2004 Mail this to a friend!
Scott,

I compliment you for your curiosity and interest in antennas. I've been licensed for more than 30 years and I'm still learning about and experimenting with antennas.

There are some GREAT antenna books which you should try to acquire, but don't buy them all at once.

I'd have to recommend the ARRL Antenna Book as one which every amateur should have in his/her library. It's a great theory book to check for specific questions. If you're just starting out, look for some of Bill Orr's (W6SAI) paperback antenna books. [There are several.] These have a lot of practical information and combine well with the ARRL Antenna Book.

A personal favorite of mine is ON4UN's book, "Low Band DXing"--lots of good antenna information there.

On the internet, W4RNL's site is a huge source of data; he includes a lot of antenna math plus practical ideas.

Keep experimenting, keep reading and keep asking questions. Remember, the only dumb question is the one you didn't ask!

73, Blair k3yd
 
RE: Why 1/4 Wavelengths?  
by N6AYJ on April 19, 2004 Mail this to a friend!
I'm not an electrical engineer, but I impersonate one on eham.net! No, seriously, I always wanted to understand Maxwell's equations, but I'm not that strong in calculus, so I used to always look around for books that would explain antennas and RF radiation in non-technical terms, but I could seldom find anything. But then, luckily for me, my son Ben, KB6ZD, graduated from Berkeley and Yale in physics, did about 8 years of post-doc work at various high-energy labs, and then started teaching introductory physics for non-majors at a southern California college. He didn't like any of the available physics texts, so he wrote one of his own, and to avoid any impression that he was profiting from a captive audience he made it available by free download at lightandmatter.com. It is called the "Light and Matter" series of 3 volumes: Conservation Laws, Newtonian Physics and Simple Nature. I downloaded all three and burned them to a CD-ROM. They will answer almost all your questions about RF, electricity, resonance, inductance, reactance, refraction, difraction, etc. in layman's terms, but extremely rigorously with tight analogies to the physical world. (That's where I first learned how antennas radiate.) Also about a whole bunch of other stuff. And he throws in some really interesting and sometimes kinky biographical details about all the scientists he discusses. But then again, maybe I'm biased!
 
RE: Why 1/4 Wavelengths?  
by WB2WIK on April 19, 2004 Mail this to a friend!
There are minor exceptions, but for the most part if something is obvious and not in popular use, there's a good reason. If an 80m dipole worked well on all bands, that's what everyone would be using.

A confusion factor enters with the wild claims made about omnidirectional VHF-UHF gain antennas. You'll find "7/8 wavelength" two meter omnis having claimed gains of 7dBd+, which is ridiculous. In reality, a 1 wavelength vertically polarized antenna isn't particularly useful unless it's carefully made to be a stacked vertical colinear array, in which case a bit under 3dB gain is achievable. That gain, as others mentioned, doesn't come from the antenna's "length," but by the compression of its radiation pattern to eliminate wasted energy in areas not useful for making contacts. The resulting very low-angle radiator is actually very undesirable for working stations located above the antenna's location, or working air mobiles flying overhead!

WB2WIK/6
 
RE: Why 1/4 Wavelengths?  
by KB9WIS on April 19, 2004 Mail this to a friend!
So, if one wanted an omni directional antenna with "only" low angle radiation, he or she should indeed "phase" and/or stack the elements on top of one another and make them "collinear" or "1/2 wavelength over a 1/2 wavelength".. This antenna would yield the higher performance at longer ground wave paths and long distance DX contacts by compressing more of the signal in the low angle radiation only.. However, a "non-linear" non-phased (or stacked) vertical antenna would have higher performance in more local DX and shorter Line-Of-Sight paths/propagation, in comparison to the "stacked" collinear vertical.. Am I correct with these assumptions?
 
RE: Why 1/4 Wavelengths?  
by W8JI on April 19, 2004 Mail this to a friend!
All of the following might be plain English, but unfortunately it totally wrong:


RE: Why 1/4 Wavelengths? Reply
by N6AYJ on April 19, 2004 Mail this to a friend!
The reason an antenna radiates is because, as you increase the frequency of an RF signal, the electric field of the antenna current is able to change direction faster than the magnetic part. This is because the magnetic lines of force resist the change in current direction of the magnetic part of the field as the magnetic field collapses with change in the direction of the antenna current, but the electric field does not resist the collapse of the electric part of the field. When the frequency gets high enough, the magnetic part of the field can't follow the electric part as the direction of its current flow changes, because it takes too long for the magnetic field to collpase; then the magnetic part of the wave gets cut off from the electric part, and the magnetic part gets launched off into space from the antenna. Therefore, anything we can do to make the electric part of the wave change direction faster and more efficiently will cut off the magnetic part of the wave more effectively and will therefore increase the amount of radiation from the antenna.>>>

Unfortunately that's not even the least bit factual.

