[Elecraft] Coax Cable Interconnections

[Edward R Cole]

Jim and all:

Coax quality -- absolutely makes a difference.

I inherited some "previously driven" (aka used) coax from work 
thinking to save money -ha!  It dates from before I began working at 
my last employer as comm tech else would never have been 
purchased.  I found in preparing for installing connectors the shield 
density so poor one could not make good connections -- only useful 
buried in ground as radial.

I ran into the coax shield quality issue in a different situation but 
is the same isolation or "radiation" problem.  When I increased power 
from 600w to 1400w on 2m eme I suddenly started blowing preamps.

There were multiple "suspects":  My TR relays were not rated for 
1200w nor had the Rx port isolation needed.  I try to keep RF into 
the preamp < 0 dBm (1mw) and 1200w = +61 dBm.  My relays were only 
spec at 45-dB isolation.  Also turned out only good to 800w.  I 
replaced all after melting down three of them!!  Now using NARDA 
sem123N rated 1200w & 80 dB isolation.

But to the point the high emf of being installed at the antenna 
subjects connecting coax jumpers to high RF fields so shielding is 
critical.  I was using RG-58.  I swapped for RG-142 double-shielded 
coax.  I also fed 12v to the preamp thru shielded power wiring (RG-6 
or RG-58).  Preamps has feedthru bypass caps so that path should not 
as critical.

So changing three things means I do not know which was the "cure" but 
no longer blow up preamps.  Double-shielded coax is commonly used as 
jumpers with repeater duplexers for the same reason cited by Jim.

I wonder if better coax jumper in the shack would improve isolation 
from wifi type RFI?  Probably couldn't hurt.

73, Ed - KL7UW
http://www.kl7uw.com
     "Kits made by KL7UW"
Dubus Mag business:
     dubususa at gmail.com