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RF Problems and the HF Vertical Antenna

from N4ZOU on August 16, 2006
View comments about this article!

"Editor's Note: Due to the popularity of some of eHam's older articles, many of which you may not have read, the eHam.net team has decided to rerun some of the best articles that we have received since eHam's inception. These articles will be reprinted to add to the quality of eHam's content and in a show of appreciation to the authors of these articles."

RF Problems and the HF Vertical Antenna

Are you having RF problems in the shack since you put up your HF vertical antenna? This is a common problem with an easy solution. RF energy traveling into your shack on the outside of the coax feed line causes this problem. The solution is adding an "ugly" 1:1 Balun in the coax to choke off the RF energy before it enters your shack.

So what's an "ugly" Balun? Nothing more than a few turns of the coax forming a coil. Don't scramble the turns; they must be side by side for the coax Balun to operate properly. 5 turns or more will be enough for 80-meters and up. Use caution not to coil the coax so tight that it causes the inside conductor to merge into the shield shorting it out. 4" PVC pipe coupler works well and it's cheap. Normally a 1:1 Balun would consist of multiple turns of three wires on a coil form and the simple coax Balun is also multiple turns of three wires when coiled, the center conductor, the inside of the coax shield, and the outside of the coax shield. Now your asking how this could possibly work. RF energy flows on the conductor and not in the conductor. This is the reason silver-plating or copper-clad steel works so well. The RF energy only penetrates the conductor slightly and for plated parts the RF flows entirely in the silver or copper plating and almost never in the conductor below it. In the case of coax this property separates the RF flowing inside the coax from the RF flowing on the outside of the coax.

We can use this problem to an advantage by careful placement of the "ugly" Balun. Most of us would think we would want that Balun at the base of the vertical and try and prevent all RF from flowing on the outside of the coax. In reality this is the worst place you could put it! Why? Simply use the outside of the coax as another radial. Putting the Balun at the base of the antenna would choke off the RF but the vertical element would simply put it there again. The Place to put the Balun would be 1/4-wavelength from the feed point of the vertical on the lowest band the antenna is capable of operating on. If your coax run is 1/2-wavelength or more you will want to place a Balun at each 1/4-wavelength point. You will also want to put a Balun on the feed line just before it enters the shack.

The formula for figuring out where the Balun should be placed is simple. 234 / frequency in MHz * the velocity factor of the coax = length in feet. An example is a Butternut HF6V 80 through 10 meter vertical. As the antenna is capable of operating on 80 meters simply input 234 divided by 3.5 times .75 would equal 50.14 feet assuming your using modern foam type coax with a velocity factor of .75. The HF6V uses a length of 75-ohm coax as a matching transformer. Simply ignore the velocity factor if different from the 50-ohm coax and measure 50.14 feet from the feed point of the vertical for the proper place to coil the coax forming the 1:1 Balun. If your feed line is shorter than a 1/4-wavelength on the lowest band the antenna is capable of operating on, place the Balun on the next highest band where the coax would be longer than 1/4-wavelength. This should cure RF problems associated with RF flowing into your shack on the outside of the verticals feed point.

N4ZOU

n4zou@yahoo.com

http://groups.yahoo.com/group/Butternut-antennas/

Member Comments:
This article has expired. No more comments may be added.
 
RF Problems and the HF Vertical Antenna  
by LNXAUTHOR on August 16, 2006 Mail this to a friend!
- love these antenna articles! i'll let others comment on the content, but for me, playing with antennas is the biggest draw to ham radio, and involves all bands and license classes!
 
RE: RF Problems and the HF Vertical Antenna  
by KG4RUL on August 16, 2006 Mail this to a friend!
Unless CC&Rs limit you to what you can sneak in through the back door. My vertical has to look like a flagpole to get by.
 
RE: RF Problems and the HF Vertical Antenna  
by K8MHZ on August 16, 2006 Mail this to a friend!
Since this is a vertical fed with coax, would one not need an ugly 'unun'?
 
RE: RF Problems and the HF Vertical Antenna  
by K0BG on August 16, 2006 Mail this to a friend!
There are a few fallacies in this. First, coiling up the coax does indeed work, but it certainly is not a wide band solution. In other words, it works well on one given band, not all of them.

Secondly, the only reason you would have current flowing on the outside of a coax feeding a vertical antenna is simply due to a lack of a good ground plane (radial field) under it.

A radial field is the other half of a 1/4 wave vertical, and a simple ground rod or a few radials will not suffice. In fact, until the number passes 25 to 30 (depending on their length), you won't notice much difference in performance. Commercial broadcast installation typically have 100 or more.

This is another case of treating the effect, not the disease.

Alan, KØBG
www.k0bg.com
 
RF Problems and the HF Vertical Antenna  
by KL7HF on August 16, 2006 Mail this to a friend!
The Common-Mode choke can be made in many ways.
WHile the coiled Coax version does work, care has
to be taken to assure resonance doesn't occur on
any of the used bands.

A real good method that is cheap and looks cleaner
is to enter the transmission line into the shack
through a length of PVC Conduit. After the cable is
run, stuff the conduit full of inexpensive Steel
Wool which is a very Ferrous material.
 
RE: RF Problems and the HF Vertical Antenna  
by WA2TNO on August 16, 2006 Mail this to a friend!
To KL7HF and other good readers of these comments:

Steel wool is a good RF absorber. Too good in some cases, as steel wool in an intense RF environment can start an impressive fire. I'm not knocking your suggestion, it does work after all, I just believe people should know that there are issues involved. Incidentally, the steel wool in a tube approach has been used for years by defense contractors as a way to introduce loss over cable shields entering test facilities. They used to call this a "Stuffing Tube", but it has been a while since I've heard that phrase used in industry. At the time that I became familiar with this approach, people were not using it on lines that passed high levels of RF...it was most often used for low level signal conductors.

Keep up the good comments!!!
 
RE: RF Problems and the HF Vertical Antenna  
by K3AN on August 16, 2006 Mail this to a friend!
The steel wool solution was published a few years ago in QST, and the following month it was shown to be an "urban legend" through actual testing. A steel wool choke balun did not attenuate shield current by any measureable amount.
 