A magnetic field is produced by moving charges. It "moves" at the speed of light through freespace.

An electric field is produced by a DIFFERENCE in charge distribution. The force or effect moves through freespace at the speed of light.

Radiation is caused by charge acceleration. The effect or force on other charges moves through freespace at the speed of light.

The reason an antenna has current and voltage, is the boundary at the antenna end. If the boundary is an open circuit, voltage is high and current is zero. There simply isn't anything for the current to flow into. The mismatch is such that source and reflected current is out of phase, and source and reflected voltage is in phase. The voltage adds and the current subtracts at an open end termination.

The large mismatch from the wires open end (power has nothing to flow into) causes a reflection, and 1/4 wl away (electrical distance) inciddent or forward voltage and the reflected voltage is out-of-phase and nulls or subtracts, while incident and reflect current is in phase and adds.

I have some stuff at

www.w8ji.com if you search for radiation and fields.

73 Tom



radiation occurs because of chanrge accleeltaion

 
RE: Why 1/4 Wavelengths?  
by W8JI on April 19, 2004 Mail this to a friend!
refraction, difraction, etc. in layman's terms, but extremely rigorously with tight analogies to the physical world. (That's where I first learned how antennas radiate.) Also about a whole bunch of other stuff. And he throws in some really interesting and sometimes kinky biographical details about all the scientists he discusses. But then again, maybe I'm biased!>>>>>

Either you misunderstood what he wrote, he doesn't understand EM radiation at all, or he made an error in writing.

One particularly disturbing thing is your comment "And he throws in some really interesting and sometimes kinky biographical details about all the scientists he discusses".

Technical papers or articles that contain personal information or stories about other people are almost always written by people with severe lack of scientific thought processes. A techninal paper or technical reference that is on a personal attack level or contains "kinky biographical details" automatically belongs in the trash heap.

73 Tom

 
RE: Why 1/4 Wavelengths?  
by WB2WIK on April 19, 2004 Mail this to a friend!
Well, the assumptions aren't phrased well for my interpretation, but I think you're on the right track, generally.

"Ground wave" is an MF-LF phenomenon rarely encountered in amateur radio work, so I'd probably avoid using that term. "Direct wave" is very applicable to most of the really strong-signal work we do on VHF-UHF-SHF, stuff that's either line-of-sight or close to it, just a bit over the horizon. Most of our contacts on HF are ionospheric, other than really close stuff which is mostly tropospheric scatter. Even contacts only 100 miles away on 40m or 75m are ionospheric. We can't normally run enough power to get a ground wave signal to go that far.

Vertically polarized antennas having gain are nearly always designed to have that gain at low radiation angles, by pattern compression; however, it's certainly possible to design an antenna for a higher radiation pattern, with an intentional uptilt, and a null at lower angles. Almost nobody would want one.

Since ionospheric propagation which represents the vast majority of what we actually use on HF loses polarization identity after reflection, there's no particular reason to use a high-angle vertical antenna, especially when a much simpler horizontal dipole will do the same job, usually better. I can think of only a couple of reasons to use a vertical antenna on HF, those being an attempt to achieve a lower-angle radiation pattern or having a lack of space and suitable supports for a horizontal antenna.

WB2WIK/6
 
RE: Why 1/4 Wavelengths?  
by N6AYJ on April 19, 2004 Mail this to a friend!
Tom, you are right! I am pretty dumb about antennas, and I am sure that I did make the mistakes that you mentioned. I'm sorry. (Oh well, I tried!) I will check out your website and see if I can learn more. 73 from N6AYJ
 
RE: Why 1/4 Wavelengths?  
by W8JI on April 19, 2004 Mail this to a friend!
So, if one wanted an omni directional antenna with "only" low angle radiation, he or she should indeed "phase" and/or stack the elements on top of one another and make them "collinear" or "1/2 wavelength over a 1/2 wavelength".. This antenna would yield the higher performance at longer ground wave paths and long distance DX contacts by compressing more of the signal in the low angle radiation only.. However, a "non-linear" non-phased (or stacked) vertical antenna would have higher performance in more local DX and shorter Line-Of-Sight paths/propagation, in comparison to the "stacked" collinear vertical.. Am I correct with these assumptions?<<<

Not really.

