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Author Topic: Quarter wave dipole?  (Read 4582 times)
KB9QDC
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Posts: 13




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« on: January 31, 2002, 04:59:10 PM »

Is there such a critter? Would it be just the measurements for a halwave cut in half? Thanks for your replies.

KB9QDC
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KG6JEV
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Posts: 65




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« Reply #1 on: January 31, 2002, 05:17:27 PM »

You bet! If the formula for a halfwave dipole is:

length in feet = 468/freq (in MHz)

then the formula to find the length of a quarterwave antenna would be:

length in feet = 234/freq (in MHz)

Here's an example for a quarterwave dipole at 52 MHz:

234/52 = 4.5 feet
Each leg is 2.25'

If you have the room, try to make the antenna longer. Obviously, if you have the room for a halfwave dipole, you'll have better luck. Also, the formulas for finding the length of a dipole are approximate, and should get you close to a decent VSWR. To improve the VSWR, you may have to trim the dipole a little.

73,

Steven
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AC5E
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« Reply #2 on: January 31, 2002, 05:51:20 PM »

Hi: Like all physically short antennas, the problem is not in the antenna, it's in the matching. A quarter wave dipole will have a very low radiation resistance with a great deal of capacative reactance.

  If you can match it at the feedpoint it will radiate. And it will radiate almost as well as a full half wave dipole.

  The "devil" is getting it to match any reasonable feedline. Because if you don't - feedline losses will be very high. Heating feedline works no DX.

  73  Pete Allen  AC5E

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AG4DG
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« Reply #3 on: January 31, 2002, 05:56:04 PM »

< If you can match it at the feedpoint it will radiate. And it will radiate almost as well as a full half wave dipole.

The "devil" is getting it to match any reasonable feedline. Because if you don't - feedline losses will be very high. Heating feedline works no DX. >

Why not use ladder line to feed the antenna?  This will keep the feedline loss down.
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NB6Z
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« Reply #4 on: January 31, 2002, 06:33:56 PM »

A dipole is a balanced antenna (fed at the center). You need a high current node (low impedance point) inorder to achieve a strong RF field (magnetic field) on the wire. You will not get that on a 1/4 wave wire unless you feed it at one end...
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WB2WIK
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« Reply #5 on: January 31, 2002, 06:44:25 PM »

NB6Z is right.  A quarter wave of wire is put to much better use simply end-feeding it.  Making a doublet (center fed) from a quarter wave of wire is a poor use of the wire.

WB2WIK/6
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KB9QDC
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« Reply #6 on: January 31, 2002, 07:06:45 PM »

Thank you gentlemen.

It appears, however, that there is a difference of opinion regarding this subject.

Perhaps it would be of value to let you know that I was wondering about a 20 meter dipole.

Thanks again for your time and help.

KB9QDC
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AC5E
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« Reply #7 on: January 31, 2002, 08:01:40 PM »

Well - first of all any dipole driven at one half it's design frequency will give almost identical results. It matters not whether that's a 10M dipole on 20, a 20M dipole on 40, a 40M dipole on 80, or an 80M dipole on 160. Feedpoint impedance will be around 15 ohms at about -1,000 ohms reactive. Losses will be a little higher on the higher bands, a 10M dipole on 20 instead of a 20M dipole on 40, but the differences are trivial.

A lot depends on individual conditions, height above ground, etc., etc., but a reasonable estimate of results can be obtained without much trouble.

  For a 34 foot dipole on 40 M, feedpoint impedance SWR with 450 ohm line will be around 50:1 - and if you were using a 20 M dipole on 40, using 100 feet of 450 ohm twinlead, hmm, let me get the decimal right, you will have just about 6 dB of loss in the feedline.
 
  So 100 watts out of the tuner will be 25 watts at the feedpoint. BUT the sticking point in that situation is that every tuner design I know of will have about 10-12 dB loss. So you are looking at turning 100 watts into 2 watts. Even across town, that's worse than a cheap vertical.

  The situation is worse with 50 ohm coax - where the feedpoint SWR will be around 900 to 1, more or less. Feedline losses will be 15-18 dB, tuner losses around 15 dB, and output with 100 watts in will be in the milliwatt range.

  If you are the kind who enjoys driving in traffic blindfolded, the experiment might be worth trying. I think I will pass.

