Topband: Post contest season: TX antenna vs RXantenna cros-stalk. What do you do?

Mark van Wijk, PA5MW pa5mw at
Thu Mar 24 08:36:45 EDT 2016

Good point.

On 160 there was also a relatively nearby 829KHz AM BC station.
The RDX PI4YLC Field-Day-style  station used an OM-Power 6 BPF in its TX 
But no additional BPF or AM BCB HP filter in the RX path.
A mistake I should have recognized.

Thanks for the test procedure.
Will use that additionally  for sure at home and at any FD QTH in the 

73 Mark, PA5MW

-----Original Message----- 
From: JC
Sent: Thursday, March 24, 2016 1:14 PM
To: 'Mark van Wijk, PA5MW' ; 'william radice' ; topband at
Subject: Re: Topband: Post contest season: TX antenna vs RXantenna 
cros-stalk. What do you do?

>>I was wondering what people do besides the obvious 'kill the TX antenna
input during RX mode' ?<<

There are several reasons why it is necessary to detune the TX antenna
during RX, however disconnecting the center of the TX coax during RX on 160m
is a MUST.

If you don't understand the problem you never implement the solution. Some
broadcast signal can reach -10 dbm or even 0 dbm. Most radios have very poor
isolation and pin 1 problem on both the RX input and TX input.

In order to open the center pin at the antenna feed point you need  a fast
vacuum relay to avoid hot switch. For an inverted L it will improve all RX
antennas removing re-radiation noise from the TX antenna.

Open the circuit between the transverter and the amplifier is a good idea.
You need a fast relay but it can be small.

Filters must be strong enough to hold at least 200w  with low insertion

The test to know if you have this problem is simple, disconnect the RX
antenna and use a 50ohms, shielded load to terminate the RX input. Tune on
the AM band and if you hear strong BC signals coming from you TX antenna
during RX with the RX port terminated with 50 ohms you have some homework to

No carriers at all indicate that you have shield, grounding and choking in
good shape, if you hear strong AM signals means that you have isolation and
common node noise problems. If the BC signal can come in all QRN and other
noise sources also can come in and raise the noise floor of the receiver.

My station is quiet, I cannot detect any trace or light carrier doing the
test above.


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