[RFI] TV RFI
Ian White GM3SEK
gm3sek at ifwtech.co.uk
Tue Jan 29 02:54:47 EST 2008
Jim Brown wrote:
>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.
* Where are those RF currents that are causing interference?
* Which wires are they flowing on?
* How much current is flowing?
* How much difference does a choke make?
A clamp-on RF current meter will SHOW you the answers to all of those
questions. It really is like taking the blinders off - suddenly you can
SEE what's happening!
Beg, borrow, build or buy a clamp-on RF current meter, and you'll see
exactly what I mean. It will become the first thing you reach for when
RFI rears its head.
You can throw one together in about 10 minutes - I'll bet that
everything you need is already in the junk box: a clamp-on ferrite bead,
one or two detector diodes, a small 50-100uA meter and a few other
components. These pages show some practical examples:
http://www.ifwtech.co.uk/g3sek/clip-on/clip-on.htm (also in Japanese
If you'd rather buy than build, the only clamp-on current meters for the
amateur market appear to be the MFJ-854 and 853 (Palomar used to sell
one, but don't seem to any more). I reviewed both of the MFJ instruments
for Radcom. The 854 is a nice instrument, well designed, solidly built
and performs well. The 853 (unless they have radically changed it) was
junk - quite literally, you couldn't even trust the meter needle to move
in the right direction!
The only criticism of the MFJ-854 is that it's over-specified for basic
RFI investigation, and therefore more expensive. There's a real
commercial opening for a basement business to market a kit for a basic
clamp-on meter. It is also an ideal "weekender" construction project...
will someone *please* write an article for QST, featuring parts that are
readily available in the USA?
Once you have one of these instruments in your hands, and can SEE where
your RFI problems are coming from, you will re-read Jim's publications
with eyes newly opened.
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
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