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
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