[Elecraft] Interesting K3 QRM problem

[Vic Rosenthal 4X6GP/K2VCO]

It isn't necessary to filter the loop. What is happening is that BC RF 
is coming into the ANT1 connector of the K3 from the vertical, where it 
is mixing up with every other signal around, probably in the t/r switch 
section. Then the IMD products (which are on numerous frequencies) are 
getting re-radiated by the vertical, and they are picked up by the loop.

You can see this by hooking the loop to RX IN and listening. The noise 
is only there when you plug the vertical into ANT1.

I made a 4-pole Butterworth filter that cuts off at 2.5 MHz using the 
filter design program that came with the ARRL handbook, husky enough to 
transmit through, and put it on the ANT1 connector. That did it -- no 
more noise on either antenna.

If Wayne would like me to test the Elecraft filter I will be happy to do 
so. It will be interesting to compare to my homemade one.

On 23 Oct 2014 20:09, Jim Brown wrote:
> On 10/23/2014 3:58 AM, Vic Rosenthal 4X6GP/K2VCO wrote:
>> I have a BC band filter which is used with the MFJ antenna analyzer
>> when there are nearby BC signals. Sure enough, putting this in series
>> with the K3 ANT 1 connector kills the noise. Of course I can't
>> transmit through it.
>
> Hi Vic,
>
> A few suggestions. First, try the BC Band filter on the K3 RX loop. If
> that works, work out a switching arrangement so that you can apply it to
> either antenna. Second, build or buy a BCB filter that can handle the K3
> output. Based on the frequency of the BCB transmitter, one of the lower
> cost filters should work. I've measured the vintage ICE BCB filter, and
> it's good for at least 40 dB on 1080.
>
> Another thought is that this might be common mode on the coax, in which
> case a serious ferrite choke tuned to 1080 might help. I'd start with at
> least 20 turns of RG58 on a #31 2.4-in toroid, and I'd put it by the rig.
>
> 73, Jim K9YC

-- 
73,
Vic, 4X6GP/K2VCO
Rehovot, Israel
http://www.qsl.net/k2vco/