Re: Optimum Coax Coiled Turns Choke Baluns for Antennas

[n2bc]

Hi Tom, great stuff, thanks for posting it.   Question....  Since I have a bunch of 9913F7 and zero RG-213, will the difference in characterists  between the two types change the choke parameters?   

                    RG-213    9913F7
Vel Factor       66%       85%
uH/ft             .077       .059
pF/ft              30.8       22.5   

I think the main reason RG-213 is usually specified is it's solid dielectric, but 9913F7 has a non-migrating dielectric that keeps the center conductor 'in place' unlike earlier foam dielectrics.

73, Bill  N2BC

[K1JJ]

Hi Bill,

Yes, that's a good question. I don't know if an coaxial RF choke's effective impedance is changed due to velocity factor.  I'll bet someone here knows -  or we could do some reading on the web.

Though, the impedance is not critical as long as it's at least 500 ohms for a 50 ohm coax, according to the author.


Another thought:
Looking at some of the phase shifts, I wonder how stable it would be using ferrite beads for a set of phased antennas?  If we were feeding three 10M Yagis and one group of beads had a higher permability than the others, it appears like there could be phase feeding differences. It seems as if coax would be more stable considering how beads can be saturated or damaged by EMP.  I use only coaxial baluns here.

T

[WA1GFZ]

The purpose of a balun is to balance the shield current with the center conductor.
You want the RF to flow on the inner surface of the shield not the outer surface.
the balun acts as a RF choke for outer shield current to balance the inner shield surface with inner conductor. The performance of the coax is not changed when wound as a balun as long  as the minimum bend radius of the wire isn't violated.
Mil cable states this in the mil C17 spec.
The coax is in the near field so some RF will be picked up unless the coax is run through pipe. Who knows how effective a balun is in the near field.......

[K1JJ]

If the coaxial balun is oriented so that its open ends are in the same direction as the element, the pickup is minimized -  there is less RF stray coupling into it. I've seen some antenna installations that had the balun shifted 90 degrees so that the coupling was like two links parallel to each other.

I made that mistake on my first log installation, but later corrected it.

So, Frank... looking at Bill's question above, does the effective impedance of a coaxial choke balun change with the coax velocity factor, or is there no relationship?  ie, If the velocity factor is slower, does the choke exhibit a higher impedance at a given frequency?

T

[WA1GFZ]

Tom as long as you don't wind the coil smaller then the min bend radius the coax does not change.
Remember the coax hanging below the balun can pick up RF. The log is different because the cable is against the tower leg. I was thinking of the cable hanging off a dipole. I suspect the balun is ineffective on a dipole.
The balun on my log is back at the mast. Tennacrap guy suggested that was a better spot for it. I have no problems except for a small segment near 22.1 MHz.

[K1JJ]

Thanks for the info, Frank.

Let me try again to ask Bill's original question. I don't seem to be able to make it clear..


Let's say you use RG-213 (velocity factor .66) for a choke coil and the measured impedance is 500 ohms.

Then you switch to 9913F7 coax which has a velocity factor of .85. You wind the exact same coil. Will the measured impedance change to something other than 500 ohms?

BTW, I've always thought that the balun should be placed right at the feedpoint. We might want to axe Chuck about his opinion on that. Except for accessibility, I can't see an advantage to putting it at the mast. If that worked, why not just place it in the shack?  :-) 

I wonder if that 20' piece between the feedpoint and mast can develop its own RF ground loop unbalanced effect?

T

[WA1GFZ]

Tom remember my boom is the feed line. The shield is up against the shield side of the boom. The balun is where it breaks away from the boom. The balun at the feed point would not be effective in my case unless the feed line was hanging below the antenna. I was told that would mess up the pattern. It worked fine both ways when it was sitting between the kids swing set and the picnic table.
The best way to do it is have the feedline inside the boom I am told.
The tow types of feedline would generate the same reactance if they are the same size and spacing. The inductance of the shield outer surface is making the Rf choke.
BTW you can check this with a RF generator series resistor and a scope. Just compare the voltage across the reisitor and balun to get the reactance. fc

[K1JJ]

OK, good.

So the shield is what makes the RF choke, so any velocity factor will be the same impedance.  Tnx.

BTW, my 32el VHF/UHF log uses the feedline as the boom too.  What I did was to mount a 2' fiberglassrod  up front and hanging straight down. There the balun was mounted and the feedline came back to the mast. Everything was at right angles to the log elements.  I talked to Chuck about this and he seemed to think it was the right way to do it, at that time.

I originally had the coax running down the boom/feedline which was the worst thing possible, in hindsight. But that's what the manufacturer suggested. Just imagine running coax down your openwire line... geezz..

Yes, inside a round boom is the best for the coax. And inside one of the feedline tubes would also work. The problem is having to drill a big hole in the boom near the mast  at it's weakest stress point... NG.

T

[WA1GFZ]

I did the same thing when the log was on the ground.
A hole drilled in the bottom of the boom would be in compression so not as much stress but I would use a big boom to mast plate and a couple extra clamps.
I had a number of conversations with the crapadyne guy about feed lines and he told me about a number of tests he ran. The coax running under the boom didn't seem to be a problem but it needed a choke when it got to the mast. It works so can't worry about it. I checked the pattern and it works. the high swr point at 22.1 didn't change either.

[K1JJ]

Probably the reason it works OK is because the boom feedline is of low impedance, say about 75 ohms. If it were high like normal open wire, the nearby coax would cause a balance problem.

What I learned from this article is when there are too many turns in the coaxial  balun, the impedance goes to hell and there is a phase shift. Look at the phase numbers when, say, 12 turns are used at 30mhz. Or ferrite beads are used up there.

My question is, let's say you used a well designed coaxial balun (no phase shift) on one Yagi and used a balun with too many turns on another. The second balun had a huge phase shift, but still used the same total coax length for both ants. If you ran these Yagis together fed at zero degrees, would this phase shift show up in the effective power fed to the second Yagi?  ie, is it possible that you are feeding both coaxes at zero degrees but the second Yagi with the poor balun is getting  minus 60 degrees, etc ?

If so, it shows a good  reason to make both baluns identical when stacking antennas.

T

[WA1GFZ]

The larger balun would have more phase shift due to the longer length of wire.
The phase shife isn't related to the coil size only the length of coax acting as a delay line. I would want to keep all things the same because I'm not all that sharp myself.

The crapadyne guy once told me he mounted a tube under the boom to carry the coax and found no difference in performance. He was thinking of selling it as a modification but it did't pan out.

BTW the new tennnadyne owner added 20 pounds to the antenna...thicker wall on the boom

[K1JJ]

OK Frank-

My question was this:

Let's say that you had two EQUAL lengths of coax feeding two Yagis.  If one coax was wound for 6 turns and showed "89 degrees" on the tabular listing here and the other coax was wound for 30 turns and showed -56 degrees, would the Yagis be getting zero degrees of phase each?  Or, would one be getting the big phase shift from too many balun turns?

I am refering to the listing in my first post that shows the big phase shifts when too many truns are made in a balun. Bear in mind that the two cables are the SAME length.

T

[WA1GFZ]

Two equal lengths of coax will have the same phase delay through the coax as long as you don't violate min bend radius. It does not matter how you wind it if the coax is good cable.
The RF choke performance will vary in the way you wind the balun.