[Elecraft] spitting distance QSOs: why the diodes are needed by HF Packers

[Wayne Burdick]

Robert,

Here's the scenario. If you transmit a VERY large signal from a transmitter
whose antenna is within a few feet of the K2's antenna, the signal will exceed 3
volts peak-to-peak at pin 4 of the K2's I.F. amplifier, an MC1350. When this
happens, the MC1350's AGC range will be exceeded. It takes an admittedly HUGE
signal to cause this problem. Normal on-air signals rarely rise above 100 mV at
pin 4. But if you did as the HF Pack group does--operate on the same frequency
while standing a few feet of another station, with your antennas mutually couple
by being in their near fields--you can get signals that exceed the AGC range.
(They do this because they all have independent back-pack mounted transceivers,
and occasionally they both talk on the HF Pack net frequency even when standing
right next to each other. We wanted the K2 to be well-behaved in this situation.
Read on.)

The new modification, a simple one, is to put two diodes (one in one direction,
one in the other) across pins 4 and 6 of the MC1350. This clamps signal input to
the 1350 at 1.4 Vpp. Yes, this is a hard limiter, but since the 1350 is followed
by another crystal filter, harmonic distortion will be reduced to a level where
it is almost inaudible. And of course the diodes have no effect on normal on-air
signals--just "extreme" ones, on the same frequency.

The net result is that you can literally use the K2 as a monitor receiver for a
transmitter at the same station, with both on different antennas just feet
apart, even if the transmitter is putting out 100 watts. It won't hurt the K2
even if the diodes are *not* used, but with them there, that collossal signal
will sound just like another S9+40 one, even if in reality it is more like S9+80
 ;)

73,
Wayne
N6KR 

robert parker wrote:
> 
> What kind of diodes are you referring to?  How many volts are we talking
> about?  Wouldn't this de-sense the receiver by quite a bit?  Before a
> diode even gets into full conduction, it enters a non-linear region.
> All sorts of mixing products will be present.  I'm guessing the
> distortions you are referring to are the mixing products that occur.
> Are you thinking of using small signal diodes? What would be the FV?
> 
> Regards,
> Robert VE3RPF>