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Re: [TowerTalk] Balun Recommendation

Subject: Re: [TowerTalk] Balun Recommendation
From: Jim Brown <>
Date: Wed, 18 Apr 2012 09:29:53 -0700
List-post: <">>
On 4/18/2012 2:25 AM, Jim Thomson wrote:
> ###  yes, but can you hit 5K  from 160-10m with a single balun design.
> Sure, you can easily optimize for a single band, or maybe 2 bands.

Sure -- there are several designs that that. Look at the bifilar-wound 
THHN chokes.

> Your typ balun manufacturer  will want to build a balun that covers
> all bands, so they end up with a compromise.

Most end up with lousy products because they don't UNDERSTAND how chokes 
work, and/or they don't know how to measure them properly. . It's 
"monkey see, monkey do" for most of them.

>   Can you mount it
> on a 3 inch diam boom, without adding a bunch of stray C.

Sure, I've done it (and helped friends do it) many times.

> Can you hang it in mid air, on your 80m dipole, up 100 feet, fed
> with 213-U.   To do this right, no one single design will work.
> At a bare minimum, you really need one design for the lower bands,
> and another for the higher bands.  One for yagis  and one for dipoles.

You somehow seem to be committed to buying a commercial product, yet one 
of your other interests is BUILDING big power amps from scratch.

> ### My understanding is the 5K  MIGHT lower local noise only, like
> within a 500-1000 foot radius.  The 5K  wont buy you anything for band noise.
> Some folks see nothing, some see  1-2  S units with real high Z baluns
> of your various designs.

What the very high Z choke does is prevent the feedline from acting as 
part of the RX antenna.  That allows whatever directivity the antenna 
has to be fully realized.  We work hard to get antennas as high as we 
can and as far away from noise sources as possible, and we use 
directivity both to focus TX power and to reject noise and QRM from 
other directions. If the feedline is part of the antenna it fills in the 
nulls of the antenna pattern, which could be thunderstorms from another 
direction or switching power supplies down the block.  Or in our 
neighbor's home. Or a noisy street light, or power line.  I have 
Beverages in four directions, and when I switch between them, I often 
see/hear differences of 10 dB in the background noise level, usually the 
result of  weather conditions in a given direction. Same when I swing a 
beam on the higher bands.

> ##  If the balun is mainly resistive, and you want it to handle high power,
> and with high duty cycle modes like RTTY, AM, 10M FM, etc,  and also
> typ swr’s,  then it will have to be high Z, as in several thousand ohms.
> If it’s  just 1-2 k ohm, and mainly resistive, it will overheat.

You took the words right out of my tutorial.  A second major reason for 
5K ohms is power handling. But SWR has NOTHING to do with dissipation in 
a common mode choke. What matters is the common mode voltage, which is 
directly related to IMBALANCE in the system, and also to feedline length.

I'm a serious contester, running 1.5kW on CW and SSB, and run RTTY 
de-rating my Titans to 1 kW. The chokes you see on my high dipoles in my 
Power Points have been in place for 4-5 years now, and many of the guys 
in our local contest club are using them in the same manner I have shown 
here, most running high power.  So far no one has told me they had one 
blow up, and you can bet that I would have heard about it -- these guys 
aren't shy. :)

> ##  I see on one of your dipoles, you have a 2nd identical balun,  a few feet
> below the 1st one.   How close can you install  2 x identical baluns.

The only limitation is a practical one -- how close can you wind them.  
And they do NOT need to be identical -- they can be wound for different 
frequency ranges to increase their effective bandwidth. This is 
discussed in the text.

> I take it the idea is to end up with a high Z  by using 2 of em, nose to tail.

In the particular photo you were looking at, it was both to get higher Z 
and greater dissipation.  The photo is from the "torture test" I did to 
place the highest possible voltage across it that I could. The two 
chokes form the end insulator of an end fed half wave dipole, where the 
outer conductor of the coax between the feedpoint and the chokes is half 
of the antenna, so there is a VERY high voltage there. With 1.5kW, a 
single 5K choke will melt the coax. A second identical choke doubles the 
impedance, which divides the current by two, and the dissipation is 
equally divided between the two chokes. As a result, the two chokes 
combined can handle the resulting stress without overheating.

73, Jim Brown K9YC


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