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
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