RF Problems and the HF Vertical Antenna  
by K8AG on August 16, 2006 Mail this to a friend!
I use a 2-liter pop bottle. Cheaper and lighter than PVC pipe.

My 2 cents.

73, JP, K8AG
 
RE: RF Problems and the HF Vertical Antenna  
by WA4DOU on August 16, 2006 Mail this to a friend!
Lets use our heads here. Steel wool and a little moisture in a while will become nothing more than a pile of rusty residue. Its also unnecessary and pointless. With a good radial system the current on the shield of a coax feeder will be small. I have used many verticals over decades and never had trouble with rf in the shack on the shield of a coax feeder. Baluns today are frequently overrated. They are supposed to resolve a problem but aren't always necessary and serve no purpose if they aren't needed. Over engineered exercises are pointless. If your coax is on or in the ground, the rf level on it is low.
 
RF Problems and the HF Vertical Antenna  
by W4VR on August 16, 2006 Mail this to a friend!
Your formula for calculating placement of the balun is incorrect. It should be 246 divided by the frequency in MHz quantity times the velocity factor of the coax.
 
RE: RF Problems and the HF Vertical Antenna  
by WA4DOU on August 16, 2006 Mail this to a friend!
The outside of the coax doesn't involve velocity factor. That only comes into play inside the coax.
 
RE: RF Problems and the HF Vertical Antenna  
by W0IPL on August 16, 2006 Mail this to a friend!
It seems you disagree with some of the manufacturers on installation of chokes. I have a Hustler 6BTV and they STRONGLY recommend a choke at the base of the antenna and another in the shack. Using their recommendations, I have had no problems (20 years now) since I installed them.
 
RE: RF Problems and the HF Vertical Antenna  
by WA4DOU on August 16, 2006 Mail this to a friend!
Indeed and I've never used a choke on a vertical.
 
RE: RF Problems and the HF Vertical Antenna  
by WA4DOU on August 16, 2006 Mail this to a friend!
That includes a Hustler 4BTV.
 
RE: RF Problems and the HF Vertical Antenna  
by KL7HF on August 16, 2006 Mail this to a friend!
WA2TNO and all, Yep - all ferrous materials
change rf to heat. That's it's value. As to the
fire hazzard, if a transline has enough rf on it's
shield to start the Steel Wool on fire at Amateur
levels, the antenna isn't working at all.

As to it's "Urban Legend" report - c'mon - all
iron based material is ferrous to one degree or
another. If a test was run, it was performed with
the wet finger method!
 
RE: RF Problems and the HF Vertical Antenna  
by WA4DOU on August 16, 2006 Mail this to a friend!
Putting a choke in line with a vertical antenna isn't a bad practice. It is a practice designed to prevent problems that sometimes happen with certain installations, installers and certain practices. Its just unnecessary in many cases, adds nothing and takes nothing away. If you don't know whether or not you need it, then you need it.
 
RE: RF Problems and the HF Vertical Antenna  
by WA2TNO on August 16, 2006 Mail this to a friend!
I agree that steel wool is dust after minimal environmental exposure, and in commercial applications where a stuffing tube type solution was appropriate (very rare), I always preferred commercial RF absorber material and good environmental enclosure practices.

As for RF exposure and steel wool ignition possibility...I'm standing by that one, even at modest power levels levels...and it happens very quickly when it happens...it is gone in a flash...quite literally. No, I'm not speaking of my ham experience, but commercial observations at similar power levels and frequencies. We hams operate over a very wide range of frequencies...and sometimes with impressive power levels.

As for the comment to the effect that your antenna is not working if you've got ignition possibility...that's a very good comment. That happens, often in the commercial world, the antenna being damaged or such. And that's not always clear in the first seconds that a transmitter is turned on. It's always a good idea to select system components based on thinking about the things that can go wrong!!!

As for attenuating signals on the exterior surface of a cable shield, that is determined to a great extent by frequency and boundary conditions. I would like to see the ARRL debunking of that...what boundary conditions were established. It may be that the ARRL test was in a different context from my experience. The people I saw doing this in the defense contractor world...were not doing it because it didn't work!! They were doing it because it was a simple way to solve their immediate problem...under their conditions...which in many ways were similar to conditions some hams might encounter.

All, keep up the excellent commentary!!
 
RF Problems and the HF Vertical Antenna  
by WA1RNE on August 16, 2006 Mail this to a friend!
Some of the advice in this article is not quite technically correct.


Scramble winding the turns will work fine on most of the low bands, but from 15 or 10 meters and up and depending on the way the choke is wound, the interwinding capacitance essentially shorts out the turns and decreases its effective impedance, reducing the choke's ability to isolate common mode current from the outer shield.


The correct position for a common mode choke, especially for a 1/4 wave vertical is at the feedpoint. Adding another choke 1/4 wavelength away will not hurt, but current maximums occur every 1/4 wavelength starting at the feedpoint, so decoupling at the feedpoint is priority #1.


Common mode current flows on the outside of the transmission line, so calculating the 1/4 interval does not require taking velocity factor into account. Velocity of propogation is a measure of a transmission line's ability to transfer signal through the cable referenced to the speed of light. This is affected mainly by dielectric properties of the line.

Simply use the standard formula 234/F, Mhz to calculate the position of the choke at 1/4 wave intervals. The length to diameter ratio comes into play as the frequency increases, so the 1/4 point will become shorter and the formula will need to adjusted. The ARRL antenna Handbook has graphs that provide approx. compensation factors for this purpose.


Chris, WA1RNE


 
RF Problems and the HF Vertical Antenna  
by KE7IMB on August 16, 2006 Mail this to a friend!
Thanks for the time to share such good topics.
 
