Radio interference

strebo, Apr 9, 10:09pm
I have a small TV set with about 10 metres of coaxial cable to the aerial. I have small powered speakers plugged in to improve the sound quality. These are permanently on. The TV is always on standby.
From time to time I receive garbled voices from the speakers. Could this be from a ham radio operator two houses away whose ham signal is being picked up by my TV speakers acting a a receiver or have I got my own audio ghost?

ians2, Apr 9, 10:28pm
Your speakers will be built as cheeeeply as possible and will have no filtering so it is real easy to blame the nearest source of radio energy as at fault.
Do taxis also produce this interference when parked outside?

kenw1, Apr 9, 10:31pm
Put a few turns of the speaker lead around a ferrite rod at each end of the lead, that will probably get rid of the pick up.

strebo, Apr 10, 12:03am
Thanx for that.

cjdnzl, Apr 10, 12:09am
Realise that if that is the case, it is not the ham's (radio amateur's) fault. The fault is almost always domestic radio/TV/phone equipment being unable through cheap design to reject out-of-band signals, especially where longish leads are part of the installation, as they can act like an aerial and pick up any and all transmissions that happen to be around.

kenw1, Apr 10, 7:36am
You are so right.

Many years when operating in G land. I had persistent complaints from some very anti neighbours, they really did not like them beam and tower. Anyway got the Radio Inspector van round, with spectrum analyser the whole 9 yards.

I was running a very full 400 watts into a full size 2 ele 20M delta loop pointed straight at their TV aerial. The RI cleared me, nice clean transmitter, told them to buy a TV made this century.

cjdnzl, Apr 10, 11:40am
That's the way, in NZ the general approach by the RI was to tell the ham to stop transmitting, even though it wasn't the ham gear at fault, "in the interest of reducing the problem"- in other words he didn't want the aggro from the complainant and would sooner shut the ham up. I had a few problems with a neighbour who turned out to have a galvanised T-joint in the power conduit (old house) which was rectifying and mixing the ham sigs on 80m with the local bc station. I bonded it to the adjacent pipes and cured it, but the RI didn't have a clue about that.

shall, Apr 13, 1:20am
rectifying ?

phil1943, Apr 13, 4:16am
To convert AC to DC.

cjdnzl, Apr 13, 12:07pm
For power, yes. For signals, it is one method of recovering the audio from a radio signal, usually called detection, but it's basically a rectifier circuit using diodes. If you apply two different signals (f1 and f2) to a rectifier ciruit, the action of rectification will produce two more signals, (f1+f2 and f1-f2). In the case I mentioned, the two frequencies were a broadcast station 1YA on 690 kilohertz, and an amateur radio signal on 3.7 megahertz. The rectifying joint then produced two more signals, 3.7 MHz minus 690 KHz, and 3.7 MHz plus 690 KHz. These mixing products were audible on a transistor radio that would tune to one of the product signals and was used as a search receiver, the signal getting louder as we approached the source of the signal. It was unmistakeably coming from the galvanised T-joint, and a thump with a pair of pliers on the joint instantly cured the problem. Bonding the joint to the conduit produced a permanent fix. This may sound like gobbledegook to an untrained person, but them's the facts, as they say.
I am a qualified electronic technician and also have a ham license, though I am seldom on the air these days.

shakespeare6, Feb 16, 10:07pm
Good find, passive Inter-modulation can be cause all sorts of issues that can be hard to detect, I'm a teleco engineer - RF. over the years spent many hours chasing interference from all sources on hill top sites feeders banging on towers, rusting u bolts are a good ones simple loose tower or rack earth link. Lack of filtering on transmitters are another good one get some good Inter-modulation on busy sites all that mixing in the transmitters generates some ugly bi products.

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