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Author Topic: Interference with stereo/TV speakers in my home  (Read 9029 times)
AC4RD
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« on: March 30, 2011, 08:52:56 AM »


I've been inactive for almost two years; didn't have a HF antenna up during that time.  Recently I put up a simple 40meter (33' per leg) dipole, fed with ladder line down to my shack window.   In place of my old manual antenna tuner, I ordered a new automatic one, thinking that would be a nice change.  There's a good Bencher low-pass filter in the radio's output, before the autotuner.  Every major piece of the radio gear inside the shack is connected to my ground buss, a brass plate.

I've never had any significant problems with TVI before, but since putting this new antenna up, I am interfering with my own TV/stereo speakers.  (Much to my wife's displeasure.)  I'm concerned that I may also cause interference to my neighbors, and I like staying on good terms with the neighbors.  I've asked the neighbor closest to my antenna and shack to call me right away if he gets interference.   He hasn't complained, but I've only had a few very very brief tests from this setup (15m phone)--my wife's report of bad interference on our system convinced me not to try anything more.

My last HF antennas were big horizontal loops, not doublets.  I thought a twinlead-fed doublet ought to be nicely balanced and not create any RFI problems, but apparently not.   Would going back to a loop help?  Or is that not likely to be part of the problem?

So far I haven't done or tried anything significant to solve the problem, though I'll start on that this weekend.  I've read some information about interference on the ARRL website.   So I don't miss anything, I'd welcome input about what to look at, and what to try, in troubleshooting this.   Does it sound like the new tuner could be the problem?  (I'm going to try putting my old manual tuner back inline this weekend and see if the interference goes away.)   TIA!   --Ken AC4RD
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W8JX
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« Reply #1 on: March 30, 2011, 10:41:12 AM »

Well for starters a 300 ohm line is not a good match for a 33 ft leg dipole even on 40 as it will have a feed point impedance closer to 75 ohm not 300. The mismatch is adding stray currents from feed line that is likely cause of problem in house. I would either change feed line or design a antenna that is a better basic match to a 300 ohm feed. You could try a 40 meter folded dipole which would match 300 line and should also work on 15 too as a 3/2 wave antenna. (and other bands via tuner)
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KF7CG
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« Reply #2 on: March 30, 2011, 10:48:17 AM »

The problem may well be that your dipole is too good at getting out a signal. Let me assume the the TV/stereo speakers are driven by a home theatre type arrangement and are not the TV speakers.

Some tests for more help.

1) Turn off the amplifier driving the speakers and the TV. Do you still get interference on the speakers? If so you have too much signal getting onto the speaker leads. They may need toroids or to be sheilded.

2) Check with just the TV and its internal speakers. If interfence then the problem is with the TV.

3) Check the amplifier with the TV disconnected. If interference, it is probably in the amplifier (audio for speakers).

As you can see the trick is to isolate each component to check for problems. It may well be that your radio is not at fault but the TV/stereo is.

KF7CG
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K8AC
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« Reply #3 on: March 30, 2011, 11:25:19 AM »

There's nothing wrong with your radio or tuner.  The days of harmonic interference to TV stations is essentially over as most of the stations have moved to the UHF band and your low pass filter is of little or no value.  Your problem is that the fundamental signal is getting into your audio chain somewhere.  The fact that you're having such a problem says nothing about whether or not your neighbors will have a similar problem.  You may or may not be able to solve the problem without replacing a piece of equipment, but first you have to identify which piece of the audio chain is at fault. 

The ARRL publishes a good RFI book, and there's good info here: http://www.arrl.org/audio-1 covering your situation.  Even expensive audio gear can be susceptible to RFI, so don't be led down the path of blaming "cheap" devices.  I have a very expensive sub-woofer that required a couple of ferrites on the audio and control lines to resolve.  The split-core ferrites from Radio Shack did the job in that case.
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W8JX
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« Reply #4 on: March 30, 2011, 11:28:11 AM »

The problem may well be that your dipole is too good at getting out a signal. Let me assume the the TV/stereo speakers are driven by a home theatre type arrangement and are not the TV speakers.


I do not think so here. It is likely in feed line because when it is not properly terminated with correct load/impedance it will radiate more from feed line in and close to house unlike coax. I would wager that if he switched to 75 ohm coax the problem would go away. You have a resonant antenna with a non resonant feed line impedance mismatch that you then use a tuner to make it work.  Could try making antenna non resonant too as that would change line to antenna impedance match too.
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K1CJS
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« Reply #5 on: March 31, 2011, 07:13:25 PM »

Did you have the same TV/stereo system when you were operating before as you have now?  If not, that system may need to be worked on to stop the 'reception' of your signals.  Have your neighbor actually look at his electronics (radio, stereo, TV) while you're transmitting.  If he doesn't have a problem, then your station isn't at fault.

Another thing to check is the circuit you run your station off of.  If it is the same circuit that your TV/stereo runs off, it may be an easy fix--just use another circuit to run your station if you can.  That one step may relieve the interference.
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AC4RD
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« Reply #6 on: April 01, 2011, 04:21:52 AM »

Thanks to all for the suggestions; it gives me a nice start on looking into the problem.   My neighbor tells me he hasn't noticed any interference so far, but I haven't operated on HF--just a few tests that my wife says caused interference to our stereo/tv  (which IS the same setup as previously.)   Thanks, everybody!
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K5LXP
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« Reply #7 on: April 01, 2011, 11:05:36 AM »

It is likely in feed line because when it is not properly terminated with correct load/impedance it will radiate more from feed line in and close to house unlike coax.

