[Elecraft] KPA500 RFI Problem

[Randy Farmer]

Thanks much for your thoughtful and informative reply, Jim. My comments 
are interspersed.
On 1/26/2014 10:32 PM, Jim Brown wrote:
> Some other thoughts. It is critically important that the shield of all 
> cables be connected to the shielding enclosure at both ends, and ONLY 
> to the shielding enclosure. You should make sure that the cables you 
> are using do that, and it may not be easy.  I terminate the signal 
> returns of all lines in my DB-connectors to the DB connector shell. 
> This requires that the connector shell be bonded to the shielding 
> enclosure, and with most gear, that works. It did NOT work with a 
> cable I made to go between the K3 and a KAT500, so I suspect that 
> there is a Pin One Problem at the AUX jack one or both of those 
> products. What you are describing is symptomatic of a Pin One Problem.
I agree. It seems the K3 is responding badly to the RF on the ACC cable, 
whatever the coupling mechanism. I can't imagine RF getting into any 
other component of the system that would cause the K3 to change its 
power output. If the KPA500 was being bombed it would probably show some 
truly bizarre behavior. It seems to just faithfully amplify whatever RF 
gets delivered by the K3. I haven't looked real closely at the K3, but 
since they're using D-sub connectors connected to printed circuit boards 
a Pin One problem is practically guaranteed. I'd sure like to see 
manufacturers return to using chassis mounted connectors with internal 
connections to the PCB via multiconductor cables that plug into headers 
on the board. I don't think it will ever happen, though.
>
> Pin One Problems are tough to diagnose, because the places at which 
> they inject RF are at the whim of the circuit layout designer. The 
> choke you've put on the AUX cable sounds about right for 20M, and 
> should be effective from 30-10M. At this point, I'd look at what other 
> cables are connected to the radio and choke them.
I'm using the microHAM factory cable. See below.
>
> There's another thing that might be causing your problem that I ran 
> into only a few days ago helping W6OAT solve a similar problem -- his 
> big SteppIR is about 60 ft above the shack on a tower, and he gets RF 
> feedback into the radio that locks up CW keying . He's using a 
> Microham box (don't know model number) and the interface cable between 
> that box and the K3 is a train wreck waiting to happen. There's a big 
> multi-pin plugged into the Microham with a lot of wires coming out of 
> it that have labels on them like Line In, Line Out, Key, etc. but they 
> are all unshielded, and there's no signal return -- the setup is 
> depending on the coax shield for that. That works for DC, and maybe 
> even for audio (if there's no big power transformer nearby), but NOT 
> for RF.
According to the microHAM drawing, the factory-supplied K3 interface 
cable (type DB37-EL-K3R) is composed of individually shielded lines that 
have their shields tied back to the ground side of the RCA DC power 
pigtail (which I am NOT using) and are also connected to the DB37 
connector shell and the pin 15 Ground line. The Line In, Line Out and 
Mic cables run their returns to dedicated pins on the DB37 connector. I 
opened my MK2R+ up to have a look and of course found the usual PC board 
mounted D-sub connector stacks with no direct connection to the metal 
enclosure, although they do show DC continuity to the case. I put a 
small solder lug under the jackscrew sockets at each of the D-sub 
connectors and ran short #16 bus wires between all the connectors on the 
outside of the case to bond them together and then took a ground wire 
from the nearest connector frame to a heavy copper bus bar that connects 
the ground lines from radios, amplifier, and other major boxes in the 
station. I don't *think* RF is getting into the MK2R+ in my case.
>
> The problem with using coax (or even a chassis bond between the 
> equipment) as the signal return is that total circuit forms a big loop 
> consisting of the signal wires, the chassis, and the coax shield. You 
> are certainly in the near field of your antennas, and in the near 
> field of a current maximum (in this case, the antenna feedpoint), the 
> magnetic field dominates and couples into that open loop, and the 
> strength of the coupling is proportional to the loop area!  In 
> addition, that loop forms a receiving antenna, which picks up the 
> electromagnetic field.
How well I know. I spent five years doing CDMA base station design for 
Motorola, and we were constantly fighting unintended radiation problems 
that were caused by improper digital cable routing and termination.
>
> I have no idea where Rusty's cable came from, but it's clearly 
> commercially made, and seems to be specific to the Microham and the 
> K3. Cable adapters like this may be simple to hook up, but they are a 
> sitting duck for RFI.
I would think it's the microHAM factory cable I've described above. 
Thanks again for the help.

73...
Randy, W8FN