If you focus the wave along the earth losses can greatly increase. While that does increase ground wave signal strength, it does NOT necessarily increase DX signals.

A vertical with out-of-phase sections would have a higher radiation angle because low angle signals would be out-of-phase, and that antenna might be better for short skywave distances but it would be worse for groundwave.

The AMOUNT of wire in the antenna really doesn't matter, the distance, phase, and spacing of current in the sections does.

Radiation all comes exclusively from charge acceleration, and so it all boils down to how many ampere-feet of area the antenna has and the phase and spacing of the highest ampere-feet areas.

That's why a shorter antenna, for a given power, has higher current. That's also why an antenna with out-of-phase radiation, like a small loop or close-spaced Yagi, has very high currents for a given amount of power.

Radiation absolutely always comes back to time-varying current and the spatial (space) distance that current flows over. 1000 feet of wire in a one foot space is a one foot antenna!

73 Tom
 
RE: Why 1/4 Wavelengths?  
by W8JI on April 19, 2004 Mail this to a friend!
Tom, you are right! I am pretty dumb about antennas, and I am sure that I did make the mistakes that you mentioned. I'm sorry. (Oh well, I tried!) I will check out your website and see if I can learn more. 73 from N6AYJ>>>

I didn't say anyone was dumb! It isn't dumb to make a mistake, that's the only way anyone ever learns.

What I did say is you either misunderstood him, he wasn't clear, or HE doesn't understand how radiation works.

I strongly dislike any technical paper or article that gives "kinky details" about others. That type of stuff has no place in a technical paper. That unfortunately seems to be a modern trend that we need to not encourage.

73 Tom
 
RE: Why 1/4 Wavelengths?  
by N6AYJ on April 19, 2004 Mail this to a friend!
I strongly dislike any technical paper or article that gives "kinky details" about others. That type of stuff has no place in a technical paper. That unfortunately seems to be a modern trend that we need to not encourage.

I think I disagree with you on this. I think we need to be fully aware of how freakin' weird most scientists are. (Not to mention most ham radio operators.) Otherwise we are going to promote the impression in the minds of the public that this kind of behavior is normal.
 
RE: Why 1/4 Wavelengths?  
by AE6IP on April 20, 2004 Mail this to a friend!
I know a lot of scientists. They tend to be less weird than the population in general, but more nerdy.
 
RE: Why 1/4 Wavelengths?  
by AA6E on April 20, 2004 Mail this to a friend!
Whereas hams are nerdy AND weird? :-)

Personally, I have found that CopperWeld makes pretty kinky antennas...

Enough of this!
 
RE: Why 1/4 Wavelengths?  
by N8VB on April 20, 2004 Mail this to a friend!
W8JI wrote:

<<<"Either you misunderstood what he wrote, he doesn't understand EM radiation at all, or he made an error in writing.
One particularly disturbing thing is your comment "And he throws in some really interesting and sometimes kinky biographical details about all the scientists he discusses.
Technical papers or articles that contain personal information or stories about other people are almost always written by people with severe lack of scientific thought processes. A techninal paper or technical reference that is on a personal attack level or contains "kinky biographical details" automatically belongs in the trash heap. ">>>

It is clear that N6AYJ does not understand EM radiation. But nowhere in his son's textbooks is EM radiation explained in that way.

Before you go shooting your mouth off about the quality of his son's physics textbooks you should really download a few and read them. These are physics TEXTBOOKS, not "technical papers". Furthermore, no where does the author make personal attacks on scientists. Here is a typical exerpt from the textbook describing historical events about certain scientists:

---
It took just a moment for that head to fall, but a hundred years might not produce another like it.

(Joseph-Louis Lagrange, referring to the execution of Lavoisier on May 8, 1794)

The Republic has no need of scientists.

(Judge Pierre-Andr´e Coffinhal’s reply to Lavoisier’s request for a fifteen-day delay of his execution, so that he could complete some experiments that might be of value to the Republic. Cof-finhal was himself executed August 6, 1794. As a scientific experiment, Lavoisier decided to try to determine how long his consciousness would continue after he was guillotined, by blinking his eyes for as long as possible. He blinked twelve times after his head was chopped off.)
---

I seriously doubt that you would be able to write a useful textbook on the wide range of physics topics that Benjamin Crowell, KB6ZD has.

