Topband: High Impedance RX Antennas

[Don Moman VE6JY]

A used Tek 1502 TDR is the "gold standard" to me anyway and they can be
quite cheap at flea markets etc. They are "arm held" and self contained
portable so ideal for up to 1000' runs and have enough resolution that you
can "see" defects that you don't really need to worry about.  The advantage
over some of the VNWA's that have a TDR function, is that the 1502 is real
time so intermittents can easily be spotted. And it measures impedance so
you can easily see how long and what Z that unknown roll of flea market
coax is!

I built the scope based TDR from the QST articles years ago, but that
version isn't even close to the 1502.  I also have a Tek 1503 for checking
longer runs of control cable etc.

73 Don
VE6JY

On Mon, Apr 3, 2023 at 11:21 AM Tree <tree at kkn.net> wrote:

> 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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