RF Problems and the HF Vertical Antenna  
by KL7FH on August 16, 2006 Mail this to a friend!
To: KL7HF, I think you have it backwards Del, Just like your callsign! NOT! just kidding buddy! Good answers thanks for your input...that is all..
73
Frank
 
RE: RF Problems and the HF Vertical Antenna  
by KL7HF on August 16, 2006 Mail this to a friend!
For WA2TNO and all:

I agree with everything you say, I use Ferrite
sleeves from Amidon, but they are spendy if you
buy the ones that will handle large coax.

You all are correct about Steel Wool deteriorating
from moisture. It would have to be sealed of at both
ends and even then you would have some moisture from
any air inside.

However, I would like to see the ignition at play.
Some day when I'm bored maybe I'll try it and see what
levels are required.

By the way - even if you don't need them, it is always
a good practice to eliminate all paths of rf back
into the shack, so don't knock the guys doing it.

I see my old nemisis, KL7FH is watching. Tough to have
other engineers editing my comments!
 
RF Problems and the HF Vertical Antenna  
by W1YW on August 16, 2006 Mail this to a friend!
Uhhh...NO!

This is horrible advice.

MONOPOLES should have as many radials as possible; be ELEVATED (unless you have many dozen radials); and have the coax CHOKED AT the feed.

Letting the coax radiate like this on a MONOPOLE will cause a current imbalance that will severely degrade the omidirectionality.

Also the formula is wrong and does not include velocity factor.

Antennas DO exist that use radiating coax; this (a MONOPOLE) is not one of them.

73,
Chip W1YW
(ex N1IR)
 
RF Problems and the HF Vertical Antenna  
by W1YW on August 16, 2006 Mail this to a friend!
RE: VELOCITY FACTOR

Of COURSE you have to include velocity factor!

The RF current on the braid is induced by the current flowing in the center wire. That's how coaxial systems work. The dielectric has a velocity factor.

73,
Chip W1YW
(ex N1IR)
 
RE: RF Problems and the HF Vertical Antenna  
by K2WH on August 16, 2006 Mail this to a friend!
Had many vertical with many radials. 70' 80 meter 1/4 wave with 60 radials. No problems at all at 1.5kw.

However, if need be go the Palomar Engineers way. They make balun kits of ferrite material tubes that just slide on the outside of the coax. The kit includes a length of heat shrink to hold it all together. $20.00.

K2WH
 
RE: RF Problems and the HF Vertical Antenna  
by WA4DOU on August 16, 2006 Mail this to a friend!
The velocity factor of the shield of coax is just about exactly 1 and it does not matter that the rf energy got on that shield via the inside of the coax. The shield of coax is no different than just a piece of wire in this case. RF energy flowing inside of coax is affected by a velocity factor of .66-.85 depending on the dielectric. The outside of the coax - NOT!
 
RF Problems and the HF Vertical Antenna  
by W1YW on August 16, 2006 Mail this to a friend!
Would you like me to cite the page from Halliday and Resnick's PHYSICS that deals with the induced current on the shield-- and dielectrics?

Remember, the objective is to produce an induced current (in the shield of the coax) that is equal in magnitude and opposite in phase from the conducted RF current in the center wire.

Since the induced current is an out of phase version of the center conductor current, then the velocity factor that affected the (AC) RF current on the center conductor by definition also affects the shield. If not the shield current and center conductor current would be partially out of phase (not 180 degrees out) and the coax would be an antenna and not a transmission line.

The objective of chokes is to change the effective impedance on the shield. May be that is where the confusion lies.

73,
Chip W1YW
(ex N1IR)

 
RE: RF Problems and the HF Vertical Antenna  
by WA4DOU on August 16, 2006 Mail this to a friend!
Chip, you don't seem to realize that the differential mode current flowing between the shield and the center conductor of coax and influenced by the dielectric is entirely different than the common mode current flowing on the outside of the shield. Indeed the entire purpose of the choke is to change the impedance of the outside of the shield at the point of insertion. The outside of the shield is "just wire".
 
RE: RF Problems and the HF Vertical Antenna  
by NL7W on August 16, 2006 Mail this to a friend!
Ditto... On my HF verticals of the past, I've used these larger ferrite beads, only from Palomar Engineers.

73.
 
RE: RF Problems and the HF Vertical Antenna  
by W8JI on August 16, 2006 Mail this to a friend!
Steel wool does not work.

It does nothing. Zero. Zip. Nada.

The reason is the steel strands are too large and the eddy currents simply make the steel have virtually no effect at all except to reduce inductance.

If you take steel wool and wad it up and place it in an RF coil, the inductance actually decreases at higher radio frequencies.

Even the article's concept of velocity factor does not apply. Why? Because the choking is on the OUTSIDE of the cable, not the cable core. The only dielectic on the outside is the cable jacket, and since it has very little concentrated electric field it has a small effect on velocity factor.

73 Tom
 
RE: RF Problems and the HF Vertical Antenna  
by N3OX on August 17, 2006 Mail this to a friend!
I see a good bit of this "I can build a 2/3rds size dipole 'cause I'm using V.F. 0.66 coax" stuff. Those that actually try it find that the resonant frequency is ... umm... a bit high.

Chip, how about that citation, because I remember H&R as containing actual physics.

Consider an infinite coaxial line built with perfect conductors. Look at the electromagnetic field on the inside. Does it ever see the outside? Stick whatever dielectric you want in between the center conductor and the shield; the rest of the universe doesn't know what's inside that cylinder; it's called a shield for a reason. You can couple waves in and out at the ends, that's it. Remember, the fields have to to ZERO inside a conductor; this admits a set of solutions on the inside and a set of solutions on the outside and they just don't have to talk to each other.

That said, you might use the coax as a "radial" but you're going to need a fairly high impedance choke out at the end in an elevated system. This may be why the "Cut to 1/4 wave * the velocity factor" rule works out for the original poster... a moderate impedance choke at a higher current, lower voltage point than at the "radial end". It's a good idea to use a choke or two in your feedline anyway, so no harm done.

73,
Dan
 
RE: RF Problems and the HF Vertical Antenna  
by W4LGH on August 17, 2006 Mail this to a friend!
"The solution is adding an "ugly" 1:1 Balun in the coax to choke off the RF energy before it enters your shack."