N8VB
 
RE: Why 1/4 Wavelengths?  
by N6AYJ on April 20, 2004 Mail this to a friend!
It is clear that N6AYJ does not understand EM radiation. But nowhere in his son's textbooks is EM radiation explained in that way.

I plead guilty. I didn't mean to sound like a "know-it-all", and I didn't realize my understanding of EM radiation was that bad. But I think it works SORTA the way I said. Can someone who understands RF radiation better please correct my explanation in layman's terms?
 
RE: Why 1/4 Wavelengths?  
by N6AYJ on April 20, 2004 Mail this to a friend!
To Tom, W8JI: I have a lot of respect for you as a hard-core ham, but in all candor, I read over your posts again in reply to my original post, and it really isn't clear to me how you are disagreeing with what I said. To me, your posts seem to be saying the same thing I said, but in different words. But maybe that is because I don't really understand what you are saying. Would you please specify again exactly what was wrong with my original explanation? I really would like to understand RF radiation better, but I am resigned to the fact that I am never going to understand Maxwell's equations. And I'm sorry if my comments about my son's books did not do them or my son justice. Yeah, my kid understands this stuff, and I don't! 73 again from N6AYJ
 
Why 1/4 Wavelengths?  
by KW4N on April 20, 2004 Mail this to a friend!
Let's cut the squabbling so we don't victimize KB9WIS or discourage others from asking straightforward questions. This is about KB9WIS, not you squabblers.
Dave
 
RE: Why 1/4 Wavelengths?  
by N8VB on April 20, 2004 Mail this to a friend!
KW4N:

KB9WIS's problem has already been disposed of - he needs to pick up a good antenna reference (such as the ARRL Antenna book)... What more needs to be said?

N8VB
 
Why 1/4 Wavelengths?  
by KL7HF on April 20, 2004 Mail this to a friend!
Why does a very legitimate question from
a gentleman who wants to learn, have to
be so confrontational?

I'm glad I didn't respond technically. I catch
enough flack from locals!


 
RE: Why 1/4 Wavelengths?  
by N0SP on April 20, 2004 Mail this to a friend!
KL7HF wrote:
>Why does a very legitimate question from
>a gentleman who wants to learn, have to
>be so confrontational?

Because someONE always likes to jump in and try to prove that he knows more than anyone else. The overriding emotion here should be to help Scott. But certain big ego(s) get in the way and belching contest begins. So what if someone posts something a little wrong... if it's not destructive, LET IT GO for god's sake. They should go hibernate in their own forum for grandstanding instead of seizing someone else's plea for help.

On a broader note, the character of this thread has become insane!! It's clear that Scott needs some BASIC understanding of antennas. But look at some of these responses.. Suddenly it's IMPEDENCE IS THE INVERSE OF THE SQUARE-ROOT OF +J REACTANCE IN THE IMAGINARY COLINEAR COMPARED TO THE RESISTIVE CHARGE-DISTRIBUTED MAGNETIC ZEPP. Then it's KINKY COPPERWELD and a guys chopped-off head blinking his eyes. Get real guys...

If I had asked these questions I might be taking up crochet right about now.
The books I mentioned earlier start simple and no math is required to gain a basic understanding of antenna patterns, gain, and feed methods. The best posts on this threads were the nice short ones with recommended reading and websites.
73,
Dennis
 
Why 1/4 Wavelengths?  
by W3DCG on April 20, 2004 Mail this to a friend!
Yeah...simple good questions like this need not be confrontational, and I commend those willing to ask them.

I like simple.

For me, it means that generally feedpoint impedance depends on several factors, such as- height above ground, distance to other conductive objects like wet branches, fences, parallel lines/gutters...given a center-fed resonant half-wave wire, and also, generally, one shouldn't fret about it too much at first.

As far as traps, the best I can figure is-
They are incorporated primarily to give a person an ability to predict where signal is headed, especially a 1/2 wave 80m wire, on say, the 20 meter band and higher- at the expense of gain.

As far as the need for resonance,
from what I've been able to gather in 3 years, it matters most for the sake of reduced SWR, which makes a difference when feeding with average runs of coax, like 100' or more, with average kinds of coax, such as RG-8x.

