[TenTec] RFI question for Orion and other TT rigs

[Robert & Linda McGraw K4TAX]

Roy:

Any rig is subject to RFI under certain conditions.  Some are more prone 
than others while some fair better in random RF fields.  The general 
conditions which contribute to RFI issues are those rigs that have lots of 
inputs and outputs.  Inputs and outputs not only mike but key, accessories, 
remote keypads, data ports, serial ports, external audio in's and out's, 
power supply leads and etc. just to name a few.

Some of the rigs add bypass caps to the in's and out's and some add ferrite 
beads or chokes.  In each of these applications, implementation of 
"filtering" does reduce the acceptable bandwidth of the port of concern. 
Rolling off or reducing the high frequency response of a digital port or 
data port will limit the upper limit to which data will pass.  High speed 
data has sharp edges thus requiring wide bandwidth of the port.  This is an 
open invitation to RFI.  Audio lines, key lines and such are no different.

Grounding is always a question of concern.  In true practice today, most 
antenna systems do not need a ground to properly radiate.  This includes 
dipoles, beams, and center fed balanced antennas.  Those that require a 
ground are unbalanced wires, verticals, off center fed Windom's and such. 
Today, most stations only need a safety ground or one supplied by the 3rd 
pin of the AC mains service.  Any attempt to "ground" the station can meet 
with problems which can actually make RFI worse.  Look at it this way, a 
ground lead from the radio to a driven ground that is approximately 8 ft in 
length is approximately 1/4 wave, electrically, above ground.  That's 1/4 
wave on 10M.  If the ground wire is some 17 ft then that is 1/4 wave on 20M. 
In looking at a 1/4 wavelength, one will see that if one end is at zero RF 
potential then the other end is at a maximum RF voltage point.  Yes the rig 
is a DC ground but at a very high potential point with regard to RF.

A second point where RFI may occur is one where the radiating portion of the 
antenna is very close to the station.  One where a leg of a dipole runs over 
the house which then will produce a very high level of RF in the operation 
position.  Here is a case where lead length, connections between radios, 
tuners, computers, mikes, keys and keyers and such will serve as small 
antennas and thus these may inject RF into the radio.  The point here is the 
longer the feed line, the further away the antenna is the less likely one is 
to experience RFI issues.

In my case, my operating position is on the 2nd floor of a wood frame house. 
I have no ground other than the AC 3rd pin ground for safety purposes.  The 
antennas are either fed in a balanced configuration with open wire feed or 
coax feed with a 1:1 current balun at the feed point.  Running legal limit 
on any band, any frequency I have NO RFI issues with any of the 3 Tentec 
radios that I own and use.  Same true for the Collins S-Line.  (See my 
call/pix on QRZ.COM).  I have 2 operating positions with the 2nd one being 
at my shop here on the property.  There I run a Paragon for the receiver and 
the exciter.  The transmitter is either a CCA AM-1000D or a Western Electric 
250B.  These are retired and restored broadcast rigs capable of 1KW 
modulated carrier.  Again, the feedline is 125 ft of Andrew 1/2" hard-line 
with a 1:1 current balun at the feed point for the center fed wire.

Regarding grounding because of concerns for lightning, most hams have a very 
poor ground system, more correctly poorly designed to the point of actually 
being dangerous, not only to  there personal selves, but their equipment and 
their homes.  To that end, I offer a copy of my research paper on Lightning 
Protection for the Home and Station.  It is in Microsoft Word format and is 
about 175K in size.  If you want it, ask, and I'll send it along.

In conclusion, if you have RFI issues, it's not the radio's fault.

73
Bob, K4TAX