[RFI] High Pass Filter and Broadcast Interference

Jim Brown jim at audiosystemsgroup.com
Thu May 10 23:29:24 PDT 2012

On 5/10/2012 9:48 PM, Hardy Landskov wrote:
> I have a 22KW BC station about 5.5 miles away

What frequency?

> from me and it turns the K3 into Hamburger until Sundown.

What do you hear on the K3?  On what frequency(ies)?  Could these be 
intermod products generated outside the radio? Lots of ways this could 
happen.  FCC oversight of broadcasting is not what it used to be, and 
some stations aren't always clean.  I could hear a second harmonic from 
WBBM-AM (780 kHz, 50 kW) at a distance of about 20 miles. Not loud, but 
I heard them. And that was a pretty well run station at the time.

> I talked to N6KR at Visalia and he said I was
> in a minority of people having this problem and putting additional filtering
> on the front end would solve the problem. He is right but my FT1000D never
> did it nor did my FT990. I have looked at the interior and IMHO it's just
> poor grounding. So all that "buckshot" you might be hearing is not you
> towers but the best radio ever made, did not do their homework very
> well.....

I've had some issues with bonding in the K3, but I'm not convinced 
that's the problem. An easy way to figure this out would be to add a 
broadcast band high pass filter at the RX insert point.  Decent filters 
don't cost much. I bought a couple of them ten years ago when I had 
broadcast intermod on 160M.  At that time I was mostly using a TS850 and 
a K2. My problems were outside the radio, and I never found the 
culprit(s) that were creating the mixes. BUT -- I still leave them in 
the RX loop of my K3s, even though I'm not close to any broadcast 
stations running any power, under the hope that they might help a bit 
with front end overload from very strong lightning static.

73, Jim K9YC

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