"I use a 2-liter pop bottle. Cheaper and lighter than PVC pipe. My 2 cents. 73, JP, K8AG"

The easiest and cheapest way to add a 1:1 balun is to simply wind between 6 & 8 turns of coax in a 6" diameter circle (open air) and you can either use electrical tape to hold it, or I prefer, using nylon
wire ties. This method truely works well, and your only cost is about 9 extra feet of coax!

I have personally found that 6 turns works for me, but I have had others say they used 8. Try it you'll see it really works, and it is NOT "URBAN LEDGEN"!
This method is also great to use between your radio and your RF amp to add a isolation between them too.

Coax is also GREAT to wind antenna traps for multi-band antennas. So there is no need to go out and buy fancy stuff, when you have everything you need already in your shack! Hey you could build your entire HF vertical out of PVC and coax.

73 es cu later, de W4LGH - Alan
http://www.w4lgh.com
 
RE: RF Problems and the HF Vertical Antenna  
by W9OY on August 17, 2006 Mail this to a friend!
Thank God steel wool doesn't work. Once it's ignited it makes a handy little fire starter. Choke = AC resistor

73 W9OY
 
RE: RF Problems and the HF Vertical Antenna  
by N3HKN on August 17, 2006 Mail this to a friend!
As is usual in antenna discussions there is no consensus. To the Amateur radio person it appears that no REALLY KNOWS how antennas work. Theories have been developed to fit either assumptions or observations. Formulas follow to substantiate them. However, when discussions such as the above occur it becomes obvious to an "unwashed" observer that mysteries do exist.

Dick N3HKN
 
RE: RF Problems and the HF Vertical Antenna  
by W8JI on August 17, 2006 Mail this to a friend!
Dick,

A lot of people understand in great detail how antennas and RF systrems work. There reason there is no consensus is because a few people who don't know make untrue statements that sound reasonable based on half-truths.

The velocity factor of the choke for example has nothing at all to do with the dielectric factor of the cable INSIDE the shield. Understanding that would require understanding how current and voltages are distributed in a transmission line operating in TEM mode. At radio frequencies, all common mode current moves to the outer wall of the shield. It always has been that way, it always will be that way. We knew that since the 1800's or earlier, so the information is at least 200 years old. Faraday even knew it.

That of course doesn't stop people from thinking and saying otherwise.

The steel wool test is very easy. The ARRL even had, in the Handbook of all places, a steel wool balun. That's utter nonsense. What they forgot is Lentz's Law. Lentz's Law has been around a long long time. Lent'z law is why the soft iron in transformer cores are laminated AND insulated from each other, and why the laminations become muct smaller and smaller as frequency increases.

This is why the iron in RF cores is powdered and placed in an insulating binder. This is why a core that increases inductance at 60Hz will DECREASE inductance at 100,000 Hz. All those people making powerder iron cores aren't just spening money for nothing to powder and insulate the iron fragments. If they could sell steel wool, they would be puting it in a binder.

If you want to learn something Dick don't read a non-peer reviewed opinion in an E-ham article and the replies.

73 Tom





 
RF Problems and the HF Vertical Antenna  
by KD5PSH on August 17, 2006 Mail this to a friend!
I use aluminum or copper wool; it last much better than steel wool.
 
RE: RF Problems and the HF Vertical Antenna  
by KL7HF on August 17, 2006 Mail this to a friend!
For Tom -

This is a poor forum for attempts to build
self esteem, especially with insults to others.
Leave that to Dave Letterman.

You ran with rf transformer theory, he isn't
making transformers, merely an rf choke, and
hopefully not a resonant one.

Del, KL7HF
 
RE: RF Problems and the HF Vertical Antenna  
by N3OX on August 17, 2006 Mail this to a friend!
Dick,

I'd like to add to Tom's comment. I think that the lack of concensus on the part of antenna builders, experimenters, gurus, and so forth comes from differing experience, lack of theoretical footing, and bad scientific method.

If I build a steel wool balun and I get no RF problems in the shack, I go ahead and claim the balun works, right? Nope. If I just stuff my conduits as a matter of course and never make some measurements in the situation without the steel wool, what have I shown? I've shown that my particular overall installation doesn't have enough RF currents flowing in places I don't want them to cause distortion on my audio or a hot mike. You can't go saying it's because of the steel wool unless you try it both ways.

Few people really try to get into the physics of antennas. Some of the concepts involved admit simple mathematical descriptions, but a lot of them, well, the math is just really, really hard. Antennas themselves require a great deal of numerical computation to figure out what's going on; programs like EZNEC are making that easier, but you still have to know the limitations and sensitivities of the code. Some people are writing very nice programs where they take a page full of equations and write them into a small, easy to use interface... but if you don't know the questions to ask, how do you even start to calculate the answer?

So what do you do? You go over to an engineering point of view... you've got some criteria for an antenna design, say 6dBi gain and >18dB front to back for some phased array you're building. You want to lose no more than 20% of your power in your grounding system. All of this can be met with a good engineering approach. You take previous experience, plans, and design equations (simplified physics with all the fudge factors necessary to make it work in the real world) and you cut a bunch of tubing and coax and lay down a bunch of radials, and then you measure and tweak until the array meets or exceeds your specs.

Maybe you don't have any measuring implements beyond your rig's SWR meter. Maybe your criterion for your antenna system is that it gives strong signals into Europe and doesn't mess up the kids' computer. With these limitations and relaxed criteria, you can still make progress. You can copy antenna designs that aren't very fussy about adjustment. You can do A/B tests among antennas to see which is better for what you want, you can add and subtract chokes and grounds to see if your RFI gets better.

You can ALWAYS move up to a more detailed, more exact description of your antenna system. I could try to set up a fully 3D calculation of the electromagnetic fields in my backyard including every tree, house and powerline and add a term describing the average dog flux through my yard per unit time. I could, instead, just throw up a low dipole and talk to my heart's content. Or I could take a path through the middle and do some calculations and some theoretical reading and some trimming and measring and figure out a good antenna system for my situation.

