[Elecraft] Re: FM Broadcast Station Interference

Wed May 29 13:14:00 2002

At 12:18 05/29/2002 -0400, [email protected] wrote:
>Many thanks to all of you who have responded to my post about FM broadcast
>crud. I've also noted that others have the same problem. Just to further
>clarify the issues here at my QTH:
>
>1. I am receiving the crud with NOTHING attached to the K2 but the power
>cord. Right now its coming in especially well on 14.068 MHz.
>
>2. I've tried different power cord lengths, line operated dc power supplies,
>and external battery supplies (both gel cells and NIMH) and still experience
>the same level of interference. I am planning to fabricate a shielded,
>twisted pair power cable as was suggested.

I don't remember from your earlier postings, but do you have the problem 
with headphones, internal speaker, external speaker, or all three?

You might want to consider a couple of additional things.  Fabricate a cord 
that runs both leads through a ferrite core, (common mode) a choke or core 
on the hot lead, and tinkering with the grounding.  We have some real pros 
on here that can recommend the best core type and number of turns to 
suppress 88-108 Mhz.

Try putting a capacitor across the power input of the K2 to ground.  The 
rev C manual shows C196 as a .047.  Try a .47 and a 47 across the power 
input as well.  If that is where it is getting in, some more suppression 
won't hurt.

Find the ground for your house, and set the K2 up there.  Hook PS negative 
lead and K2 ground to house ground with as short and stout of a lead as 
possible.

There were some words of advice on here recently about keeping HF RF out of 
the K2, and that may be sage advice for this as well.

It is possible that the same thing in the K2 that is suceptable to AC 
fields is suceptable to very high RF fields at FM frequencies.

You are probably in what used to be referred to as the "blanket coverage" 
area of the FM station.  This is an area that has rf fields that exceed the 
normal limits imposed on devices that are approved under Part 15 and other 
FCC regulations.  There may be some help from the FM station.  Drop a dime 
to them, ask for the Chief engineer, explain your situation, and ask him if 
he has any suggestions.  Don't expect miracles, but he might have something 
up his sleeve from working with others that will help you as well.

>3. I went portable with the K2 and NIMH battery pack inside my house and
>around the lot. Again with NOTHING connected to the K2 except the battery
>pack on an 8 inch cable. There were slight variations in the strength of the
>interference as I walked around the house and lot but no major differences.
>
>4. I will soon be installing the KBT2 perhaps that will help. Just as an
>aside I installed the KAF2 last night. What a super option...that thing works
>great!
>
>5. Perhaps this problem has something to do with my call. Oh well.

Look on the bright side, somewhere there is a TV2RFI that probably does.

>I have easy 5-min access to the backcountry from my QTH but the FM
>transmitting tower is visible from just about every trail on this side of the
>mountain.
>I'm going to take the K2 about 20 miles away and see if that makes
>a difference! Stay tuned.

I suspect that you will be able to find sites within a couple of miles of 
your home that the problem goes away.  You will hit nulls in the pattern of 
the FM station, and the problem will vanish.

>Tom KI5FM

Good luck Tom.

Tim Raymer
73 de KA0OUV

Timothy A. Raymer               <[email protected]>
Missouri Department of Health   <[email protected]>
and Senior Services