[Elecraft] OT: Advice re: blocking RF from rx-only antenna

[Edward R. Cole]

As one who deals with isolating very sensitive low-noise preamps from 
QRO levels at VHF I can offer a little more background on receiving 
on the same antenna as one transmits.  At HF probably most coaxial 
relays exhibit adequate isolation, but most eme'rs are familiar with 
the situation that it isn't necessarily so at VHF and higher where 
relay specs are derated.

Your goal should be that the receiver never sees RF above 0 dBm (1 
mw).  So how much isolation does the relay require if you run 
500w?  500w = 57 dBm, so you need at least 57 dB isolation.  Many 
relays only offer about 45-dB isolation so check the specs.  They may 
state good for a Kw or more but that is the transmit rating and not 
isolation.  I initially used Tohtsu CX-600NL relays rated at 1200w 
(but only had 45 dB isolation).  That meant my expensive low-noise 
preamp was seeing +7dBm (5mw) which is marginal.

I upgraded to relays with over 100-dB isolation for use with my 1400w 
2m-8877 amp.  At these levels it is most important that the relay is 
not switched while 1400w RF is flowing thru it (called hot 
switching).  A sequencer is the absolute solution for that.  MY 
sequencer also detects relay position to enable the K3 via the 
inhibit line.  Last week I had a failure on that ckt and no RF was 
transmitted (worked as designed).  I had forethought and added an 
inhibit bypass switch which allowed me to verify if it was a relay 
issue of just control failure ( SWR was fine indicating a failure in 
the inhibit control ckt).

Locating preamps at the tower top inside the near field of QRO 
antenna arrays has additional problems.  I ended up using 
double-shielded coax from the TR relay to the input of the preamp and 
shielded 12vdc cable to prevent RF pickup on the power leads.  This 
in addition to RF chokes and bypass caps being used on the preamp 12v 
connection.

At <600w I did not encounter these issues!

73, Ed

-------------------------------------
Date: Sun, 22 Apr 2012 20:32:17 -0400
From: Don Wilhelm <w3fpr at embarqmail.com>
Subject: Re: [Elecraft] OT: Advice re: blocking RF from rx-only
         antenna
To: David Herring <dave at ah6td.com>
Cc: elecraft at mailman.qth.net
Message-ID: <4F94A311.5050000 at embarqmail.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

Dave,

That situation is dealt with routinely by VHF folks with mast mounted
preamps.
Using a coax relay to connect the amplifier to the antenna and a second
relay to connect the preamp to a 50 ohm load will usually provide
adequate isolation.

73,
Don W3FPR




73, Ed - KL7UW, WD2XSH/45
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