QRZ! Ham Radio 4/QRZ Ham Radio Callsign Database - Volume 4.iso/files/arrl/tis/tvi.txt

Text File | 1994-11-27 | 15KB | 298 lines

Reprinted from March 1994 QST Lab Notes column
Copyright 1994 American Radio Relay League, Inc.
All rights reserved.

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Subject: Lab Notes:  TVI, CATVI and VCRI

Ed Hare, KA1CV, ARRL Laboratory Supervisor, chats with his friend,
Joe Hamm.  Once again, the subject is interference. It seems as
though Joe is inflicting some grief on the guy next door.  This time
it's a double whammy: he's trashing two television sets at once!
--WB8IMY

Q: Hello, again, Mr EMI-Fix-It.  It's me, Joe Hamm.  I really
appreciate the advice you have given me about my other EMI problems.
It seems I can't win, though.  My new neighbor, Bruce, just came to
talk to me about some interference problems he's having with his TVs
(TVI).  It seems that when I transmit on HF, I'm getting into his
VCR and both of his TV sets.  One is hooked up to cable, the other
is hooked up to a "rabbit-ear" antenna.  When I transmit on
2 meters, I'm tearing up channel 18 on the cable TV.  I heard that
TVI is the fault of the transmitter.  Should I buy a new transmitter?

A: Don't buy a new transmitter just yet, unless you're looking for
a good excuse to spend your money.  First, I'm going to send you
back in time to the February and March 1992 issues of QST.  Those
important issues will refresh your memory about the technical and
interpersonal basics that are part of any EMI problem.  Reprints are
available from ARRL Headquarters along with a pamphlet for your
neighbor and a list of companies that sell filters and other helpful
products.  This compilation is the Technical Information Service
"EMI/RFI Package."(1)  I also suggest that you buy the book Radio
Frequency Interference--How to Find It and Fix It to get a more
complete picture.  Contact your ham-radio dealer or refer to the
ARRL Publications Catalog.

Let's see what we can do right now, though.

Interference can result from several different causes--electrical
noise (such as a vacuum-cleaner motor), spurious signals from your
transmitter, or the inability of the TV set to "tune out" a strong
local signal.  As an analogy to the last point, if you were to
tune a TV set to channel 3, and saw channel 13 instead, you would
probably conclude that the TV was broken.  If you tune your TV to
channel 3, and see a local HF transmitter instead, you could reach
the same conclusion.  That is, the TV is "broken" in the sense that
it needs additional filtering or shielding to work near a strong
RF source.

If you can demonstrate to Bruce (and yourself) that you are not
interfering with an antenna-connected TV at your house, your
transmitter will be off the hook.  (It would still be a neighborly
thing to offer your help to find a solution, however.)

Q: Well, I suppose I must tell you that when I transmit on HF, I
interfere with the TV in our den.  It is connected to an outside
antenna, located about 30 feet from my beam.  Does this mean the
interference is all my fault?

A: Not necessarily.  Let's run through the things you can do to
find out.  Start by installing a low-pass filter on your
transmitter.  This should be installed as closely as possible to
the transmitter or amplifier output.

Q: I installed the filter and it helped quite a bit.  I still see
some interference on the video and I hear myself in the sound.
Should I add another low-pass filter?

A: Most interference problems have multiple causes.  The fact that
you made things better by installing a low-pass filter tells you
that you probably did have a problem with spurious signals from
your transmitter.  The next thing you can do is to look at your
station ground.  If you have no ground, add one.  Grounding is not
a cure-all for EMI problems, but under some circumstances, it can
help.

I suggest that you install a low-pass filter permanently--whether
you need one or not!  You can always point to it with pride,
explaining that you have installed a filter on your end.  If the
FCC is ever involved with your EMI problem, they will ask you lots
of questions about your low-pass filter and station ground.

Q: I added a good ground and it didn't seem to make any difference.
What's next?

A: It sounds as if we're dealing with a TV that is susceptible
to overload from your fundamental signal.  It's time to try the
appropriate filters for the TV.

