[TenTec] Strange FD RFI

[Stuart Rohre]

Kim,
Yes I have heard of similar problems from a nearby rig, and offer some 
suggestions on things you tried and what could have helped you did not know.

First for all to know, using Astron external power supplies, it is often 
helpful to twist no. 12 stranded power supply DC leads all the way to 
the rig being powered.  Next, for any power supply, put a disc ceramic 
by pass cap big enough to be low impedance (reactance) at 75m and up on 
the DC terminals.
The 1000's internal supply probably had a cap, but it might have gone 
open, or they relied on a big electrolytic which can be bad at RF 
frequencies for bypassing. A Corcom on the 1000's AC power input would 
keep stuff off the line, unless it is conducted on its chassis to the 
third pin ground lead of AC distribution.  Also, make sure the AC cord 
and distribution back to you was not a quarter wave total length at your 
band, or their band (20m).  Quarter waves will be high impedance usually 
at the end that is closest to your having interference.

It was good you tried a Corcom on your AC input, but both places would 
have been better.  The reason the UPS helped is that every AC 
transformer you add adds about 20 dB RF attenuation at HF.  That I 
learned in the RFI business and confirmed by measurement at 160m one time.

Yes, each station using bandpass filters helps the other stations 
nearby.  Despite the filters being bandpass at a certain band, they may 
have some out of band responses, but your using a band pass probably 
attenuated your third harmonics if yours.  But maybe the third harmonic 
was generated in their rig input, or its tuner if external, and adding 
the 15m band pass filter cut down the rig input level to where the 
fundamental could not generate the third harmonic in some non linear 
junction at their rig.

You also could have had an RF detector and re-emitter in a bad joint 
forming a diode and radiating a third harmonic from impinging 
fundamental from you.  Not your fault at all.  Could have been in 
someone's antenna connection not your own!

We have often used big Corcom or other brand AC line filters to sort out 
noise on AC lines in shipyards, or aboard ship in a drydock.  The best 
are ex submarine RTTY AC line filters from 1968 vintage submarines. 
Found that out when some became surplus while we were down the docks 
working an RFI problem aboard Enterprise.  It is good to meet and know 
the Chief of the Electronics shop!  Or any Chief Petty Officer, when you 
are a Navy contractor as we were!  They can get things down!  "How many 
filters do you want, here, take a few extra" was the response to my 
request for "A filter".  Those became a standard accessory to our 
instrumentation for shipboard work.

Now, on general FD RF problems I will share my 2012 repeat of a 1980 
problem, and the solutions we used.  First of all I had a 20m phone 
station in the same room at next desk, to a 20m digital and CW 
station.Both 100 watts.  I had a Kenwood TS 450.

The Icom 756 the CW man had, would cause overload and hash if we got too 
close to his frequency.  Sometimes, even up the phone band, I would 
cause some hash to him, on an apparent spurious receiver mixer response 
he had.  He was running a vertical at the end of 270 feet of double 
shielded coax, and a remote controlled SGC tuner, 270 feet from the rig.
(Yes that is longer than the 75 foot cable supplied for ICOM tuning, but 
we put an extension cable in use).  Within 20 feet of the CW station was 
our phone doublet, and 88 foot one inverted Vee style, with center at 20 
feet, and ends at about 15 feet. It was parallel to the table upon the 
which the rigs sat.  There was a large patio door with aluminum rim and 
frame between the antennas and the rigs.  We had double shielded coax 
for about 20 feet and then 30 feet of RG 8x.

Now, at a certain point in the FD, I was getting funny tuner settings, 
and was not getting out when calling S9 or higher level stations.

The rig I was using was a Kenwood 450 with an external Dentron Super 
Tuner.  The tuner has an aluminum case as does the Kenwood, and they 
were side by side, and by that time, we had moved them closer and turned 
the Kenwood a bit closer on one end to better see its display.  I 
switched antennas, and still had problems, so suddenly, I remembered FD 
1980, when I had a different rig and a compact B&W 300 watt rated, tee 
tuner.  That year, I had first sat the tuner on top of the rig being 
used, and not getting out like this time.

Moving the rig and tuner apart cured the feedback, which is what is was, 
and it was by magnetic coupling as best we could tell.  Again, in 1980, 
aluminum tuner case, and small spacing of coil from the bottom cover of 
the case, right above the rig.
So close they had added a piece of mu metal as a magnetic shield  under 
the end of the vertical coil.  Now the Super Dentron has no mu metal 
shields, but with its larger coil has ample ability to couple fields, 
and I surmise, I got too close to the Kenwood and caused coupling.  This 
probably caused some ALC detection and folded back the power.

Moving them apart 5 inches cured the problem and immediately the next 
strong station I called came right back to me, and the next, and the next!
It was so dramatic, I wanted to share this RFI cause with the group.

Also, on a 1990's FD, we had the experience with a Ten Tec Scout, of RF 
feedback, incidentally with the same tuner, the Dentron. That one had 
two causes, we found a bad thread on a UHF connector and changed it, and 
we had a bad shield connection in the jumper coax from rig to tuner.
There is my Ten Tec content!
73,
Stuart Rohre
K5KVH
(operated W5KA 3A)