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Re: Protecting downconverters



David Reinhart wrote:
 
> Has anybody seen a circuit, perhaps essetially a PIN diode TR switch)
> that can be placed between the xcvr and the converter that could
> shunt stray RF fast enough to save the converter?  Maybe even have
> actual "RF Out" that could go to a dummy load to soak up the power
> from inadvertently keying the rig?

There are two paths that you want to select between:
  1) antenna -> downconverter -> radio    (To receive 2.4 GHz)
  2) antenna <-----------------> radio    (To operate 144-148 MHz)

A PIN diode connected from the input to the output of the downconverter
would bypass the downconverter when the PIN diode is biased to conduct,
and would become an RF open circuit when not biased, leaving the down-
converter in the circuit.  So, theoretically, flip a switch to bias
the diode when you want to use the IF radio as the actual frequency
to receive or transmit with, bypassing the downconverter, and flip the
switch to the open position to enable the downconverter.  That, however,
still requires active intervention on your part, and does nothing to
protect against accidentally transmitting into the converter with the
PIN diode bias switch in the wrong position.  So you still need an
RF sensor that detects that the transmitter has been activated to
turn the PIN diode on automatically to shunt the RF around the down-
converter.  The 2.4 GHz antenna would become your "dummy load" for the
2M (or other) IF radio when accidentally transmitting, and it either
has to absorb full power or be a bad enough mis-match at the grossly
incorrect frequency that the SWR foldback circuit of your radio would
drop power when it "sees" the wrong antenna in place.  And of course,
all this presumes that you have some other way in place to connect the
IF radio to the 2M (or other) antenna when you INTEND to use that
frequency band.

Ramsey Electronics sells an inexpensive RF-sensing relay kit.  This
might be a helpful starting point.  In fact, you could use the RF
sensing part of the circuit to drive a pair of coaxial relays instead
of a PIN diode, to get better isolation than the PIN diode and better
power handling and SWR characteristics than the relay that is part of
the kit.  Their URL is:

http://www.ramseyelectronics.com/cgi-bin/commerce.exe?preadd=action&key=RFS1

If you have a problem with that URL (it might get wrapped by your mail
program), go to the main page
 http://www.ramseyelectronics.com
and click on:
 Hobby Kits, then
 Mini-Kits, then
 RFS1 to get to it.

It is called the RFS1 "RF Sensed T-R Relay Kit" and costs $14.95USD
plus shipping.  I have a couple of them at home that I haven't built
yet, so I have no first-hand experience with how well they work, but
the circuit is pretty straightforward and looks to be very easy to
build, even for someone who was spoiled by the old Heathkit assembly
manuals of years gone by.  They say it can handle 100 watts, although
the relay in mine looks like it would not give a good low-loss, low-SWR
match to 435 MHz, it might be OK for 145 MHz.

Hope that helps some.
John (KB0ZEV)
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