[Elecraft] 12 meter K3 RFI from nearby FM station

[Don Wilhelm]

Since the problem occurs without an antenna connected, it is a version 
of the "RF in the shack" problem, although the RF field is coming from 
the nearby FM station rather than the classic situation where the 
transmitted signal is the source of the RF.

Jim makes some very good suggestions for 'taming' that problem.

It would be very difficult to cure the K3 of the "pin 1" problems.

For those who are not familiar with the "pin 1" problem, that is a 
situation where the shields of interconnecting cables are connected to 
the circuit boards rather than directly to the outside of the 
transceiver enclosure.  That allows any pickup on attached cables to be 
imposed into the boards of the transceivers.  In days of old when we 
mounted all connectors to the enclosure rather than to the circuit 
boards, the enclosure itself provided a shield for whatever trash was 
picked up by the external wiring - because those trash signals would 
flow on the 'outside' of the enclosure and not affect the circuits 
inside.  With modern transceivers where cable jacks are connected to the 
ground plane of the circuit boards rather than to the enclosure, the 
external signals are coupled to the ground plane of the internal boards, 
and will couple the offending signals into the transceiver.

In other words, "board ground plane"  does not equal "ground" - 
variations in the ground plane reference will cause "funny happenings" 
within the interior circuits.  In other words, the enclosure does not 
offer a sufficient shielding effect for the circuits inside the enclosure.

The K3 is not alone in this problem, it is shared by any transceiver 
that does not ground the connectors directly to the enclosure.

73,
Don W3FPR

73,
Don W3FPR

73,
Don W3FPR

On 2/10/2016 6:57 PM, Jim Brown wrote:
> Yes, this hit me between the eyes as well. The K3 has Pin One Problems 
> at many (most?) connectors, so RF current flowing on shields of cables 
> plugged into those connectors will couple into the K3. I've been 
> preaching to Wayne about Pin One Problems since 2004, when we met in 
> Dayton.
>
> If the problem is around 90 MHz, I'd use multiple one-turn and two 
> turn chokes with #43 material on every cable except the SO239 output, 
> which IS actually bonded to the chassis. Also choke the power cable.
>
> 73, Jim K9YC
>
> On Wed,2/10/2016 2:53 PM, Fred Jensen 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.
>