  73  Pete Allen  AC5E
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WB6BYU
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« Reply #8 on: February 01, 2002, 11:58:17 AM »

Perhaps things won't always be as bad as Pete describes.  If all
tuners have at least 10dB loss, then they would all be disipating
90 watts and would get warm rather quickly!  Not that it doesn't
happen with some loads, but I'm sure there are lengths of feedline
that will present an impedance that the tuner can match reasonably
well.  (The tuner loss is a particular problem on 80 and 160m,
where most commercial tuners just don't have enough capacitance
available to match low impedance/reactive loads efficiently.)

However, there are two commercial methods to make a quarter-
wavelength long dipole reasonably efficient, even when fed with
coax:  inductive loading, and folding.  And, for home construction,
I would add a third method - end loading.  Generally, these will
make it a single-band antenna, but additional elements can be
added in parallel for the other bands if desired.

Inductive loading can be pretty simple:  add a loading coil in series
with each side of the feedline where it connects to the antenna.
There are formulas and design tables in the ARRL Antenna Book
(and probably the Handbook, as well.)  Actually, you can match
just about any wire length shorter than a half wavelength this way,
but efficiency becomes a problem when the elements get very short.
The radiation resistance will probably be considerably lower than
50 ohms, and the bandwidth will be narrower than a standard
dipole.  I don't have my reference books handy, but here is how you
can set one up (and provide impedance matching at the same time)
with just a dip meter.  I would take a length of coil stock from the
junk box, or wind a coil on a toilet paper tube that can be tapped
every turn.  Connect this across the center of your dipole and find
the resonant frequency by coupling the dip meter to the coil.  If
it is above the operating frequency, you need a bigger coil.  Then
start tapping the ends of the antenna in towards the middle of the
coil.  At some point, you will find the amount of coil that will give
resonance at your desired frequency.  Now count the turns in the
coil, and connect the coax braid to the very center.  Tap the center
conductor about 3 turns to either side, and check the SWR.  You
will have to experiment with the tap point to find the best match,
and you may have to change the total inductance slightly to
maintain resonance (keeping the antenna wires symmetric around
the coax braid connection) as you go.  In some cases, you may
find it easier to use one coil to resonante the antenna, then wind
a link coil of a few turns around that for the coax.  The adjust the
number of turns on each coil for best SWR at the operating freq.

I use this method of link coupling the coax to the loading coil on
my 80m portable vertical antennas with an iron powder toroid core
and it works great.  Takes a bit of experimentation, but once it is
set up it is quite reliable.

The loading coils don't have to be in the center - the further out
along the wires you put them, the higher the feedpoint impedance
will be, but the more inductance will be required for resonance.
The impedance still will be lower than 50 ohms, but the losses in
the loading coils will raise it somewhat.  You can match the coax
impedance by connecting either a coil or capacitor across the
feedpoint and readjusting the loading coils slightly.  This is
commonly done for mobile antennas, and you can use the same
design equations as you would for a 16' mobile whip for 20m.
(The feedpoint impedance will be higher for the dipole because
you have two wires, but you won't have the ground losses of a
mobile installation.  So the feedpoint impedance will be different,
but the loading coil inductance should still apply.)

One commercial dipole simply folds the longest element in half and
brings the ends back to the center.  This lowers the feedpoint
impedance and probably will take a bit more care in tuning, but
should be close enough that it can be fed with coax through a
tuner without excessive losses in the feedline.  A similar antenna
is the quarter wave folded dipole (same antenna, but fed at a
different point.)  This has a very high feedpoint impedance, but
probably could be used with 600 ohm feedline without excessive
losses.  And you could also use it on the second harmonic.

Of course, you could always just install the center section of a
half-wave dipole, then bend the leftover ends of the wire around
however necessary to make it fit.  "Capacitive end loading" is an
extension of this principle, but uses two (or more) wires going in
different directions.  Generally, the loading wires are made all the
same length - as a starting point, I would guess it would take about
50% more wire total in each end than the amount of shortening you
are trying to achieve, but, because the wires can run in several
directions, the total distance required is less.

You can then combine inductive loading with capacitive loading.
If you put a loading coil at the end of each wire, and used a
perpendicular wire after the coil for added capacitance (otherwise
the coil must be VERY large) you could get it to resonate on 20m
and the coil would act as a trap on 10m, giving full dipole operation
there as well.