OCF wires, and Carolina Windoms, they are not resonant, but it matters little with a good tuner.
I hear there can be losses in the tuner at high SWR, so probably "good tuner" depends on things such as basic design, kind of "balun" and what you want to do with it. Some designs have greater loss at certain SWR values, and SWR values can read the same, for differing kinds of reactive impedences...
So I hear tell, that if you have to use a tuner to match a non resonant antenna SYSTEM (comprised of feedline+radiating parts such as the 1/2 wave wire), losses will be higher in the tuner if the reactive impedance of the system given a specific frequency/wavelength/SWR- IF THE TUNER IS FORCED TO USE THE INDUCTOR COMPONENTS INSIDE of it. So, one would want to "design" the antenna system such that if high SWR is present, make sure that SWR is due to reactive impedances that are slightly INDUCTIVE in nature. That way, the tuner must utilize the CAPACITORS inside it's little box, to bring it into resonance, so that our solid state rigs are seeing a happy go lucky works pretty good for such a simple set up, 50 ohm load. What I have heard from several sources, including some most respected/talented/true professionals that have posted here on this topic already, is that capacitors by nature are less lossy than inductors. It makes sense, seems like coils and cores always tend to heat up more than several metal plates parallel to each other separated evenly by air. And so that's basic applied grade school or high school physics.

See an open-line or window-line fed "All Band Doublet" and it has very low loss, with a proper outboard tuner, because parallel wire feedline of the 600 ohm, 300 ohm twin or windowline, or 450 ohm windowline, works super, even in high SWR conditions, has practically negligible loss- it would be likely that most of the loss to be had in such a system would be sustained in the tuner, especially if the SWR is very high, the inductor very small, and the impeding reactance of the antenna at a specific wavelength/freq/SWR, is Capacitive in nature.

Hopefully someone will add to the forum how to make a length of wire more reactively Inductive as opposed to Capacitive.

I think I got off topic, so next is my simplistic understanding of why use traps, aside from providing resonance and therefore, a nice, 50-75 ohm match (no need for a tuner).

Longer than half wave lengths, will have sometimes great, sometimes dismal, radiation patterns, depending on where you want to reach.

Google Cebik, he's got great stuff on the internet, which shows premodeled typical radiation patterns of typical ham radio HF lengths of wire, such as 80m half wave lengths of horizontal, Inverted L, and T configured wires. He shows modeled effects of having such wire higher or lower than a half wave up.

I think you probably already understand, thinking in terms of wavelengths instead of actual varying units of measure such as feet or meters.

If I remember the basics correctly, I think an 80m half wave wire, on 20 meters, makes a narrow leaved clover shape- super for places where those four major lobes are aimed, not so good for reaching places that are in the nulls of that four leaved clover pattern.

On ten meters, even greater gain, lower take off angles, but rather unpredictable, with also, very deep nulls. Essentially what you would have there, is an unsteerable wire beam, probably strung up between some trees, that works either great on the higher bands, or else very poorly, depending on where the other station is located.

Which means, having one wire with traps, so that bands covered resonantly, are electrically 1/2 wave in length, creates a broader coverage area (in the sense that more compass points are covered), but- at the expense of significant gain to be had if one were to not use traps and instead choose an 80m half wave wire antenna design using low loss parallel line and a proper tuner, resulting in lower visual profile, less weight, more gain, but also, at the same time, less gain.

I hope this makes sense.

And, I could be wrong, if so, someone please do correct me.

73,

Cheers!
 
RE: Why 1/4 Wavelengths?  
by NJ0E on April 21, 2004 Mail this to a friend!
at the end of the wire, the current will be zero, the
voltage at a maximum. as z ~ v/i; that will be a high
impedance point. working back along a 1/4w wire
(towards the center), the current will increase, and
the voltage decrease, until at the center, they will
have altered by 90 degrees; now the voltage is at a
mimimum, the current at a maximum. as z ~ v/i; this
is a low impedance point. if you are feeding the
antenna with a low impedance, lossy feedline line
like 50 or 70 ohm coax, that is the point at which
you will want to feed the antenna to minimize losses
in the feedline. similarly, with an antenna that is
an odd multiple of half wavelengths long will be
have the voltage and current in each leg varying by
an odd multiple of 90 degrees; a 40 meter dipole
used on 15 meters will have voltage and currents
varying by 270 degrees along each 3/4 wavelength
leg.

in contrast, if you try to use an 80 meter dipole on
40 meters (like i did when i was a novice, doh!!),
the impedance at the center will just be the same
as that at the endpoints, because the voltage and
currents will have varied in their relationship by
180 degrees. so the centerpoint will have low
current and high voltage, just the same as at the
endpoints.