For an antenna-connected TV, the first filter to try is a high-pass
filter.  These are available from Radio Shack (catalog # 15-579 or
15-582).  Several other sources are listed in the "EMI/RFI Package."
Buy one and install it as closely as possible to the TV antenna
input.

Q: Wow! The picture is clear as a bell, but I'm still getting into
the sound.  What gives?

A: A high-pass filter will attenuate the HF signal that's picked
up by the antenna and sent to the TV inside the TV's feed line.
But you can also pick up a lot of signal on the outside of the
feedline.  The signal that is not inside the feedline is called
a common-mode signal.

You can filter the common-mode signal with a common-mode choke.
Obtain an FT-240-43 ferrite core.(2)  (Don't use unknown material;
it may not work.)  Wrap about 10 turns of the feedline (either
coax cable or twin-lead) onto the ferrite core, installing it just
before the antenna input to the TV.  The "EMI/RFI Package" also
has a list of commercial suppliers of common-mode chokes.

In some cases, you may also need to install a common-mode choke
on the TV's ac-line cord.  If necessary, you can also try an
ac-line filter, available from Radio Shack (catalog #15-1111).

Q: Hooray! It worked.  My TV is clean as a whistle, except for that
real weak channel we get from about 150 miles away.  Can I fix that
one, too?

A: Probably not.  The FCC rules do not protect fringe-area
reception.  If you are outside that station's coverage area, you
will just not get enough TV signal to be usable.  If you, or your
neighbors, are having problems receiving that station, it is not
surprising.

Q: Well, I am ready to go help Bruce.  Any other words of wisdom
before I go over there?

A: Don't forget that you do not want to service your neighbor's
equipment.  Once you have determined that the problem is not your
radio station, you want to be a good neighbor.  You're a locator
of solutions, not a provider of solutions.  If you take the back
off the TV, you may be blamed years later when the 25-year old
clunker gives up the ghost.

There are a few other things you should watch out for.  Make sure the
TV is getting a good picture.  An old, rusted TV antenna or feedline
can be a direct cause of an interference problem by generating
harmonics right there at the TV!  A bad installation will result
in a weak, snowy picture, much more prone to interference.  Fix
these things first.

Q: I'm over at my neighbor's house and he's right--things are a
real mess.  Where should I start?

A: Well, you just fixed the antenna-connected TV in your house,
didn't you?  Start doing the same things with your neighbor's TV,
in the same order.

Q: You're right.  We finally fixed it.  Should I install the
high-pass filter on the VCR and cable TV installation?

A: No. If the cable TV system is not leaking severely, there will
not be any HF signal inside the cable to be filtered.  Most of the
time, cable TV is virtually immune to HF interference problems.
If there is a problem with CATVI from HF signals, it is usually
caused by a common-mode signal on the outside of the coaxial
cable.  If you tell your neighbor to buy a high-pass filter, and
then you go back and tell them that specified the wrong filter,
he may be a little upset!

Start by disconnecting the VCR.  If you still have interference,
add the common-mode choke.  Install the choke where the cable
attaches to either the set-top cable converter or to the back
of a cable-ready TV.  If this still doesn't cure the interference,
you can try a common-mode choke and/or ac-line filter on the TV
and/or the converter.  If this doesn't do it, you could try a
high-pass filter as a last resort.

Once you fix the cable interference problem, apply the same cures
to the VCR.  VCRs can be a bit harder to fix, primarily because
the video and sound signals inside the VCR are in the HF range.

A cable-ready TV or VCR that doesn't respond to any of the above
treatments is subject to direct-pickup interference.  In this case,
the only recourse your neighbor has is to contact the equipment
manufacturer in writing.  He could also call the Electronic
Industries Association(3) to get the appropriate contact persons
for the manufacturer.

