Topband: Post contest season: TX antenna vs RX antenna cros-stalk. What do you do?
jim at audiosystemsgroup.com
Thu Mar 24 13:11:33 EDT 2016
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
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.
73, Jim K9YC
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