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QRZ Ham Radio Callsign Database - December 1993.iso
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emi_ca.tvi
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1993-11-21
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CATVI EMI/RFI advice from ARRL Headquarters.
In a CATV installation, the usual form of interference is as a
result of the common-mode signal on the cable, ie the signal
present on both the inside and the *outside* of the cable. Some
TVs or VCRs are very susceptible common-mode interference.
The best way to eliminate this common-mode problem on a 75-ohm TV
antenna or CATV system is to use an FT-140-43 or FT-240-43 ferrite
core. Wind about 15 turns of the incoming coaxial cable onto the
ferrite just before the point where it connects to the first electronic
equipment -- set top converter, VCR or TV antenna or cable input. The
free ARRL handout "EMI/RFI Package" lists several sources for this
core. Retrieve the file EMI-source from the ARRL Information Mail
Server (info@arrl.org).
Another way to eliminate this common-mode problem on a 75-ohm TV
antenna or CATV system is described in Mar 89 QST Hints and
Kinks. The write-up describes how to use two 4:1 transformers and
a 300-ohm high-pass filter to effectively eliminate the common-
mode HF signal that is causing interference. If anyone would like
a free copy of this Hint and Kink, send an 8X12 SASE to the ARRL
Technical Secretary and ask for the "EMI/RFI Package." It contains a
quite a few pages of EMI/RFI information, lists of sources for
EMI/RFI products and and several Hint and Kinks reprints. 98 cents
postage is appreciated.
An amateur that is involved in an EMI/RFI problem should contact the
ARRL section Technical Coordinator (TC). The TC will know of a
more local Technical Specialist and or local TVI Committee that
might be able to offer some assistance. The ARRL Section Managers
(SM) are listed on page 8 of any QST, a quick phone call to the
SM will locate the TC quite quickly. ARRL HQ staff can also look
up the TC in the database.
Education is also important. The ARRL and Orr Handbooks both
contain a chapter on EMI/RFI. I know of only two books on EMI -- the
ARRL "Radio Frequency Interference - How to Identify and Cure It"
and Nelson's Interference Handbook. The ARRL book covers nearly every
aspect of interference problems. The Nelson book is THE book to buy
for *electrical* interference, especially if the interference
originates from power lines. Both books are available from ARRL
Headquarters.
Ensure that your own house is "clean". You should be able to
watch an *antenna*-connected or cable-installed TV while your
transmitter is operating. This is usually enough to satisfy the
FCC that your station is not radiating any harmonics, and it is a
real powerful point to be able to tell your neighbors that if
they can't watch TV at their house while you are on the air they
can come over to your house and watch while you are operating.
When troubleshooting EMI, simplify the problem. Instead of
dealing with 3 Tvs, 2 VCRs and a satellite system all hooked up
together, work at first only with the cable drop and one cable-
ready TV or the cable-company converter box. After that is all
properly debugged, add the other components back to the system
one at a time, and if the problem recurs at least you know which
piece of equipment is the culprit. VCRs are notorious for their
susceptibility to fundamental overload. These basic principles
apply to any interference troubleshooting.
Grounding the shield of the cable outside the house can be
effective, especially if used in conjunction with a high-pass
filter AND common-mode choke right at the TV antenna terminal.
The HF common-mode (translate: signal on the *shield* of the
coax) will take the low-impedance path to ground rather than go
through the high-impedance filter. CATV grounds should only be
installed by cable-company repair personnel.
Either use the filter described in the March 89 QST Hint and
Kink, or use a conventional high-pass filter with a common-mode
choke made from a ferrite core or rod. It is a bit big (2.4"
o.d.), but I would use a FT-240-43 core, with about 15 turns or
so. Ocean State Electronics, POB 1458, Westerly, RI, (401) 596-
3080, has them available for $9.75.
A "free" source of a ferrite material is the old AM-radio loopstick
antenna, or an old television yoke, although this has its element
of risk. One does not know how this MF or AF material will perform
at RF. If it works, fine business, but if it doesn't, that may mean
that it is not suitable material for RF. In that case, try the
*correct material*.
Get the ARRL EMI/RFI Package. It is free (except for the stamp) and
will answer many questions. The bulk of the text is contained in the
file emi-gen available from the ARRL Information Mail Server (info@arrl.org).
The members and HQ staff would like to thank the following people for
their contributions to this information file:
KA1CV
Send any additional information or changes to mtracy@arrl.org.
73 from ARRL HQ.