if you use a low loss, balanced feedline such as
low loss tv twinlead, window line, or open wire
ladder line, the feedline has very low loss, and the
main consideration is whether you can achieve a good
match with your tuner (and keep the feedline away
from nearby metallic objects that could cause the
parallel wires to become unbalanced, or appear
electrically different). so you could use that 80m
dipole on 40m if the feedline losses will be low
enough. also, you need a special type of tuner to
work with balanced lines, the internal tuner sold
in alot of rigs today won't be up to the job.

if you are just beginning to assemble your amateur
radio library, i would recommend a copy of 'the arrl
handbook for radio amateurs' (used to be called 'the
radio amateur's handbook' in earlier editions). it's
somewhat expensive, but you can find older ones
second hand at swapfests that are also good. the new
editions, though, are really very excellent, and
include alot of background about the mathematics
involved that can be useful to understand.

any technical advice you receive in forums such as
this (including mine), you should try to verify
from a reputable source, such as the arrl
publications.
 
Why 1/4 Wavelengths?  
by WA2JJH on April 22, 2004 Mail this to a friend!
nO cw/ANTI cw war yet?
 
RE: Why 1/4 Wavelengths?  
by KC0JBJ on April 22, 2004 Mail this to a friend!
W3DCG said:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
"So I hear tell, that if you have to use a tuner to match a non resonant antenna SYSTEM (comprised of feedline+radiating parts such as the 1/2 wave wire), losses will be higher in the tuner if the reactive impedance of the system given a specific frequency/wavelength/SWR- IF THE TUNER IS FORCED TO USE THE INDUCTOR COMPONENTS INSIDE of it. So, one would want to "design" the antenna system such that if high SWR is present, make sure that SWR is due to reactive impedances that are slightly INDUCTIVE in nature. That way, the tuner must utilize the CAPACITORS inside it's little box, to bring it into resonance, so that our solid state rigs are seeing a happy go lucky works pretty good for such a simple set up, 50 ohm load. What I have heard from several sources, including some most respected/talented/true professionals that have posted here on this topic already, is that capacitors by nature are less lossy than inductors. It makes sense, seems like coils and cores always tend to heat up more than several metal plates parallel to each other separated evenly by air. And so that's basic applied grade school or high school physics."
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Well, the most important lesson I learned in high school physics is "You can't get something for nothing"

Part of what made my high school physics class fun is my teacher's favorite past-time was debunking bad science, such as "perpetual motion machines", the 100 MPG carburator, etc. He would call your notion of making an antenna inductive just so you could "tune it" with capacitance to be more efficicient, "BALONEY!"

If inductance is inherently less efficient than capacitance, adding any more to an antenna should make it LESS efficient not MORE efficient. Oh, I know you are going to say that you said only make it inductive if you already know it will have a high SWR. You must not understand the concept of SWR, because the number indicates the degree of mismatch in impedance, not whether the mis-match is due to inductance or capacitance. If it is due to inductance already, then sure, it will be matched with capacitance, and vice-versa. However, you will not have any more efficiency by trying to match an inductive system with capacitance than vice versa. Since, by your own statement, the capacitive system is more efficient than an inductive one, the matching inductor will not create any more inefficiency than already exists in the inductive antenna system. As my Dad used to say "Six of one, half-dozen of another."

Now perhaps there are some slight differences in inductors, something about "Q" or somesuch, which gives a tradeoff in efficiency for bandwidth, but your notion of making an antenna inductive just so the tuner will be capacitive is just plain silly!

But what do I know, after all, I am only a Mechanical Engineer! <grin>

Oh, by the way, "several metal plates parallel to each other separated evenly by air" sure looks like cooling fins on a heat sink, mechanically anyway. Of course that would be cooler than a tightly wound coil of magnet wire, usuually covered in shellac or some newer insulating polymer sealant, etc. Almost all good electrical insulators are also excellent thermal insulators. Having worked in the area of electrical heater design I can attest to that. What we wouldn't have given for a new material that was electrically insulating but thermally conductive at high temperatures! We usually referred to this material as "Unobtainium".
 
RE: Why 1/4 Wavelengths?  
by W8JI on April 22, 2004 Mail this to a friend!
N6AYJ,

If your son understands EM radiation, then you should copy you post and send it to him. He'll have the time to explain it to you.