If you are able to determine that the converter box is subject to
direct pickup, or that a cable leak exists, contact the cable
company for assistance.  Often the cable repair personnel are not
familiar with all of the technical issues surrounding a
cable-television interference (CATVI) problem.  If that's the case,
there are two things you can do: call the cable company and discuss
the problem with the repair supervisor or the system engineer.
The ARRL has written a two-part article about CATVI that was
published in Communications Technology, the journal of the Society
of Cable Television Engineers.  The cable company will recognize
this publication!  A copy is also available in the "EMI/RFI -
CATVI" package from the ARRL Technical Information Service
Secretary.(1)

Q: Well, it took quite a combination of filters, but I managed to
fix most of Bruce's problem.  The only thing that's left is my
2-meter interference to cable channel 18.  Why is that problem
still there?

A: In a nutshell, cable television makes use of frequencies
allocated to over-the-air services.  Cable channel 18, or channel
"E" in CATV parlance, is from 144-150 MHz.  The amateur 2-meter
band falls within that channel.  Your 2-meter signal is clearly
leaking into the system.  The hard part is finding out where.

Start by determining if the signal is leaking either in (ingress)
or out (egress).  In most systems, the video carrier will be on
145.25, plus or minus 25 kHz.  If you hear a strong carrier on
that frequency, the cable system is probably leaking.  The cable
company will appreciate your calling this to their attention.  This
type of leakage can be very expensive if they fail their cumulative
leakage tests or if they're cited for violations of FCC rules.  If
their signal is leaking out, you can be sure your signal is leaking
in.  In the case of cable-plant leaks, it is the cable company's
responsibility to cure the problem.

Even if the leak from the cable system is below the FCC leakage
requirements, the interference could still be caused by a leak in
the cable system.  An interfering signal that's more than 40 dB
below the video carrier level can still cause interference!  If
the leak is not severe, the cable company may try to tell you that
they don't have to fix it.  Don't be fooled.  The cable company
must adhere to several regulations about picture quality.  FCC
regulations 76.605(a) state, among other things, that the
carrier-to-noise ratio must be at least 40 dB and the
carrier-to-coherent disturbances ratio must be at least 47 dB.
These regulations, and the ARRL CATVI package, will help your cable
operators to understand their responsibilities.

Unfortunately, it is not all black and white!  The leak could be in
the cable system, the cable company's set-top converter, or a
customer-owned cable-ready TV or VCR.  The cable company is
responsible for the first two, but cannot be held responsible for
the design of the TV.  If the TV leaks, Bruce will have to contact
the manufacturer.

Sometimes, a common-mode choke does help.  Try one on the incoming
cable, just before it contacts the first piece of electronic
equipment.  Try a common-mode choke on the ac line connected to
each piece of equipment, too.  Other than that, there is no
filtering you can do.  If you filter the 2-meter signal, you will
be filtering cable channel 18, too!

The manufacturer and cable company can get involved in a real
finger-pointing contest in some situations!  There is an easy way
to determine if the cable operator is "off the hook."  Ask the
cable operator to demonstrate good performance on channel 18 using
a cable-company supplied set-top converter.  If they can, the leak
is clearly in the cable-ready TV.  If they can't show that their
system is working well, they have a bit more work to do.  Suggest
that they buy the ARRL RFI book and contact the ARRL "RFI Desk"
(that's me!) here at Headquarters.

Q: Thanks for all your help.  Bruce is happy now.  His dog Bucky
even likes me again!  Is it always this easy?

A: Unfortunately, no.  The proper application of personal diplomacy
and technical solutions sure make it a lot easier, though!

Notes:

(1) Send a 9x12-inch self-addressed, stamped envelope with 4 units
    of First Class postage for each package to the ARRL Technical
    Information Service Secretary, 225 Main St, Newington, CT  06111.
    Include a specific request for the "EMI/RFI Package" and/or the
    "EMI/RFI--CATVI" information packages.

(2) Ocean State Electronics, 6 Industrial Dr, Westerly, RI 02891,
    tel [phone].  Catalog # FT240-43 ($8). Add $4.50 for
    shipping and handling.  VISA, MasterCard or COD accepted.

(3) Electronic Industries Association, 2001 Pennsylvania Ave.,
    Washington, DC 20006 [phone]


The ARRL 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 [e-mail].

73 from ARRL HQ.