Thanks Jim,
Our learning point so far:
Our fixed setups usually had harmonic coax stub filters per individual
antenna.
Antenna interaction was carefully measured with help of a spectrum analyzer
(worst case ever found = -17dB)
The RX inputs often had individual BPF's (home made or Dunestar)
The TX lines always had 6-band 200W BPF's
Often the TX antenna was never used for RX and switched to GND to avoid
noise coupling to RX antennas.
As such the TRX internal RX/TX cros-talk was never noticed.
In some other cases this event was perhaps overseen?
For me this is a new challenge.
Our typical field-day setups only have the TX 6-band BPF's
None of the other measurements/precautions have been taken as the building
of the FD-style setup already takes a lot of time and the QTH offers limited
access as it is being used for commercial workshops where participants fear
electronics and antennas.
One ham recently switched to 4O3A hardware and noticed similar positive
performance results.
Sheet metal boxes perform better than aluminium?
About coax and plugs/sockets; we are already for years busy replacing old
stuff by Amphenol or Kabel Kusch hardware.
For my Wellbrook Loop antenna I used a home made 3-pole 160m BPF with a DC
bypass for the head-amp. That was effective for the Topband contest.
Measurements using calibrated generator ( Marconi 2955) and now the new
Elecraft XG3 (very handy in the field), together with a analyzer , or even a
good SDR like the Microtelecom Perseus (lowest phase noise ) is very helpful
making steps forward at home.
A VNWA sure can be helpful too. At least it is more portable than our XXXXL
HP analyzer.
In the field the challenges are different and take a lot of debugging time.
Thanks for all the tips
73 Mark, PA5MW
-----Original Message-----
From: Jim Brown
Sent: Thursday, March 24, 2016 6:11 PM
To: topband@contesting.com
Subject: Re: Topband: Post contest season: TX antenna vs RX antenna
cros-stalk. What do you do?
On Wed,3/23/2016 11:38 PM, Mark van Wijk, PA5MW wrote:
The problem is that while listening to your receiving antennas, there is
also some signal from the transmit antenna getting through thanks to the
internal cross-talk of your rig's TX/RX relay.
There are many heads to this snake, but also many fixes. This post is
about crosstalk, NOT interaction. First, there is coupling between the
antennas themselves. Cross-band interference between transmit antennas
can be greatly reduced by the use of bandpass filters, but these filters
don't help with RX antennas not within the filter loop.
Second, there is coupling due to leakage capacitance between relays,
poor layout of relay boxes, and currents within our stations. My station
uses a 6x2 relay box to distribute monoband antennas between rigs for
SO2R, but some have a lot more crosstalk than others. Last spring, I
replaced an Array Solutions Six-Pak with a 4O3A box. I measured both
boxes for isolation using the DG8SAQ VNWA -- the 4O3A unit had more than
20 dB better isolation than the SixPak.
But even with that improvement in hardware, I still found isolation
insufficient, so I bought a spool of BuryFlex and a box of Amphenol
83-1SP (solder-type coax connectors) and replaced every piece of coax in
my station and made sure that all were wrench-tight. (I didn't count,
but I'd guess about 25 cables). That provided another 10 dB or so (not
measured, but looking at P3 traces while operating).
Another issue I still need to address is bandpass filters on RX
antennas. I often use my Beverages up to 30M. My K3 can handle the
signal strength without damage, but while contesting, I hear very strong
harmonics there.
In one of the early chapters of his classic book, "Managing Interstation
Interference," W2VJN advises us to begin by measuring the coupling
between our antennas so that we understand the levels of crosstalk that
can be present. It's easy to do that with a vector network analyzer like
the DG8SAQ VNWA. [Note that antenna analyzers are NOT network analyzers
-- they are single-port devices. The difference is that a VNA is a
2-port device, so it can measure BOTH impedance and the gain (or loss)
between input and output ports.] We can, of course, make these
measurements with gear as simple as an amplitude-calibrated RF generator
and a calibrated voltmeter (anything from a scope to a spectrum analyzer).
The VNWA 3e is a real bargain. It's full spec to 500 MHz, reduced spec
to 1.3 GHz, and self-powers from the USB connection to your computer. I
paid about $750 shipped to my home in W6 about 3 years ago for a unit
with calibration kit.
http://sdr-kits.net/VNWA3_Description.html
73, Jim K9YC
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