FM station QRM

[Rick Dwight]

Mike,  I have not had this particular problem with my Elecraft rigs, but have solved similar problems on other equipment.  The first question I will ask do you have resonant antenna for 15 meters.  If so often this in itself will attenuate an FM signal by at least several S units.  If you have a transmitting type low pass filter with a cut off frequency above 10 meters, or above 6 meters if you use this band, you could try putting it between your rig and your antenna.  However these filters would not be appropriate if the antenna has any significant SWR…say over 2:1.  This might give you enough attenuation of the FM signal and I suspect the filter would introduce less than a dB of loss in signal.  If this does the trick, but introduces too much loss on TX, you could move the filter inside the rig, but route it so it would only be in the RX path.  Another idea is to try a coax trap in the rx path only, would only involve a T connector and possibly a few adaptors….research this in an antenna handbook, etc.  You could try this on the rx jack on the rear panel first then move it inside.  Another idea instead of a coax stub, try an LC trap…just a series C and small L (a few turn coil…very small) shunted across the RX path.  Either of these ideas may or may not give you approximately the 30 dB attenuation you will need.  
  Once again, if you have something like a random wire, endfed half wave, etc type of antenna it may let in lots of the FM signal, but a resonant Yagi, or even a dipole often, but not always, will be a rather poor antenna for the FM broadcast band.  Just another thought.  Try turning off your preamp for 15 meters….probably not needed for 15 meters…however since the QRM is only S6, I doubt that this is the problem.  
         Rick   KL7CW



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[k6dgw]

Mike,

I'll bet everything is working OK.  The K3 1st IF is at 8.2 MHz [or
so].  The VFO injection is above the recv'd freq on 15, the 3rd harmonic
of that plus the 1st IF comes out to 90 MHz.  When I was in FM/TV b'cast
while in college, the FM deviation was +/- 75 KHz for an occupied
channel of 150 KHz [may have changed, that was pre-stereo/pre-subcarrier
days].  The JA rig's are probably up-conversion and the math doesn't
work out.   If the station is close it's very strong.  My HF mobile did
very strange things when I parked at the TV transmitter at work.  This
may have already been suggested, as soon as I saw your email I pulled
out the calculator and voila!

I think you said it got much weaker when you powered the K3
differently.  If so, the 90 MHz signal may be sneaking in on the power
lead and a 90 MHz trap at the PP connector might suppress it enough. 
You may not need to supress it very much before it disappears.

73,

Fred ["Skip"] K6DGW
Sparks NV DM09dn
Washoe County

On 7/24/2018 4:22 PM, Frederick Dwight wrote:
> Mike,  I have not had this particular problem with my Elecraft rigs, but have solved similar problems on other equipment.  The first question I will ask do you have resonant antenna for 15 meters.  If so often this in itself will attenuate an FM signal by at least several S units.  If you have a transmitting type low pass filter with a cut off frequency above 10 meters, or above 6 meters if you use this band, you could try putting it between your rig and your antenna.  However these filters would not be appropriate if the antenna has any significant SWR…say over 2:1.  This might give you enough attenuation of the FM signal and I suspect the filter would introduce less than a dB of loss in signal.  If this does the trick, but introduces too much loss on TX, you could move the filter inside the rig, but route it so it would only be in the RX path.  Another idea is to try a coax trap in the rx path only, would only involve a T connector and possibly a few adaptors….research this in an antenna handbook, etc.  You could try this on the rx jack on the rear panel first then move it inside.  Another idea instead of a coax stub, try an LC trap…just a series C and small L (a few turn coil…very small) shunted across the RX path.  Either of these ideas may or may not give you approximately the 30 dB attenuation you will need.
>    Once again, if you have something like a random wire, endfed half wave, etc type of antenna it may let in lots of the FM signal, but a resonant Yagi, or even a dipole often, but not always, will be a rather poor antenna for the FM broadcast band.  Just another thought.  Try turning off your preamp for 15 meters….probably not needed for 15 meters…however since the QRM is only S6, I doubt that this is the problem.
>           Rick   KL7CW
>

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