Re: K3 on-channel strong signal overload?

Posted by Jim Brown-10 on May 04, 2009; 6:07pm
URL: http://elecraft.85.s1.nabble.com/K3-on-channel-strong-signal-overload-tp2785612p2788626.html

On Mon, 4 May 2009 13:36:18 -0400, Randy Downs wrote:

>I see distortion with a 40 over signal. 35 to 40 over seems to be when
>I see it. Preamp on.

So turn the preamp off!  Preamps are for weak signals on the higher HF
bands. Preamps should NEVER be used below 20 meters, and rarely on 20M.

Steve said:

>This "Duhing" is entirely unfair. Didn't you read what he said?

Yes, and he said he got rid of most of the overload by minimizing the
gain of the K3. The receiver in the K3 is a thoroughbred, "souped up" to
copy weak signals with other strong signals AROUND them. It uses a lot of
IF selectivity to do that. 100W ON FREQUENCY in the same driveway is an
extreme case -- the IF filtering can't do anything at all. I suggest that
you compute the voltage at the antenna terminals of the radio for the
scenario described in the post. It is unreasonable to expect a souped up
and well protected receiver to be completely free of overload under this
condition.

Have you ever worked a ham through a VHF repeater when you're less than a
city block apart?  It is VERY common for the other ham's transmitter to
desense your receiver so that you can't hear the repeater!  What's
happening is that the other guy's transmitter is biasing your front end
into cutoff (even with 600 kHz spacing on VHF, 5 MHz spacing on UHF).

The K3 doesn't do that -- it keeps on working.  But there is protection
circuitry in the front end that prevents a big signal from frying
something. Remember -- the K3 is not ONLY a QRP rig. Many hams use K3s in
big contesting stations with legal limit amplifiers and SO2R -- that is,
listening on one frequency while transmitting on another with a second
radio. Those field strengths can fry the front end of a receiver that
isn't well protected. The K3 IS protected. The small distortion he's
hearing is part of the cost of that protection.

73,

Jim Brown K9YC


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