Topband: High Impedance RX Antennas

[Frank W3LPL]

Another highly recommended tool is a Time Domain Reflectometer.
Tt easily detects any abnormalities in my 8-circle and 4-square
arrays and stacked HF Yagi arrays and switches.

Other than my hand held antenna analyzer, my hand held TDR is my
most often used and most valuable tool for proving that the performance
of my antennas and feedlines has not changed from prior measurements
and identifying and isolating faults before they cause partial or
total failure.  A TDR can detect, diagnose and locate feedline and
antenna faults from the comfort of your ham shack.

Affordable TDRs are often available on eBay. Some antenna
analyzers have rudimentary TDR capabilities but I've never used
an antenna analyzer with TDR capabilities comparable in
performance to a TDR.

Long before moisture causes coax failure, it causes the impedance
of the coax to change, a potentially big problem in a phased array
or stacked Yagis.  A TDR easily detects and locates the problem
before it causes a failure.

An antenna analyzer can also detect that moisture has caused
an impedance shift in coax cable.  Place a dummy load (with the
same impedance as the coax) at one end of the coax cable.  Sweep
the antenna analyzer frequency.  As you do a frequency sweep the
VSWR and impedance should not change.  If it does, moisture is
probably eating away inside your coax.

73
Frank
W3LPL





----- Original Message -----
From: "Dennis W0JX via Topband" <topband at contesting.com>
To: "topband" <topband at contesting.com>
Sent: Monday, April 3, 2023 4:42:37 PM
Subject: Re: Topband: High Impedance RX Antennas

There is an old saying "The enemy of good is perfect."  You will be driven crazy by chasing down every slight variance from the ideal. That being said, it is important to check out the obvious areas where flaws can create big problems. Indeed, the feedline coax, typically RG6 CATV line, is a major contributor to array performance problems.  The cheap, big box cable is covered with PVC and that stuff hardens and tightens up over the years. This allows water to enter the shield and the water easily migrates for long distances, creating a high resistance. Spend the money upfront and buy quality cable that is flooded.
The idea of using cable that has a solid copper conductor has merit. But even with that, a bare copper conductor will oxidize over time. I find myself cleaning these conductors on an annual basis with the eraser end of a pencil (recommended by Lee K7TJR).
I have also had F connector failures on my combiner box and on a common mode choke. Sometimes the imported female connector center contact just fails for no reason. The alternative Amphenol connectors are very expensive at about $8+ each but I am considering replacing the connectors with these higher grade parts.
Also, the so-called "water-proof" compression connectors are often not. The connection must be waterproofed effectively.  Also, condensation from humidity can be a big problem. Here is northeast Ohio, the environment is humid for the majority of the year. I have poured water out of a 75 ohm preamp that was sheltered in a good enclosure!
These complex phasing arrays are by no means plug and play. Each installation will be unique depending on layout and surrounding objects. There are three tools necessary to get them and keep them working:  1.) a good portable signal source to feed the antenna preamps (I use an Elecraft XG-3 with a home brew impedance transformer at the input);  2.) a portable transceiver to measure signal out of the system at various points (My KX3 does a great job); and an accurate antenna analyzer to measure impedances and to measure feedlines and phasing lines for the proper electrical lengths. N6RK is right. There can be considerable difference in RG6 impedance so you need to check it out.  
 
 73 Dennis W0JXMilan, OH  
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