The universe gives you your desired concensus. Write down Maxwell's equations and the necessary boundary conditions. Solve.

Since this is essentially impossible, just keep learning. There's lots of good stuff out there. There's lots of contentious stuff ONLY because we all need to learn. For example, what IS the velocity factor for electromagnetic waves propagating on the OUTSIDE of the coiled ugly balun?

*ducks*

73,
Dan
www.n3ox.net
 
RE: RF Problems and the HF Vertical Antenna  
by N3OX on August 17, 2006 Mail this to a friend!
I also have to add, after seeing KL7HF's comment, that part of the problem in discussions of this sort is that the participants tend to be a bit too thin-skinned.

Let all percieved insults roll off when you're in a technical discussion. Keep self esteem out of it. Don't feel bad if someone tells you your idea was dumb... look at the technical content of what they're saying. Don't take a tone of disdain for others' ideas when they're incorrect, you'll just shut them down when you try to show them the right way.

If you don't have something technical to say, don't say it at all. All this stuff is written in the laws of the universe, folks. Every single one of us is at least subtly wrong about everything.

73,
Dan
 
RE: RF Problems and the HF Vertical Antenna  
by WA4DOU on August 17, 2006 Mail this to a friend!
In the final analysis, it's not what we know but what we can do with what we know that counts. The rest is cocktail party chatter.
 
RE: RF Problems and the HF Vertical Antenna  
by NB3O on August 17, 2006 Mail this to a friend!
"I use Ferrite sleeves from Amidon, but they are spendy if you buy the ones that will handle large coax."

Fair-rite (www.fair-rite.com) part number 2643102002 slips over RG-8 style coax and provides around 80 ohms impedance per bead at 10 MHz. They are less than a buck and a half in small quantities at elnamagnetics.com or lodestonepacific.com. Use approx. seven of these at both the antenna and at the entry point to the shack. For less than a large pizza and six pack of cheap beer, you get a better wide band solution for attenuating common mode RF current without having the wind load and mess of a coax coil. To note, it still does not replace a good radail-counterpoise system on a vertical; it only solves the "hot mic" problem and helps quiet computer noise from the shack. 73
 
RF Problems and the HF Vertical Antenna  
by W4VR on August 17, 2006 Mail this to a friend!
There are other problems with the concept presented in this article but others have pointed that out already with some confusion. I was referring only to that portion of the article dealing with the incorrect use of 234/f...always use the speed of light formula 984/f(MHz) with the appropriate velocity factor of the material you're working with to determine the physical wavelength or fraction thereof.
 
RE: RF Problems and the HF Vertical Antenna  
by W8JI on August 17, 2006 Mail this to a friend!
by KL7HF on August 17, 2006
This is a poor forum for attempts to build
self esteem, especially with insults to others.
Leave that to Dave Letterman.>>

No one insulted anyone. I just stated the facts.

1.) Multiple people assume the velocity factor inside the cable has something to do with the impedance or speed of a wave on the outside of the cable. We've known it doesn't work that way for many years. That understanding started with Faraday and is still true today, and it will be true forever more.

2.) A chunk of steel or iron does not increase impedance of a choke or coil operating at high frequencies. It actually decreases it. Lentz explains why. It has to do with the counter magnetic fields caused by currents flowing in the iron. These currents and the fields they create prevent the iron from concentrating the magnetic field. If you wad up steel wool and place it in a coil and operate it at radio frequencies, it actually DECREASES the inductance. It does exactly the opposite of what it does at audio frequencies unless the particles are very tiny and isulated from each other.


<You ran with rf transformer theory, he isn't
making transformers, merely an rf choke, and
hopefully not a resonant one. >

A choke works the same way Del. Chokes use powdered iron at radio frequencies and they use insulated laminated cores at audio frequencies for exactly the same reason a transformer does. You can't have significant eddy currents in the core or the countering magnetic flux reduces impedance.

If you wind up an air core RF choke and measure the impedance to the flow of RF at say 1 or 2MHz and you stuff the center of the coil with steel wool the impedance decreases.

If you pack steel wool over a cable shield and measure shield attenuation for common mode radio frequency currents there is virtually no change at all. This is why they sell ferrite shield beads instead of using cheaper steel wool for RFI suppression.

That's why everyone goes through the expense of suspending small iron particles in an insulating substrate. The size of the insulated iron particle is actually what determines the upper frequency limit of a core.

There's no need for anyone to take this personally. It's just how things work.

73 Tom
 
RF Problems and the HF Vertical Antenna  
by KL7HF on August 17, 2006 Mail this to a friend!
For WA2TNO:

When you're right, you're right.

I used all of my xyl's Brillo Pads and all of the
Steel Wool I could muster from the garage, made a
neat wrapping around a section of RG-8 to the
40 meter sloper and loaded the amp up on 10 meters.
I used ten because it's dead and I wouldn't QRM
anyone except maybe some truckers plus the mismatch
would cause some high voltage on the shield.

At any rate, it got pretty durned hot. I don't know
if I could have caused ignition with a longer
test run, but it did get warm enough for the
Scotch-88 to get pretty soft and peeled loose.

Now, back to Costco to replace the Brillo Pads.

Del, KL7HF
 
RE: RF Problems and the HF Vertical Antenna  
by W6FO on August 17, 2006 Mail this to a friend!
After multiple failed attempts to work 3Y0X barefoot with my HF2V, I was getting desperate. I was willing to try anything when I had an idea. I quickly went outside and arranged the radial field in a pentagram, sacrificed three virgins and, while stroking the vertical softly, chanted "I worship qrz.com and eham.net" four times. When finished, I quickly ran back into the shack and 3Y0X came back to me on the first call.

This worked for me so I must apply for a patent and write and eHam article aboot it like everyone else who has little technical knowledge.

Sheesh.

W8JI is one of the few people making sense in this thread. You guys do know his background in this stuff, right? :rolls eyes:
 
RE: RF Problems and the HF Vertical Antenna  
by W8JI on August 17, 2006 Mail this to a friend!
I just made a few impedance measurements of the shield impedance of two feet of coax with and without a mixture of 000 steel wool and brillow pads along 6 inches of that cable.

