Topband: High Impedance RX Antennas

[Tree]

I guess most people have some kind of small antenna analyzer that has a TDR
kind of function.

I have not warmed up to them yet as the results I see seem to be "all over
the place".  Maybe I just don't know how to use them yet.

My goto TDR is homebrew - using an analog scope.

Here is one design pretty similar to what I am using:

A very simple TDR from W2AEW – PierpaLab.com
<https://pierpalab.com/2022/04/30/a-very-simple-tdr-from-w2aew/>  I put a
switch on it to slow down the rep rate as an option since some of my
"stuff" is pretty long.

It works wonderfully to check feedlines and beverages.  I have one beverage
that has transformers and feedlines on both ends and I can see the
difference between open/short/terminate on the "far end" (which is
physically in the shack).

73 Tree N6TR

On Mon, Apr 3, 2023 at 10:11 AM Frank W3LPL <donovanf at starpower.net> wrote:

> 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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