
Reprinted from March 1994 QST Lab Notes column
Copyright 1994 American Radio Relay League, Inc.
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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 401-596-3080.  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 (202) 457-4977


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 mtracy@arrl.org. 

73 from ARRL HQ.

