[TowerTalk] Six meter balun

[Jim Brown]

On 7/2/2022 3:47 PM, Dave Sublette wrote:
> My six element 6M LFA is showing a high SWR and severe loss of receive
> signal.  This happened this afternoon as I was working the band with 500
> watts from my KPA500.
> 
> To this point, the antenna was very nicely matched with a 1:1 SWR over
> almost 1 MHz of bandwidth. The antenna is home brew and I made the matching
> balun per the instructions from K9YC using two toroids with six turns each
> of high quality teflon insulated, double shielded silver braid. 

Hi Dave,

How many of these chokes are you using? My choke recommendations are not 
intended as a substitute for for the mfr's matching element, but in 
addition to it. Power rating of my chokes is strongly dependent on the 
total choking impedance, and my general recommendation is for 5K ohms. 
The photo shows the string of five that I'm using, with Teflon RG8. 
Accounting for loss in the 350 ft run of 7/8" line, they're probably 
seeing about 1,100 W (I run my KPA1500 at about 1,350 W on 6M keydown 
modes).

Many of the old hands I know say that the first things to check for 
problems like this are every in the transmit path to the antenna, and 
one of the most common point of failure is connectors, both their 
quality and their installation. I concur with W3LPL and many other good 
engineers that Amphenol 83-1SP is the gold standard, and for RG400, I 
use the silver inserts.

Yes, RG142 and RG400 should be good for legal limit on 6M. I settled on 
RG400 for the HF chokes wound on 2.4-in o.d. cores because the bend 
radius is tighter than spec, and as W7RY noted, the difference is its 
stranded copper center, rather than steel. I've bought Harbour 
Industries RG400 on the great auction site, choosing the vendor 
carefully. Harbour Industries is a major cable mfr, based along the 
VT/QC border. Like John, I use it for all the jumpers in my station.

Yes, it's important to give the chokes and the coax separation from the 
boom; capacitive coupling to the boom will de-tune the choke, moving it 
down in frequency.

Also, RG-numbers are a generic description, not a spec. The current MIL 
spec designators are quite different, as Grant noted. Many years ago, an 
engineering colleague working in pro audio went into business 
manufacturing a better kind of audio cable, he produced a top quality 
product that was better than what Belden was doing! 15-20 years later, 
he started manufacturing coax; he said it was VERY difficult to do it 
right.

In my professional life, I designed very large sound systems for public 
places, writing serious specs for contractors to bid on. I specified a 
LOT of cable for these projects, and soon learned that there was a lot 
of "brokered" no-name cable being sold by vendors who knew nothing about 
it. I eventually wrote a spec that limited cable mfrs to several 
specific mfrs that I knew could be trusted. Investigating one requested 
substitution, I called the cable vendor and asked to speak with their 
chief engineer. Eventually I reached a young lady who said that was she. 
When I asked where she got her engineering degree, she responded, "IBM."

73, Jim K9YC