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Author Topic: Air wound RF choke  (Read 14952 times)

Posts: 223

« on: January 03, 2008, 12:13:59 PM »

Dear Elmer,

I wonder if you would mind reading the writings of Ed Gilbert, WA2SRQ at...

...then lend your answer to a few questions for me.

1) Is that pretty much the bottom line on RF chokes for the higher HF bands?

2) If I use RG-8x instead of RG-213, will the results be the same?

3) I have read that RF on the coax can play havoc with the radiation pattern of a j-pole antenna, and I have seen photos on the Internet showing 6M j-poles with a choke balun under the feedpoint. I have also looked at alot of 2 meter j-poles, yet pretty much none of them have a choke balun on them. Not even in the advertising pictures of well regarded professionally made 2 meter j-poles. Does this mean that at 146MHz an RF choke is not needed?

4) If in fact a RF choke is required at 146MHz, what is the formula for winding one? How many turns on what diameter.

5) What is the accepted formula for a RF choke at 50MHz?

6) Do you know of a particular website that addresses this issue across all bands?

Thanks for your time.

Graham Welch - K5PEW (In memory of Paisley Elizabeth Welch)

Posts: 3482

« Reply #1 on: January 03, 2008, 02:03:45 PM »

Since we're not talking about a critical resonant circuit, the actual inductance of a coaxial choke is pretty much insignificant.  What goes on between the center conductor and the INSIDE of the shield won't be affected, assuming you're using good quality coax, with close to 100% shield.

What you're doing is choking off RF current that may be trying to flow on the OUTSIDE of the shield, where a coaxial choke can have a very positive effect.

I would say a word or two about the DIAMETER of the coil.  Coax manufacturers specify a minimum bend radius for each of their cables, but sometimes this crucial information is hard to find.  The US Navy has for many years mandated that, regardless of what the manufacturer says, the acceptable bend radius for new coax is at least twelve times the cable's outside diameter.  For RG-8X, which has an OD of right around 0.25 inch.  12 x 0.25 = 3 BUT REMEMBER that this is the RADIUS of the bend; the diameter would have to be at least 6".  For RG-213, which has an OD of 0.405 inches, we're looking at a radius of 5 inches (rounding up), or a diameter of 10 inches.

In either case, 6 to 8 turns of whatever coax you use should be just fine for 80 through 10 meters.  If you find you still have common mode problems on 40/80 (but above that all is well), just add a couple turns.  There is no "formula"; it's an experiment.


Posts: 999

« Reply #2 on: January 03, 2008, 03:09:33 PM »

 Actually, at high frequencies - above 20 meters, the way you wind the choke makes a big difference.

 A good coaxial choke can be made by winding it as a solenoid or single layer coil and:

 1) Selecting the proper amount of turns

 2) Spacing the turns accordingly to keep inter-winding capacitance to a minimum.

 From a 2/2007 post I submitted concerning a choke for a triband yagi:

 Using RG-8, a choke could be optimized to work up to 29.7 Mhz using the following specifications:

Diameter: 6.0"

Length: 7.5" (versus 1-2" for a scramble-wound choke)

Turns: 8

Spacing between turns; 0.625

This will provide about 714 ohms at 14 Mhz and several thousand ohms on 10 meters with a reasonable Q.

 That was for an HF yagi. For a 6 or 2 meter vertical, common mode currents can also be reduced by decoupling the feedline from the antenna using 1/4 wavelength radials. This is more efficient than a choke as the radials actually contribute to the radiation efficiency by keeping currents off the feedline and maintaining a cleaner antenna pattern.


Posts: 350

« Reply #3 on: January 03, 2008, 04:24:12 PM »

I wrote an article awhile back on common mode chokes where I did actual measurements of their effectiveness ( I only covered HF, however, not VHF.

Pete W6OP

Posts: 1112


« Reply #4 on: January 03, 2008, 07:20:00 PM »

Hello Graham,
 Yes, a choke is required for VHF j-poles. I use 6 turns at about 2" OD in rg-58. For the upper HF bands, I use 5 turns at 5" dia in rg-58 or rg-8x.