I used a calibrated laboratory standard HP4191A RF impedance analyzer. This is the type of device that is accepted widely as a standard for measuring RF impedances of inductors, capacitors, resistor, transistors, diodes, and other components. If you purchase an RF part that is characterized for value at a certain frequency, it was probably done on a 4191A or similar.

1.) The coax itself measures R=268 milliohms X= +33.7 ohms Z= 33.7 ohms at 7 MHz

The RF impedance is 33.7 ohms, that would be the "opposition" to RF current flow.

2.) The coax wrapped over a six inch length of brillo and 000 steel wool measures R=1.067 ohms, X=32.54 ohms, and Z=32.5 ohms.

Results:

The RF resistance increased from .268 ohms to 1.067 ohms.

The RF reactance (inductance) decreased from 33.7 ohms down to 32.54 ohms.

The RF impedance, or opposition to flow of 7MHz energy, decreased from 33.7 ohms down to 32.5 ohms.

A spot check of 1MHz and 30MHz showed a similar pattern. In all cases resistance slightly increase, inductance DECREASED, and the overall impedance DECREASED.

If anyone wants to see pictures, I can trasfer them from my camera to my web page tonight.

The only thing the steel wool does is slightly increase resistance and slightly decrease impedance and inductance of the cable shield. On the other hand a single 1" shield bead caused resistance to dramatically increase resistance from 268 miliohms to over 100 ohms, and impedance to quadruple.

73 Tom
 
RE: RF Problems and the HF Vertical Antenna  
by W4LGH on August 17, 2006 Mail this to a friend!
I find this discussion very interesting, and some good points have been made. I do believe that most peoples skin is way to thin, and they tend to take things personally when they are not personal at all.

The main issue of this article is RF in the shack, from a vertical antenna. One has to assume that this vertical is using the ground as it counter poist. The earth ground in this country is very low and a poor condutor. There fore we put ground radials in the ground to make up this poor resistance. Remember when it comes to ground radials, MORE is always better, and one should have 360 radials spaced at 1 degree, and tuned to the radiating frequency. This in most cases is NOT practical, smaller lots, time and money.
So we have to make a compromise, but thats ok, as all antennas are a compromise to start off with, especially antennas for multi-bands, or antennas tuned to be very broadband.

In the very basic of theories, an antenna is one side of an air core transformer. The receiving antenna is the other side. Energy radiated from the transmitting antenna is picked up by the receiving antenna. Again, basic theory says that if the impedance of the transmitting antenna is the same as the receiving antenna, equal energy is transfered. Now some of you are saying what if that aren't matched? Again if the xmait antenna is 50ohms and the receive ant is 25 ohms, it will in theory pick up more energy, however when switched and the 25ohm ant is xmiting to the 50ohm ant, what happens? First off @ 25ohms you are going to have at least a 2:1 SWR and 20% of your power is going up in heat and NOT being radiated. The receiving antenna now being 50ohms looks like a 1:2 ratio xformer and will attn the signal again. These are all factors that most don't think about when building and testing their antennas and one factor that makes it impossible to say one antenna works better than another one, unless you are testing with someone who has the same exact ant installed with teh same conditions. The other reason we all know is band conditions, which can change faster than you can switch antennas.

As someone said earlier, the math can get very complicated, conditions, location, ground impedances can change these calculations. To try and sum it up, the best cure is to achieve a 50ohm match AT the antenna, not your antenna tuner, as this is doing NOTHING at the antenna, unless placed at the antenna.
And also remember that just because you see a perfect 1:1 match on your antenna, this doesn't mean its radiating. A 50ohm resistor at the other end of the coax will prove this.

Just have fun...design and play with your antenna if it doesn't work, figure out why, learn from your mistakes, ask advise from several others, and above all remember that over the past 100yrs many many very smart people have been trying to design the "PERFECT" antenna, as it simply hasn't been done. In 40+ years of working with electronics, radio communications, and others areas...I have never heard of using nor used steel wool to try an make a balun. Theory tells me that it would have to be a VERY BIG ball of steel wool, making it very impractial. (sorry I am sitting here LOL thinking about how big this ball would have to be, also thinking about what the neighboors would be saying) Think about it...Kinda reminds me of a customer I had about 30 yrs ago, who installed a CB in his car and it wouldn't work. He came to me for help. Upon inspecting his car, I found a 5 gallon bucket of dirt in his trunk, with 2 wires runing into it. When asked why it was there, he replied..." The book said the ant and radio HAD to connected to the same EARTH ground!(true story) Now it was funny to me, and probably most of you, but he thought he was doing what was necessary to make it work.

Again, just have fun, and if it works they way you want it to, don't try and explain it, just use it!


73 de W4LGH - Alan
http://www.w4lgh.com
 
RE: RF Problems and the HF Vertical Antenna  
by W8JI on August 17, 2006 Mail this to a friend!
Anyone interested in steel wool baluns, I have measurements up on my website now.

http://www.w8ji.com/steel_wool_balun.htm

73 Tom
 
RE: RF Problems and the HF Vertical Antenna  
by WA2JJH on August 18, 2006 Mail this to a friend!
I have found a good counterpoise can make up for a plethoria of imperfect radial systems.

True, one can say if you have a counterpoise, your vertical is just a vertical dipole.

My roof space is limited. I have a few 15/10M radials
15 feet above roof level. A counterpoise just drops 180
degrees down. I found my instrinsic impedence to be close to 50 ohms 10-20M.

My counterpoise is 32 feet long. It just danngles down enough not to be in the top floor dwellings window.

I have zero RF feedback. I can put my lip on a matallic mic, and do not feel any RF heating.

I also got a new ATU. I have it in the RF sense mode.
I simply key about 50W, and the MFJ 993 intelli-tuner gives me flat SWRs on many bands.

Since the tuner can be RF tuned by a short 50W carrier, I CAN place it anywhere between the rig and the antenna.

