Great post Stuart.
I agree with your comments about user set up's. I have never had any brand
of radio, that had RFI problems, that could not be fixed by proper set up of
the station system. But it is easier to point a finger at the manufacturer
than it is to move the end's of your antenna from directly over the roof of
your operation position or to go to the 'pains' of installing an adequate
station grounding system.
Tom - W4BQF
----- Original Message -----
From: "Stuart Rohre" <rohre@arlut.utexas.edu>
To: <tentec@contesting.com>
Sent: Wednesday, September 29, 2004 8:01 PM
Subject: Re: [TenTec] Re Please tell me about the TT Omni VI+
> 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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