Rob,
If you study the receiver test reports for Omni Vi plus models, you will
find they had superior receivers to the Yaecomwoods of that era.
Also, don't be too quick to say a design is not RF resistant just because
another has ferrite cores added, and it does not. ( They are often added
after the problem has surfaced on a given design because of deficiency on
board layout, grounding, etc.) A board without cores may just be laid out
better, have a better shielded arrangement, or better ground plane.
As a long time EMC/RFI practioner in the Research field, we often have to
add cores, filters, and by pass caps to fix some commercial board after it
left the factory. Sometimes a factory catches a problem on the way to
market and the core is the quick fix to get it out the door. Since non
recurrent Design and Engineering budget has been exhausted by that time, the
core fix is left in, to avoid having to completely reanalyze and lay out a
new board. Or, their labor costs may be that a hand installed core was an
acceptable alternate design to on -board measures.
RFI can happen to any brand due to variability in user installations and
hams tendency to defy the laws of Physics and expect no RFI feedback while
using unbalanced, off center fed antennas, single wires, or random wires
with mismatched feedline arrangements. No amount of design foresight can
account for RFI when an antenna is only 5 feet above the shack, or
unbalanced, or does not have an established low impedance RF ground or
reference. Many cases of RF feedback can be traced to loose connectors,
improperly soldered antenna connections, and other failures out of control
of a manufacturer.
I have been caught by RF feedback on various brands of rigs, and so far, it
was because I failed to tighten the screw on ring of the mike connector, or
Antenna connector. Short of welding on our connections, those things
happen. I avoid poorly installed antennas, ie use doublets, or have an RF
ground for verticals, or use a beam, (balanced antenna), or quad. The only
time I have personally had other RF feedback was with Heath equipment and
Hallicrafters equipment used with a random wire antenna that was too low and
close to the shack.
Since top contesters have repeatedly used the Omni VI plus without RF
feedback problems, I tend to think such cases are a specific installation
problem.
As an RFI sleuth, I have found and fixed for other hams, several cases of
RFI that were just like my early Heath story, low antennas near an attic
shack, no RF ground, and improper installations.
Untwisted power supply DC leads are often another entry point for stray RF,
and I twist those if I make them, or find them simply paralleled wires in
others hook ups.
Another overlooked cause with older equipment, (instruments as well as ham
rigs) are loosening of grounding bolts and screws holding circuit board
grounds and chassis grounds. Periodically, or everytime I get a used rig; I
take the cover off, and inspect the internal grounding lugs/ screws, and
even back off and retighten screws to break inevitable oxidation between
them and chassis.
As was pointed out, the exit of the entire stack of trade in Omni Sixes says
something about the good reputation of that Ten Tec model in the general ham
community.
73,
Stuart
K5KVH
Old Crow of the RFI bunch
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