Posts: 9749


« Reply #5 on: January 04, 2008, 03:00:47 AM »

I've got some very sad news for anyone who thinks all there is to a balun is adding impedance. Adding a choke balun in a random environment can result in anything from a big help to actually hurting the situation.

The reason is the air-coil is a very high Q. It is a reactance. If the common mode impedance where the choke is added happens to be the opposite sign of the choke, it will INCREASE the common mode.

Let me give an example.

Suppose we have a feedline hanging down in the air to ground from the center of a dipole. If that dipole is 1/4 wave high and the feedline is grounded at earth there will be almost no common mode current at all in that feedline. Any place in the feedline I add an inductance the common mode will increase. As a matter of fact thios system doesn't even need any type of balun.

Now let's make the feeder 1/2 wave long to earth. Now anyplace I add a choke except near the middle of the line the common mode will decrease. I can even add it at earth and it has virtually the same effect as at the antenna.

So the results really depend heavily on the installation when the choke is a good reactance, and not a high resistance.

As for the J-pole, I'll bet you can find all sorts of poorly thought out experiments on Internet. The same rules apply to the J-pole. The decoupling method is an integral part of the entire antenna and feedline SYSTEM, and that system includes whatever the antenna is mounted on and how it is mounted. I can do the very same thing with the J-pole or any antenna. By patching a poor feedline interface (and the J-pole is terrible) with a reactance I can make the problem worse, not as bad, or not change at all.

This is why until someone writes a good long detailed article all the little rules of thumb will be full of errors and exceptions.

I was shocked to see the book "Baluns and UnUNs", in the very first part, start with a major error in basics. It analyzed common mode on a dipole mounted 1/4 wave high, and concluded the lack of common mode had something to do with the physical diameter of the coax!! How can a text on baluns, a text which deals critically with common mode currents, start by not grasping why a dipole with a 1/4 wave long suspended feedline does not need a balun??

It's all about what we are inserting in the path of common mode, and what the common mode impedance is at that point. Unless we consider BOTH as a system we will keep getting the wrong answers, and keep having results disagree.

The truth is if I wind a choke with 500 ohms reactance and insert it in a might increase, decrease, or not change unwanted feedline currents a bit. It depends on the entire system, not just one random component of the system.

73 Tom


Posts: 6523

« Reply #6 on: January 04, 2008, 05:15:45 AM »

"RF on the coax can play havoc with the radiation pattern of a j-pole antenna"... True.
"Does this mean that at 146MHz an RF choke is not needed?"... No.
The "air coil' type of balun sometimes does not work well on VHF, due to capacitance between the coils.  For VHF, I recommend using a ferrite, placed 1/4 wavelength down the coax from the feedpoint.
I have no figures proving this is best.
I have more concern with the air coil being used with foam dielectric coax.  If wound too tight, migration can occur over time (center conductor moving in relation to shield)... this can cause more problems than any distortion of pattern due to "line radiation".
The J-pole is not the best antenna in the world, but it is easy to build and simple to load and use!
If you are having problems with contacts on VHF, consider a Co-linear style antenna!  Still fairly easy to build and use, but with some actual gain.


Posts: 9749


« Reply #7 on: January 05, 2008, 04:51:12 AM »

If you want to stop common mode and pattern distortion in a J-pole, the base of the antenna has to be insulated from the mast. Then you can add a choke, but the most effective place to install the choke is at the connection point of the coax.

The worse place to install an isolation type balun is always 1/4 wave from the antenna connection. That because a high impedance there forces a low imnpedance at the antenna, and that assures maximum common mode current.

The J-pole is just a lousy antenna for common mode. It is really an UNbalanced end-fed half-wave that is fed by a BALANCED 1/4 wave open stub that is fed by an UNbalanced coaxial line.

Just in the antenna alone there are two improper transitions between balanced and unbalanced.

It is the VHF equivalent of a end-fed Zepp.


Of course this doesn't mean the antenna won't just means the feedline will!! :-)

73 Tom
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