What does terminating impedance have to do with radiating feedline?


I would wager that if he switched to 75 ohm coax the problem would go away.

Why would feeding a balanced antenna with coax help?


You have a resonant antenna with a non resonant feed line impedance mismatch that you then use a tuner to make it work.  Could try making antenna non resonant too as that would change line to antenna impedance match too.

Why does it matter if the antenna were resonant, or the feeder tuned, or not?


Mark K5LXP
Albuquerque, NM
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W8JX
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« Reply #8 on: April 01, 2011, 12:28:04 PM »

It is likely in feed line because when it is not properly terminated with correct load/impedance it will radiate more from feed line in and close to house unlike coax.

What does terminating impedance have to do with radiating feedline?

Everything. A open line feed radiates less with load is balance and impedance on line/SWR is correct. Otherwise it can radiate a lot

I would wager that if he switched to 75 ohm coax the problem would go away.

Why would feeding a balanced antenna with coax help?

Because a Dipole is by nature around 75 ohm and with provide a good match to coax and also greatly reduce feed line radiation near house to reduce possible RF saturation.

You have a resonant antenna with a non resonant feed line impedance mismatch that you then use a tuner to make it work.  Could try making antenna non resonant too as that would change line to antenna impedance match too.

Why does it matter if the antenna were resonant, or the feeder tuned, or not?

Again it has to do with impedance/load on open wires to reduce radiation on the wire. As it stands there is a at least a 4 to 1 mismatch between feed line and antenna on 40. 




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K5LXP
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« Reply #9 on: April 01, 2011, 01:18:22 PM »

A open line feed radiates less with load is balance and impedance on line/SWR is correct.

Is this not a balanced load?


Because a Dipole is by nature around 75 ohm and with provide a good match to coax and also greatly reduce feed line radiation near house to reduce possible RF saturation.

So you're sticking to the theory that mismatch causes feedline radiation?  And that feeding a balanced load with an unbalanced line doesn't?


Mark K5LXP
Albuquerque, NM
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W8JX
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« Reply #10 on: April 01, 2011, 01:33:14 PM »

A open line feed radiates less with load is balance and impedance on line/SWR is correct.

Is this not a balanced load?

Yes and no. It does not have correct impedance on it.

Because a Dipole is by nature around 75 ohm and with provide a good match to coax and also greatly reduce feed line radiation near house to reduce possible RF saturation.

So you're sticking to the theory that mismatch causes feedline radiation?  And that feeding a balanced load with an unbalanced line doesn't?


Open wire line radiates least amount of energy when load is balanced and impedance mismatch is low. Coax is more forgiving here.  He has a radiating source right in house that would ne be present with coax feed. He could even try used coax to get well clear of house and then a 4 to 1 balun to match it to 300 ohm line. This to move hotspot outside of house.
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K5LXP
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« Reply #11 on: April 01, 2011, 02:42:13 PM »




>> Is this not a balanced load?
> Yes and no.

It is, or it isn't.  A "40meter (33' per leg) dipole" sounds pretty balanced to me.


It does not have correct impedance on it.

Why does that matter?


Open wire line radiates least amount of energy when load is balanced and impedance mismatch is low.

Since the load is balanced, you're saying then that the mismatch is somehow unbalancing it?


Coax is more forgiving here.

How so?


He has a radiating source right in house that would ne be present with coax feed.

Wouldn't coax radiate too if the currents were unbalanced?


Mark K5LXP
Albuquerque, NM
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K1CJS
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« Reply #12 on: April 02, 2011, 08:16:38 PM »

.....but I haven't operated on HF--just a few tests that my wife says caused interference to our stereo/tv  (which IS the same setup as previously.)   Thanks, everybody!

Then if you were using co-ax before, the problem may be in the ladderline.  Have you checked the dipole since putting it up?  Could be that one of the connections at the antenna to the ladderline has weakened or failed, (try examining it with field glasses) or that the ladderline coming to your shack window is just too close to your TV/stereo system or to the speaker wiring.

Can you try placing the ladderline farther away from the area the TV/stereo is in, attach a balun and feed that with co-ax from your tuner?  It's just a shot in the dark, but that may solve your problem.
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W8JX
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« Reply #13 on: April 04, 2011, 07:01:18 AM »


>> Is this not a balanced load?
> Yes and no.

It is, or it isn't.  A "40meter (33' per leg) dipole" sounds pretty balanced to me.
It is balance for a 75 ohm feed not 450 or 300. If it was a folded dipole it would match feed line much better

It does not have correct impedance on it.

Why does that matter?

I can see you have little understanding here. With open wire feed the impedance of load sets voltage to current ratio on line and affects its loss via radiation unlike coax which just tends to absorb loss from mismatch as heat within cable.

Open wire line radiates least amount of energy when load is balanced and impedance mismatch is low.

Since the load is balanced, you're saying then that the mismatch is somehow unbalancing it?

You would not hook a coax directly to a 300 ohm folded dipole and expect good results nor should you with a 75 ohm antenna on a open wire feed and wonder why there may be radiation in line because of it.

Coax is more forgiving here.

How so?

As stated above it tends to absorb losses from mismatches as heat.


He has a radiating source right in house that would ne be present with coax feed.

Wouldn't coax radiate too if the currents were unbalanced?

Not if shield is grounded properly
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K5LXP
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« Reply #14 on: April 04, 2011, 12:03:01 PM »

I can see you have little understanding here.

Clearly, I don't get it.

Could you please provide some references I could use to learn what you're describing?  Apparently all the antenna and transmission line texts I have are wrong.


Mark K5LXP
Albuquerque, NM
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