Many make the mistake of having the ATU right next to the rig. For one CPU noise may be intoduced. You are also not getting maximum power transfer. The ATU will provide a tuning solution so your finals do not blow. However your tuning your coax and antenna for a match.

My New ATU is not WX proof. However I placed it so there is less than 30 feet between the ATU and the antenna. Before I had 100 feet of coax after the tuner.

In maritime installations, the ATU is topside with the antenna. I might WX proof my ATU for closer coax run to my homemade vertical.

Rigs with built in RF tuners have limitations. Yes, you will have the PA output to what seems to be a nice matched load. However what foward power on TX and RX is a different story.

 
RE: RF Problems and the HF Vertical Antenna  
by WA2TNO on August 18, 2006 Mail this to a friend!
For Del KL7HF...I'm not sure exactly what to think!! When I saw the original comment about steel wool, I thought maybe I would chime in with some of my experiences in industry with people tinkering with the stuff. Somehow, the idea seems to have taken off that I was actually endorsing the stuff...when in fact I was trying to say: yes, there are conditions where steel wool will absorb RF...but be careful...it can ignite and burn.

So much to my surprise, the New York Times (web), yesterday (17 August 2006) actually had an article quoting Adair on past experiences with watching steel wool lighting up with exposure to RF!! What a hoot, on the same day that some folks were saying it can't happen! So I did some other poking around on the web for similar stories, but did not find much other than a few references. And we do have to check web info carefully. In fact, now days we have to question the New York Times as well!!!

Anyway, I liked your comment, it was light and in good spirit. Somewhere in my collection of pictures, I have a photograph of the interior of an anechoic shielded room. There are black streaks on the blue absorber covering the interior walls. It seems that the coax from a power amplifier (200W) to an antenna was resting on the absorber cones, and the antenna had a mismatch. The absorber where the cable was resting burned...producing the black streaks. So there's something for thought...did the cable itself get so hot that the absorber burned...or did RF on the outer surface of the shielded cable heat the absorber to cause the burning. I visited the lab well after the fact, so I could not sort out what happened in detail. I wish I could find the picture...I would send it to you...

Oh...I should point out for the technically curious...the absorber was the pyramid cone type, carbon loaded, with flame retardent. Relatively old technology...

Have a great day...

Bruce

 
RF Problems and the HF Vertical Antenna  
by W1YW on August 18, 2006 Mail this to a friend!
I've never seen anyone use steel wool as HF absorber.

I've seen it used at VHF. Wouldn't recommend it.

There are far better material options available nowadays. For ham use the cheap ferrite works fine for most applications. For professional uses ferrite beads (depending on formulation) work at a variety of broad frequency ranges, some work up at WiFi.

Someone mentioned fair- rite. Ferrishield is another good source.

Remember, the issue on this thread is WHERE to choke. In this context, if you WANT an omni monopole, then choke at the feedpoint.

It is not common knowledge amongst hamsters that monopoles can be configured as unidirectional antennas. One way to do this is to have an asymmetric distribution of radials. I'll let you do some modeling so you can discover for yourself. The F/B is not dramatic but is real. The (forward)gain increase is not substantial.

Whips on car bumpers are the classic example of monopole (omni) pattern distortion caused by an asymmetric ground plane platform.

What's old is new.

But the steel wool thing is a bit dated--even for hams.

73,
Chip W1YW
(ex N1IR)
 
RE: RF Problems and the HF Vertical Antenna  
by AD5X on August 18, 2006 Mail this to a friend!
"...First off @ 25ohms you are going to have at least a 2:1 SWR and 20% of your power is going up in heat and NOT being radiated..."

This isn't true, at least not with typical coax runs on the HF bands. As an example, if you have 100 feet of RG-8 and have a 2:1 VSWR at 30 MHz, you will increase the loss from about 0.8 dB (matched line loss) to 1.0 dB (same line with 2:1). So in this case you do lose about 20% of your power, but this is almost all due to the intrinsic coax loss, not the additional loss due to a 2:1 VSWR.

Phil - AD5X
 
RE: RF Problems and the HF Vertical Antenna  
by KQ6XA on August 18, 2006 Mail this to a friend!
Steel wool does not work as a common mode choke. Steel wool is used to keep rodents from entering conduit and chewing up coax cables at remote transmitter site buildings. The steel wool must be replaced often in damp climates.
 
RF Problems and the HF Vertical Antenna  
by KA3DPW on August 19, 2006 Mail this to a friend!
I've been running my vertical without a balun. So far I haven't had any problem with Rf coming back down the outside. I tune flat across most bands. I get 1.3 to 1 across the 40 meter band and less than 3 to 1 on 15. Beyond that my tuner is what I use for tuning the other bands. I use tube finals. I find the electrical noise on recieve to be the biggest problem with my ground mounted vertical. The transmission line is buried underground and that absorbs radiating energy enough that it isn't a problem. I run a full-size 40 meter ground mounted vertical with 56 radials of 35 feet. The feed is 1/2 inch cellwave. I keep my ground as 3 inch copper strap length to less than 6 feet to the 8 foot ground-rod in bentonite with salt and watered regularly for a low-inductance and low- reistance ground. Even my linear poses no problems. The antenna is hidden and the neighbors don't hear me. I've got harmonic filters well grounded and common mode filters too. I'm trying to find a way to fight the recieving noise. That's why I took my station outside the house. the noise floor dropped trmendously.
 
RF Problems and the HF Vertical Antenna  
by K4UUG on August 20, 2006 Mail this to a friend!
The Double Bazooka Dipole is a very efficient single band antenna which is very quite, and does not require the use of a balun. This antenna consists of coax (RG58) or other 50 ohm type with the shield split at the center and the feedline attached to the open ends. Do not break the center conductor. With the feedline attached directly to the two open ends this acts as a half wave dipole along with the open wire end sections. This double bazooka can be cut for any operating frequency and is broad banded. It can be mounted as a flat top or an inverted vee and will handle the legal limit. As an added plus, it can be operated as a multiband antenna by using a suitable tuner. As with most antenna projects, get the double bazooka up as high as possible. Some tuning of the length for best swr may be required and you can use materials that are easily obtainable.
On the cable ends you do not need to use twin lead. You can make these antennas using a single piece of 12 gauge copper wire or larger for each end or you can also use ladder line etc.

