[Elecraft] 12 meter K3 RFI from nearby FM station

[Bob McGraw K4TAX]

Very interesting.   I would not expect the integral shielding of the K3 
radio to be that poor.  Most of the box is metal and most connectors are 
mounted on the metal box.   For an AM BC station, I would not think 
anything related to the carrier frequency, harmonics or parasitic , to 
have anything near 14 MHz existing.  So, it seems there are issues 
internal to the radio which contribute to the spurious reception.

The worst thing causing spurious is near field dis-similar metals, in 
effect diodes, excited by the RF.  I've found in more than one case, the 
metal fence around the BC tower and guy anchor points to be a 
contributor.  Better defined as poor site maintenance.  In another case, 
poor lighting conduit bond to the BC tower and also metal gutters and 
downspouts around any build in the near field causing the same effect.  
These will radiate like crazy at the resonant point.  For example, a 8 
ft downspout attached to a 33 ft metal gutter makes a nice 20M inverted 
L.  That 1100 watt station wouldn't just happen to be on 890 kHz or 
close by would it?

If one is experiencing activation of the front end protector, then a 
coax stub at the offending frequency should be of great assistance. The 
only issue is the length of coax required to make the stub. {about 125 
ft or so depending on BC frequency and velocity factor of the coax}  As 
to insertion of filters, unless attention is paid to negate the common 
mode current, i.e current induced on the feed line from the BC station, 
then the current simply flows around the outside of the expensive 
filters and they are not effective.

The problem is solvable, identifying it is the challenge.  That will 
take some detective work.



73
Bob, K4TAX



On 2/11/2016 11:33 AM, Chris Hallinan wrote:
> I have a similar problem.  I have a 1KW AM station about 1/2 mile away
> (1100W by day, 110W by night).  With nothing connected to the radio, and
> even with a 50-ohm terminator on RX Ant in, I can clearly hear the signal
> at several places, most prominently at 14.220 MHz. Nothing I've tried,
> including expensive filters, shielded cables, etc. has helped.  I get some
> relief using a RX Loop antenna basically nulled (broadside) to the tower,
> but that antenna is not always the best one for band and/or conditions. :(
>   Some frequencies are simply unusable, and it overloads a front end
> protector I purchased to try to kill the COR relay clicking when I
> transmit, making it unusable.
>
> Chris
> K1AY
>
> On Wed, Feb 10, 2016 at 5:53 PM, Fred Jensen <k6dgw at foothill.net> wrote:
>
>> I easily could have misread the original emailed problem, but he included
>> the note that this RFI occurred with all antennas [and I guess other
>> cables] disconnected which is very strange indeed.  If that is so, then I
>> doubt stubs/filters on the antenna circuits would cure the problem.
>>
>> His list of frequencies might suggest some sort of intermod between
>> constant signals, one being the FM carrier, and possibly their harmonics.
>> Very strange problem at any rate.  FWIW, and Phil can correct me, but US
>> FM/TV stations are usually authorized in EIRP, I think ... we referred to
>> KSBY-TV, where I worked while in college, as "Full-power television for the
>> Central coast, one hundred thousand watts on Channel 6 in San Luis Obispo"
>> on ID's.  That was EIRP, 10 KW to a 10 dB turnstile.  If that's still true,
>> it would seem a 2 KW EIRP FM station 2 miles away shouldn't be a big deal.
>>
>> 73,
>>
>> Fred K6DGW
>> - Northern California Contest Club
>> - CU in the Cal QSO Party 1-2 Oct 2016
>> - www.cqp.org
>>
>>
>> On 2/10/2016 2:04 PM, Ron D'Eau Claire wrote:
>>
>>> I agree. Even better.
>>>
>>> 73, Ron AC7AC
>>>
>>> -----Original Message-----
>>> From: Elecraft [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Bob
>>> McGraw K4TAX
>>> Sent: Wednesday, February 10, 2016 9:18 AM
>>> To: elecraft at mailman.qth.net
>>> Subject: Re: [Elecraft] 12 meter K3 RFI from nearby FM station
>>>
>>> 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
>>>
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>