If he can't help you N8VB probably can, he's always calm and patient with people and takes the time to read everything carefully to understand what people are saying before offering a well-thought-out response. His book suggestion was a good one.

What you basicly said was that the speed difference of the fields caused magnetic waves to leave the antenna. That just isn't true.

The speeds are all the same, and radiation is NOT a magnetic field. Radiation is a force on other charges caused by charge acceleration, it has nothing to do with speed difference between electric and magnetic fields in the antenna. Radiation is not a magnetic field, if it were you'd be lucky to talk 200 feet with 100 watts. It has to do with alternating current flowing in a conductor in this case, and the fact the charges are accelerating as current varies in level and direction with each part of the RF cycle.

Fields are how we describe the effects of these forces, and the descriptions have nothing to do with what you call "the kinky personal lives" of past scientists.
 
RE: Why 1/4 Wavelengths?  
by N8VB on April 22, 2004 Mail this to a friend!
N6AJY,

The problem is that the generation of EM radiation is almost impossible to visualize using familiar examples and analogies to our experience in the everyday world. The best that most people can do is to write down Maxwell's equations and accept that they accurately describe the real world as far as electromagnetic fields go.

N8VB

P.S. And thank you W8JI for the nice comments...
 
RE: Why 1/4 Wavelengths?  
by KN6Z on April 22, 2004 Mail this to a friend!
[A magnetic field is produced by moving charges.]

Yes, that's one way. A magnetic field can also be produced by a time varying electric field.


[An electric field is produced by a DIFFERENCE in charge distribution.]

I think what you are trying to say here is that an electric field results from CHARGE SEPARATION. A homogeneous mix of plus and minus charge, e.g., electrically neutral matter such as antenna wire, has no net electric field. If you separate the plus and minus charge to some degree, an electric field will result. In the case of an antenna, creating plus and minus charge at opposite ends of a straight wire generates a dipole field. Higher multipole fields are possible.

But charge separation is not essential. A lump of plus charge alone, or minus charge alone, gives rise to an electric field.
 
RE: Why 1/4 Wavelengths?  
by KN6Z on April 22, 2004 Mail this to a friend!
[...and radiation is NOT a magnetic field. Radiation is a force on other charges...]

Electromagnetic radiation has a magnetic field component (or vector), and an electric field component (or vector). These two vectors are perpendicular, and propagate through space perpendicularly to the two field vectors. Imagine the vertical face of an ocean wave moving toward you up the beach. One field component would lie along the vertical dimension, the other would lie along the horizontal dimension.

EM radiation could potentially exert a force on any charges it encounters, but no charge need be present. The EM field propogates along through empty space.
 
RE: Why 1/4 Wavelengths?  
by KA4KOE on April 23, 2004 Mail this to a friend!
Radio waves propogate via the ether. Thats why getting too close to antennas will fry your brain. Ether in high concentrations will cause dizziness and then unconsciousness.

Good for removing paint too, by the way.

P
 
RE: Why 1/4 Wavelengths?  
by OD5UH on April 24, 2004 Mail this to a friend!
Hi

what is the current and voltage measured at the transciever output of an 100 watt HF rig.

also is the rig generates only electrical signal only and how does transmission happen to give electromagnetic wave and thanks
 
RE: Why 1/4 Wavelengths?  
by KN6Z on April 26, 2004 Mail this to a friend!
The output signal of your transmitter will produce RF currents and time varying electric fields in your antenna, generating magnetic fields, as discussed above.
 
RE: Why 1/4 Wavelengths?  
by N0EW on April 26, 2004 Mail this to a friend!
>what is the current and voltage measured at the
>transciever output of an 100 watt HF rig.
>
>also is the rig generates only electrical signal only
>and how does transmission happen to give
>electromagnetic wave and thanks

I've never measured my output. When I transmit at 100 watts with either my Icom 746, or 706, I draw about 18 amps from the power supply while I transmit (connected directly to the transceiver). Of course some energy is lost in the transceiver. I draw about 2 amps in receive mode.

I too would like to hear from someone with an ammeter inline with the output side of their transceiver.

Regarding the electromagnetic field...

Any AC (alternating current: our Radio Frequency (RF) signal is AC) electrical signal is comprised of both electrical waves and magnetic waves (also called fields).

As the electrical field changes, or alternates, that change creates, or induces, a magnetic field. This magnetic field changes because the electrical field creating it changes. A changing magnetic field creates a new electrical field. Each new electrical and magnetic field is created a small distance from the field that created it. This is why the electromagnetic wave moves along the conductor. This is why the electrical signal created by your transceiver also creates a magnetic component.
 