THIS WILL OUT PERFORM ANY VERTICAL IN THE INVERTED VEE CONFIGURATION

 
RE: RF Problems and the HF Vertical Antenna  
by WA4DOU on August 20, 2006 Mail this to a friend!
The double bazooka had one claim to fame, it was a bit more broadband than the traditional dipole, and that characteristic is mostly desired on 75/80 meters. It isn't a particularly good performer for dx, like the vertical, because it requires height on the order of 100-140 ft. Below that height its high angle of radiation is its strong suite, just like any dipole. Its even more unremarkable as an inverted vee with its high angle, omnidirectional performance. Its not even worthy of mention in the same sentence with a vertical, who strong point is being a low angle dx antenna.
 
RE: RF Problems and the HF Vertical Antenna  
by WA2JJH on August 21, 2006 Mail this to a friend!
I would not want steel wool near any aluminum. Ever see a THERMITE reaction. Highly exothermic.

 
RE: RF Problems and the HF Vertical Antenna  
by WA4DOU on August 21, 2006 Mail this to a friend!
I use steel wool all the time to clean aluminum. I'm still waiting for something funny to happen.
 
RE: RF Problems and the HF Vertical Antenna  
by N3OX on August 22, 2006 Mail this to a friend!
WA4DOU: lack of surface area and activation energy have saved you :-)

Seriously though, it's hard to get thermite going, plus you need the OXIDE of iron (don't remember if it's FeO or Fe2O3). If you let a steel wool rust away and pour in some powdered aluminum and light it with a very hot-burning fuse (magnesium is usual), you can get some fireworks. Putting steel wool and aluminum together is just fine.

Dan
 
RF Problems and the HF Vertical Antenna  
by KU2US on August 23, 2006 Mail this to a friend!
After reading all of these posts, I must say that there is quite a bit of usefull and helpful information..All the antenna guru's are out for this one..I have a Hustler 5BTV vertical. It is ground mounted into about 3 ft. of earth. The antenna is about 100ft. from my shack and is fed with RG8u thick coax. The coax is buried under about 2" of earth (Just under the sod). There appears to be many thoughts about how to STOP interference. My one and only solution that I did was very simple. If possible, PLACE THE ANTENNA AWAY FROM THE INTERFERENCE-duh-In my case-power lines. I know this may sound stupid, but by doing this, my vertical is so quiet, it blows my G5RV away as to noise reception. Burying the coax may help too. I say all of this because most replies here, address how to STOP interference, which in some cases, because of limitations may be necessary. I have had the vertical on my roof, next to my house, tri-pod mounted to ground, mounted on a small tower, I had RF chokes, et.et.et. nothing worked! UNTILL I just moved it away from the problem source. Having 100ft. of cable seems not to bother my signal or SWR. Example..Made a QSO to JA land on 20 meters 7AM QRP with only 5 watts, and my location is LOW. I have 3 ground radials for each band cut to frequency-Thats ALL! I compared the Vertical to my tri-band yagi up 20 ft. The yagi bests the vertical by only 1Db!! as to QRN..I measured it..So as simple and stupid as it sounds, dont try to defeat the power lines, you will lose. GET the antenna away from the garbage and you can enjoy the great DX angle of radiation the vertical employs.
 
RE: RF Problems and the HF Vertical Antenna  
by WA4DOU on August 23, 2006 Mail this to a friend!
KU2US, you did good.
 
RE: RF Problems and the HF Vertical Antenna  
by N3OX on August 24, 2006 Mail this to a friend!
KU2US: You should also call the power company if you've localized the interference to the lines. They should be QUIET.

I moved in to a new place here in Maryland and didn't have cable for a few days, so I threw an antenna on the TV and saw little white dots and dashes... hooked up the FT-857 and all across 6m, BUZZZZZZZZZZZZZZZZZZZZZZZZZZZ s9+.

I got in touch with someone who does consulting on RFI investigation for Pepco and the next day a truck showed up with a little log periodic antenna swiveling around on top and within a couple of minutes, Mike, K3RFI was out of the truck with the ultrasonic microphone and had found the bad insulators.

While there are few power companies that employ someone like Mike, it's still in their best interest to fix powerline noise because it's arcing that's doing it and arcing is a problem.

73,
Dan
 
RE: RF Problems and the HF Vertical Antenna  
by K4AD on August 29, 2006 Mail this to a friend!
I use Scotchbrite pads to clean aluminum, steel wool leaves a residue.
 
RE: RF Problems and the HF Vertical Antenna  
by N2BMP on September 3, 2006 Mail this to a friend!
hought I might address the moisture Rust aspectof this thread.

Although expensive if you have the money and like to play, McMaster Carr has what you want. Research could find it a bit cheaper but who has the time!

Rustless Bronze and Stainless Steel Wool
Nonferrous wools don't deposit fibers that can cause after-rust. Pads are approximately 2 1/2" x 4" x 1/2"; reels are 4" wide x 1/4" thick and 100-120 ft. long.
Bronze— Polishes and refinishes, especially in marine applications. Uses include refinishing hardwood and polishing brass and stainless steel.
Type 434 Stainless Steel— Tough and durable, this material withstands high temperatures. Use for cleaning stainless steel. Comparable 3-Pad
To Steel Packages 5-lb. Reels
Grade Wool Grade Per Pkg. Each

Bronze
Fine 0000 7364T71 $3.56
7364T41 $98.70

Medium 0 7364T72 3.56
7364T42 87.96

Coarse 3 7364T73 3.56
7364T43 87.96

Type 434 Stainless Steel
Fine 0 —— ——
7364T44 66.76

Medium 2 —— ——
7364T45 59.43

Coarse 4 —— ——
7364T46 59.43


 
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