Oh, yes... there is "linear loading".  This comes in a number of forms,
but the easiest would be to wrap the wire back and forth between
two sets of pegs (perhaps a foot apart) to take up the extra length
at the ends.  I'd start with rather more than a half wavelength of
wire, run the center fairly straight as far as possible, then zig-zag
the remaining length and check the resonant frequency with a dip
meter or SWR analyzer.  (You can also make the entire length
of the antenna in a zig-zag pattern if you wish.)

Any of these methods would make a usable 1/4 wave dipole.  And
there are probably many more, but I've taken up enough bandwidth
on the topic already!

Good luck! - Dale WB6BYU
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NB6Z
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« Reply #9 on: February 01, 2002, 12:25:28 PM »

A 20 meter traditional dipole is good on 20 meters but not on other bands. Assuming you use coax to feed it, the impedance of the feed point will be too high for 10 meter use, and antenna theory will not support good operation on 40 meters. You may have limitted success de-tuning it for 17 or 30 meters, but it would be much better to add those elements to the dipole or make another dipole... If you plan to use open or twin lead line, you can use a balanced tuner to "tranform" the voltage/current at the end of the line such that you can match a high impedance for 10 meters; and the antenna becomes a double zepp on that band. 17 and possibly even 30 meter operation would be practicle as well, but not 40 or 80 meters.

Coax can not handle high RF voltages due to its capacitve design. It needs to be used in a low impedance circuit to reduce it's losses and prevent damage to the dielectric material. In my opinion, you should never use a tuner with a coax dipole or vertical antenna except to make minor adjustments to SWR so that you satisfy the needs of your solid state final amplifier. (This is what built-in ATUs on higher end HF radios will do for you.)
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K5DVW
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Posts: 2193




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« Reply #10 on: February 01, 2002, 03:59:40 PM »

About a 20m dipole not being useful anywhere but 20m, this is not so. I have a dipole cut for 20m which I use quite successfully on 40, 20, 17, 15 using an antenna tuner and open wire TV feedline (the best way to feed a non resonant dipole). It's performance is actually better in some cases than a trap vertical that I have for comparison.

Antenna theory says that if the antenna is physically short, it radiates more like an isotropic point source, (radiates in all directions) gain can suffer because of less antenna capture area (known as aperture.. a short rubber ducky HT antenna suffers from small aperture). The capture area gets worse as you make the antenna smaller. On the other hand, if it's larger than 1/2 wavelength, then it has a larger capture aperture and more gain (and directivity in some directions). The problem here can be the high feedpoint impedance for certain length and frequency combinations and the directivity may not be in a direction that is useful.

In either case, with proper feeding you can get decent (not always stellar) results. It's true that a 20m antenna will have a very high feed point impedance at 10m, but there are ways around that like adding a 10m dipole in parallel, then you'd have a "dual banded dipole". I've done this, it works great. Since the 10m section is shorter than the 20m section, it shouldnt pose any problems with length restricions.

Second point, as for coax not being able to handle high voltages, coax can handle a tremendous amount of voltage depending on the type. Coax or a variant is used to feed antennas for multi kilowatt radio and TV stations. Perhaps he means when it's working into a high impedance? I think with RG-8 and 100W, you won't have any problems such as this as the developed voltages won't puncture the coax dielectric. Infact, I'd have to check the data sheet, but I think RG-8X would probably work as well for 100W or less.

But to answer the original question... yes, you can run a physically short antenna with slightly diminished performance with an antenna tuner and with short lengths of very low loss coax. The formula is 300/(Frequency in Mhz) to tell you the wavelength in meters.

Anyhow, these are my thoughs on the subject. A suggestion is to just try it and see what happens.

K5DVW
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AC5E
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Posts: 3585




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« Reply #11 on: February 01, 2002, 06:35:56 PM »

  While I hate to tromp on toes, there's theory, and then there's practice. In theory, a "quarter wave dipole" will work quite well, provided you can match it to its feedline at its feedpoint.

  We know that failure to achieve a reasonable match at the feedpoint will, absolutely, positively, do what theory predicts. We know, because we can measure the results on a field strength meter.

YOU WILL HAVE HORRENDOUS LOSSES - if you hook your feedline to the antenna and try to "match" this type of antenna system to the transmitter at the transmitter. Period. Using 100 feet of quality ladder line for an example, you will have something like 7 to 9 dB of feedline loss. That means 100 watts in will leave you between 13 and 25 watts to radiate. Shortening the twinlead to 50 feet will drop those losses about 1 dB, to the 6-8 dB range.

