On Mon, 28 Jan 2008 15:59:51 -0800, Kelly Johnson wrote:
>If, on the other hand, you find TVI on multiple
>TVs then I would suspect a poorly designed antenna system which is
>allowing RF into the house.
Well, sort of. But ANY antenna that is WORKING is going to put RF in
your house. A better way to look at it is to say that some antenna
SYSTEMS inherently put more RF there than others. :) The word
"systems" is quite important.
If your antenna is something like a beam or dipole, most of the RF is
radiated by that beam or dipole -- that is, the active elements are
"up in the air." Their feedline WILL, however, become part of the
antenna if it is not fully decoupled by a REALLY good choke (often
called a balun). See my tutorial for more of that.
If what you CALL your antenna is ANY form of wire that ends in your
shack, everything connected to the chassis of your rig is also part
of the antenna system. This includes the power system safety ground
(and all of the wires connected to power system ground throughout
your house), whatever you CALL ground (the wire going to a rod,
radials, etc), and anything CONNECTED to the power system ground. All
of that wire that we like to call "ground" IS PART OF THIS KIND OF
ANTENNA, and the RF current that it carries is a MAJOR contributor to
RFI, TVI, etc.
This current causes problems three ways. First, the wires carrying
the RF current (your transmitter power) will RADIATE that RF, just
like any other antenna. Second, that current can flow INTO EQUIPMENT
that has pin 1 problems! Third, that current causes IR drop in the
shields of coax that carry it, and that RF is added to the signal on
that coax. The latter mechanism is most likely to be a problem on
coax with thin foil/drain shields (like MATV cable) and low HF band
operation.
See my tutorial for a discussion of pin 1 problems. They are the #1
cause of RFI! Lousy shielding of the equipment itself is another
biggie -- that's the likely cause of the problems that the RIGHT
ferrite choke won't fix (like what Kelly is talking about).
http://audiosystemsgroup.com/RFI-Ham.pdf
It's too easy (and overly simplistic) to blame a bad antenna for RFI.
Those of us who live on small lots MUST use less than ideal antenna
systems if we want to work the lower HF bands [somebody tell me how I
can put a 160M antenna on a 100 ft x 40 ft lot without end-feeding
some sort of wire that ends at or near the transmitter], and even our
very good tribander at 40 ft (or even 70 ft) is going to put a lot
of RF onto the wiring of the home entertainment rigs of you and your
closest neighbors. If we want to operate without RFI, we've got to
learn to blame (and fix) the real causes -- the pin 1 problems, lousy
coaxial cables, and lousy shielding in that home equipment, not our
antennas!
FWIW -- I ran 1kW from a wood frame 2-flat on a 135x40 ft lot in
Chicago on 160-10 and 100W 50-440 MHz. The only RFI issues I couldn't
fix were cheap office phones (mostly a problem on 6M, not HF) and a
toy musical instrument in my tenant's apartment. And I never had a
complaint from a neighbor, even from houses only 25 ft either side of
mine!
http://audiosystemsgroup.com/K9YC/k9ycant.htm
73,
Jim Brown K9YC
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