----- Original Message -----
From: "Doug Waller" <NX4D@comcast.net>
To: "Topband Reflector" <topband@contesting.com>
Sent: Tuesday, September 10, 2002 1:12 AM
Subject: Re: Topband: front end protection
>
> > The problem of protecting the rx front end while transmitting is a
simple
> one.
> > Its very important NOT to use an extra RX jack (which most Radios have
> these
> > days)for beverage connection> 73 de wolf -df2py-
>
> The easy way to protect the RX only front end is to shunt the receive only
> jack externally, with two switching diodes connected polarity reversed.
> I.C.E. uses this config. successfully in their preamps.
>
> IN914/4148 type, 75piv diodes were available a year ago from Radio Shack
at
> 10/$1.50. I believe these shunt at 1.7 volt and they seem to have no
effect
> on weak signal performance, when inserted in the coax lead-in system.
>
> 73, Doug / NX4D
>
> __I agree with you that that is the "easy" way, however the still very
large voltage coming in the RX port during transmit can still cause a lot of
very wierd things to happen, ie RF induced foldback...the rig thinks a high
VSWR is present and reduces output power accordingly. Spurious
transmissions...I was on 1840 ssb and got reports from east coast stations
of being S9+20 on 1865 or so at the same time. My rig didnt self destruct
because of the diode protection but problems still were there. Switching to
the transmit antenna made the problems go away. Ten-Tec suggested using a
"Shorting Relay" to ground the RX input and even sent me the parts to do it
with. That's fine until the shorting relay has a failure. Operating QSK
can take a relay to it's mechanical lifetime in short order. The other
option which I chose for my own rig, was to build a circuit that disconnects
the antenna, shorts the receive port to ground and in addition I provided a
keying relay for a linear. Old non-QSK rigs like the Heath SB220 can
"toast" a rigs built in T/R relay in seconds...de gary, kd9sv
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