  And, also in theory, the SWR at the transmitter end will be 40:1 or so, high enough to be far beyond the ability of any commercially available tuner to provide even an apparent match.

  Coax does make the situation worse, with 15-20 dB of feedline loss. For 40 Meters on a 20 Meter antenna, 100 watts in will give you a magnificent half watt out.

  In theory, the SWR will still be far, far, beyond the ability of any commercially available tuner to match.

  BUT - trying to avoid more tiresome theory here - some length of feedline will provide an apparent match to almost any mismatch. So it is well within the most remote realm of possibility that your feedline will give you an apparent match, perhaps even a "perfect 1:1" match with this sort of antenna system.

It's even fairly likely that the SWR will be low enough to be within the range of most antenna tuners.

   But it's also possible that the essentially random length of feedline between your transmitter and your antenna will provide you with an even greater mismatch, even more losses, and be beyond any tuner's ability to match.
 
   Now, there's nothing you can do that will affect feedline losses, except match that antenna to its feedline at the feedpoint. No matter what your SWR meter reads, and no matter what kind of unintentional QRP DX you have worked, you still have a very inefficent antenna system. And you can do better, much better, without a lot of trouble or expense. And without taking up appreciably more room.

  If, as the QRO ad on the left side of the screen suggests, "Life is too short for QRP," it's also much too short to handicap yourself by using the HF equivalent of a rubber duckie for an antenna.

   73 Pete Allen  AC5E  
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KB9QDC
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Posts: 13




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« Reply #12 on: February 01, 2002, 08:06:31 PM »

Wow! My first post, and I started a fight.

I think I'll just figure out a way to hide a 20 meter half wave dipole on/in my duplex.

Thanks for all your help. I am learning a lot.

KB9QDC
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AC5E
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Posts: 3585




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« Reply #13 on: February 01, 2002, 09:20:57 PM »

Why, Matt, this isn't a fight. It's a discussion.

And I hope you learned quite a bit from my mistakes - because I have been there and done that. Literally.

Try a 20 Meter trap dipole. It's a 10 M dipole with very small traps resonant at around 28.4 or so, and a short length of wire outboard of the trap to resonate on 20 M. They are MUCH shorter than a full size 20 M dipole, work almost as well, and provide a reasonable match to coax.

And if you have a few feet sideways, add a pair of 15 M wires at right angles and have a tribander!

 73  Pete Allen  AC5E
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WB6BYU
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« Reply #14 on: February 04, 2002, 11:51:31 AM »

I'll agree with Pete on this - you didn't start a fight.  You did bring
out some differences of opinion... but that is life.  We all have
different sets of experiences, and have made different trade-offs
between convenience, performance, antenna size, etc., leading
to different standards of performance.

And sometimes there are differences in how we apply the electrical
principles of transmission line theory - this is probably the one
topic in ham radio that has the most misleading information
published about it.  I know that my knowledge is not perfect, and,
at a guess, most folks fall in the 10% to 90% range in accuracy
on the subject.  So, if Pete makes a statement that doesn't match
my understanding on the subject, I owe him the courtesy of thinking
about his answer to see if there is anything I can learn from it, or
to see if I am making an error in my evaluation somewhere.  And
I presume he will do the same for my comments.  We all should be
capable of learning something new, and looking critically at our
beliefs to make sure they are consistent.

In the end, we may still not agree.  Perhaps we are looking at the
issue from different sides.  Perhaps we are both wrong.  Then it is
your job as observer to weigh all the comments and make your own
choices.

Someone who has the facilities to do so may want to try an
experiment to test some of these statements.  String up a 10m half
wave dipole fed with coax, and another quarter wave dipole some
distance away and colinear with it.  Try feeding the quarter wave
dipole with coax and/or twinlead through a tuner, or by using
loading coils or other techniques.  Compare signal levels with the
reference dipole.  All you need is some wire, feedline, a tuner, and
the S-meter on your receiver to get a reasonable sense of the
relative performances of several antennas.  If you don't have room
to do this at home, spend an afternoon in the local park borrowing
their trees.  You will learn a lot about antennas, and have an
enjoyable afternoon as well - probably much more fun (and more
practical) than listening to us haggle about theoretical performance!

- Dale WB6BYU
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