RE: Why 1/4 Wavelengths?  
by N8VB on April 26, 2004 Mail this to a friend!
"I too would like to hear from someone with an ammeter inline with the output side of their transceiver."

Uhmm, it depends on the load and the power output... 50 ohm non-reactive dummy load and 100 watts out would give you I = sqrt(P/R) = sqrt(100/50) = sqrt(2) = 1.414 Amps approx.


N8VB
 
RE: Why 1/4 Wavelengths?  
by OD5UH on April 26, 2004 Mail this to a friend!
Thanks to all who answered my question.

I am facing a problem with high swr for an inverted V at 10 meter ( without balun ) and is there any way to replace the balun by a simple method .

Is there a relation between balun and swr.
again thanks to all.
 
RE: Why 1/4 Wavelengths?  
by W0OPW on April 26, 2004 Mail this to a friend!
Sami(OD5UH), your question about the relation between baluns and SWR. My 'simple' solution is this: If using
50 ohm feedline and your rig has 50 ohm input/output, then if you add or subtract 1/4 wavelength of feedline and the SWR changes(get much worse or much better), then a balun is required. The reason: common mode currents exist on one side of the feedline, acting like a third wire. This 'third wire' is changing the effective length of the antenna, causing the SWR to change. If the SWR does not change, no balun is needed. If common mode currents exist on the line, then it will radiate and can be eliminated with a balun.

Pat W0OPW
 
Why 1/4 Wavelengths?  
by W3DCG on April 27, 2004 Mail this to a friend!
KC0JBJ,
Hello, is it Donald?
Well I appreciate your correction, though I don't think I'm going to design my simple antenna system based upon your correction.

Like I said, I like simple, I'm no engineer, mechanical or other. I'm just an ordinary newcomer, trying to make sense out of all of it, in an effort to put out the best signal I can given my very modest resources.

So, I won't get into a debate about where most loss occurs inside a typical (Pi, L, or Pi-L) antenna tuner, or what tuner circuit designs, generally offer inherently less loss. I only know what I want to do with it.

I think others have already done all the research, and if I had the time, I'd be into doing it to confirm it all, just for grins, as it is, I still have yet to have time to put down a good RF ground system since moving, and I moved nearly a month ago, am still QRT. But, when I go QRV from my little humble abode, I do plan on having a decent 100W signal on the bands important to me, which lately, is 20-80, and I'll make perhaps an ill attempt at being heard on 160.

If a one wire muti band solution is sought, I really think a 100-135' ladder or window line center fed wire through a good tuner is hard to beat, based on mere QSO's I've had in the last 2 years or so, with others using such an antenna. Pay attention to the length of the feed. It's cheap and easy. But if it's too much hassle, and coax is the only choice, resonance and SWR on the line (line loss) obviously could be a significant matter, especially on the higher bands.

However, I do still believe, regardless of your hypothetical theory, that if you make your tuner have to match an antenna system that is highly capacitively reactive, more loss will be sustained in the tuner, than if the feedline/antenna system being matched is highly inductively reactive.

Actually, I'd say don't take my word for it, I'd say look around, see what you find. I am no expert. Better still, do that, come back here, and report what you find and their sources. If I am wrong, then I'll have only appreciation for your efforts, before I measure, snip, and hanng a window line fed dipole in a less than optimal manner.

One more thing, I may not understand SWR, but I do understand that I have adjusted my tuner to perfect 1:1 (nearly) and got a lower reading on my field strength meter, and that I found another sweet combination of settings, with near 1:1 SWR, and my field strength reading was much higher (as well as my RSTs).

So if you are thinking about doing this cheap, windowline+tuner antenna, get a field strength meter too, (less than 20 bux or there abouts at Radio Shack).

Finally, the other thing I understand about SWR, is that, a dummy load offers an awesomely low, wonderful to look at, Standing Wave Ratio.

Cheers!
 
Why 1/4 Wavelengths?  
by W3DCG on April 27, 2004 Mail this to a friend!
KC0JBJ and all,
I have another very simple question:

Can a tuned circuit, be adjusted in such a manner, such that, at typical HF frequencies, said tuned circuit becomes in effect, somewhat of a dummy load?
 
NTIA REPORT POSTED  
by KA4KOE on April 28, 2004 Mail this to a friend!
http://www.ntia.doc.gov/
 
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