[Elecraft] 12 meter K3 RFI from nearby FM station

[Ron D'Eau Claire]

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