[Elecraft] 12 meter K3 RFI from nearby FM station

[Bob McGraw K4TAX]

An electrical 1/4 wavelength of coax as an open stub at the FM 
frequency, connected using a T at the receiver input, will notch the FM 
TX signal.  It won't attenuate other frequencies across the ham bands 
including 6M.  Plus one can transmit with the stub arrangement in place.

73
Bob, K4TAX



On 2/10/2016 10:13 AM, Ron D'Eau Claire wrote:
> A single frequency issue like that also can be handled with a simple trap. A
> small coil with a capacitor across it tuned to the station's frequency at
> 91.1 MHz in series with the center conductor of your coax.
>
> 73, Ron AC7AC
>
> -----Original Message-----
>
>
> Jon,
>
> I had a similar problem with a 50 kW AM BC station a few miles away. It
> turned out to be strong enough to override the bias in the K3's T/R switch.
> I built a high-pass filter which solved the problem.
>
> You would need a low-pass filter in your case. Make sure it can handle 100w,
> because you must connect it in the main antenna path -- the RX in/out path
> is before the T/R switch.
>
> 73,
> Vic, 4X6GP/K2VCO
> Rehovot, Israel
> http://www.qsl.net/k2vco/
>
> On 10 Feb 2016 16:58, Jon Zaimes wrote:
>> I had been noticing some RFI on 12 meters for some time that I
>> initially had thought was power line noise. My use of 12 meters is
>> seldom so it wasn't very high up on the list of things to track.
>>
>> When I was reminded of it while listening for VP8STI recently, I did a
>> little more snooping and realized it was actually splatter from a BC
>> signal at the low edge or slightly below the 12 meter band, centered
>> about 24.89 mHz but audible from 24.87-24.91 mHz.
>>
>> The distorted splatter is audible as a BC station, all talk, no music,
>> but not clear enough to decipher any ID.
>>
>> A few days ago I decided to try to pinpoint the source, so listened to
>> the RFI on one ear while scanning the AM and SW bands with the sub RX
>> on the other ear. But I could find no matches, though did find the
>> splatter popped up centered on these additional frequencies:
>>
>> 813 kHz
>> 996 kHz
>> 2.820 mHz
>> 3.626 mHz
>> 19.4 mHz
>>
>> Figuring it must be an FM BC station, I started going through a list
>> of Delaware FM stations and on the third one, 91.1 mHz, matched the
>> audio peaks to my splatter. Further research determined this was a
>> public radio station with a transmitter on a commercial tower just a half
> mile west of my QTH!
>> While I of course was aware of the tower, which I knew to carry a
>> couple of cell sites and trunking radio, I hadn't been aware of the FM
>> station, which has apparently been there about 2 years. FCC data shows
>> it has 2.1 KW ERP. I had not had any previous issues with the site over
> the past 17 years.
>> Thinking that mixing or rectification might be produced either at the
>> TX site or on my own QTH, I decided to track with my mobile rig, an
>> IC-706 MK2G. But there was NO evidence of the splatter on the 706.
>>
>> Further debugging found that I was hearing the same splatter on BOTH
>> of my
>> K3 radios (no. 3021 and 3057). Even when ALL cables are removed (only
>> the power lead connected). Grounding or ungrounding made no change. I
>> tried the radios on a battery but still had the RFI. Tried some
>> ferrites on the power lead but no change.
>>
>>
>> So I'm thinking now there's some internal mixing in the K3 going on,
>> and wondering if any others have had similar issues and found any
> solution.
>> 73/Jon AA1K
>> Felton, Delaware
>> www.aa1k.us
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