[TowerTalk] Best Balun

[Roger (K8RI) on TT]

On 11/27/2014 12:31 AM, Jim Brown wrote:

I've wound a number of the "choke baluns", or just call them common mode 
chokes
They don't take a lot of coax, or a lot of cores to be very effective.  
I've used them on inverted Vs, center fed, half wave sloping, fan 
dipoles on 75 and 160 half slopers.  I use Jim's numbers for core an 
turns to cover the bands of choice.
The center fed, half wave sloping fan dipole at 90  - 95 feet on top 
with the bottom near 10 feet is a long way from being balanced.  It did 
take two chokes to keep RF out of the shack, but I'm working on a 6 or 7 
core
with 1/4" spacing between cores. These are open to the elements. So far, 
the only drawback has been ice accumulation. I've been using plexiglass 
with slots milled to fit the cores, but I think I'm going to switch to 
lexan.  It's so expensive they even charge for scrap pieces, but I'm 
afraid the plexiglass is too fragile. I haven't had a chance to run the 
VNA on one loaded with ice. Usually the priority is to get the big 
generator running so we have, heat, lights, and refrigeration.   That's 
one time you really appreciate a chest type freezer over an upright,

As to the comments below, the common mode choke keeps RF out of the 
shack and on the antenna where it belongs.  I disagree with Jim on the 
transmit remark..  It can make make a difference on transmit. With a 
sloper it can make a major difference.
At first, I wanted to use a transformer balun to feed the so called 
balanced antenna, but Jim convinced me to try the common mode chokes.  I 
purchased a 100 2.4" OD #31 cores and started winding. At 5 to 6 cores 
for most uses up those cores in a hurry. A few neighboring hams saw my 
success with sloping dipoles on 40 and took home a few.  I should have 
purchased more, but that's an easy $500 or more depending on where you 
get them and the price varies. I was more than pleasantly surprised,  
The antenna appears as if it is truly balanced even though it took a 
little extra effort with the sloping, fan dipole on 75. At about a KW 
out, every LED in the shack lit up. At the legal limit, they were much 
brighter than their ratings.  One choke dimmed them. Two chokes and I 
could hit "Tilt" without a glimmer.

I did have 5 in the air at one time, but right now I'm sorta between 
antennas,  I'm waiting for the yard to firm up to pour the base for the 
LM-470 and get a climber to install the rotator, mast, and 7L 6-meter 
C3i on the 100' 45G. and replace the AV640 with an AV680.  I add a 6 
core, 5 turn choke directly under the feed point for additional 
isolation.    I have used LMR400, BuryFlex(TM) and RG-400 for the 
winding.  6 and 7 turns of LMR-400  or BuryFlex takes a lot of cable 
pulling soap, plus a lot of elbow grease.

Yes, even making your own costs more than the cheap ones, but compared 
to the good ones from DX Engineering and others they are about half the 
cost.  IOW Quality and performance costs!

73

Roger (K8RI)


> On Wed,11/26/2014 1:50 PM, Wilson wrote:
>>   As for balance, there’s a good bit of impedance in a wavelength or 
>> two of coax shield, so I doubt if the shield end at the top is 
>> actually very near ground.
>>          This is mentioned in the list of factors someone gave. No 
>> doubt, but how much does it affect the pattern?
>
> WL,
>
> A good common mode choke prevents the feedline from being part of the 
> antenna. Does that matter much on transmit?  NO, EXCEPT if it puts RF 
> in your shack. Does it matter on receive? YES! because the signal 
> picked on the feedline couples to the antenna and fills in the nulls 
> in the pattern.  Goodbye front-to-back and front-to-side rejection. 
> Hello NOISE.
>
>
>> I’m not competent to make analytical arguments about which choke is 
>> best, but rather I maintain that using the ARRL choke and getting the 
>> Z of the choke close to ten times the feedline Z is likely to be all 
>> anyone needs.
>
> In the link I posted, I showed that RESISTIVE choking Z on the order 
> 5,000 ohms (10X your "rule of thumb") is a far better design goal, 
> both for the decoupling concerns (TX and RX) and for power handling.  
> I also showed why chokes must be resistive, not inductive, which means 
> that they must be multi-turn chokes on #43 or #31 cores.
>
> 73, Jim K9YC
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-- 

73

Roger (K8RI)


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