TV working without aerial

mgfvvc

A while ago my mother tried to remove the cable from her aerial and managed to pull the plug off the end of the cable. I pushed it back on and it seemed to work for a bit, but reception on some channels was dodgy. I went back with some spare co-ax to replace the damaged cable. When I unplugged it from the aerial some channels that hadn't been working came back to life and seemed to work perfectly with just the 2 metres of co-ax and no aerial on the end.

Can someone with a bit more radio expertise shed any light on why it seemed to work with just the co-ax. I replaced the co-ax and plugged it back into the aerial, but I wonder if I could have left it with just the co-ax?

bob_a_builder

Because you live in very close proximity to a transmitter ?

Norman_Castle

The aerial is just a piece of metal shaped to get the best signal. The metal in your coax cable received the signal in the same way. If you left it the weaker signal would probably have been apparent with other channels or different weather.

George_Michael

Norman_Castle said:

The aerial is just a piece of metal shaped to get the best signal. The metal in your coax cable received the signal in the same way. 

It's still very strange that a reasonable signal was received by just the coaxial cable as it's only the inner conductor that is used for this signal. The outer screen is used to shield the inner from any interference so if the screen was in place and fitted into the connector, no signal (apart from what was received right at the end) should have been able to reach the inner conductor.

Chino

George_Michael said:

It's still very strange that a reasonable signal was received by just the coaxial cable as it's only the inner conductor that is used for this signal. The outer screen is used to shield the inner from any interference so if the screen was in place and fitted into the connector, no signal (apart from what was received right at the end) should have been able to reach the inner conductor.

The last time I made up an aerial cable I was surprised at how little braiding there was compared to what there was in cables years ago, to the extent that I wondered how effective a shield it made. Now I know!

Biggus_Dickus

George_Michael said:

Norman_Castle said:

The aerial is just a piece of metal shaped to get the best signal. The metal in your coax cable received the signal in the same way. 

It's still very strange that a reasonable signal was received by just the coaxial cable as it's only the inner conductor that is used for this signal. The outer screen is used to shield the inner from any interference so if the screen was in place and fitted into the connector, no signal (apart from what was received right at the end) should have been able to reach the inner conductor.

As stated up-thread, ... presumably a strong signal area.

Those electromagnetic TV signals are fiendishly clever;...they bounce around and re-radiate off many surfaces and if there is a way into that shielded copper co-ax core those little devils will find it.

It’s what they do.

molerat

I remember when my aerial was 4ft of coax attached to a wire coat hanger hanging on the picture rail.

MinuteNoodles

George_Michael said:

It's still very strange that a reasonable signal was received by just the coaxial cable as it's only the inner conductor that is used for this signal. The outer screen is used to shield the inner from any interference so if the screen was in place and fitted into the connector, no signal (apart from what was received right at the end) should have been able to reach the inner conductor.

Actually both are used for the wanted signal, the outside surface of the inner conductor and the inside surface of the braid as RF flows over the surface of a conductor, not through it, and RF is an AC current/voltage. When you have a high standing wave ratio, when there's a big enough mismatch between the coax and the antenna or an insufficient RF ground then you can get RF current flowing on the outside surface of the braid, something known as common mode current, and you get that not only when transmitting but when receiving too, for amateur and CB radio it is experienced as unwanted noise on receive. When this happens the outside surface of the coax forms part of the antenna or an antenna if there's not one at the other end of the coax. 

mobileron

Thats why my neighbour claimed my Ham radio was affecting his tv,stange i was on holiday in the USA at the time.

MinuteNoodles

mobileron said:

Thats why my neighbour claimed my Ham radio was affecting his tv,stange i was on holiday in the USA at the time.

I got a visit from the planning officer about my antenna and he said the complainant claimed I was knocking off his internet. Fortunately for me the planning officer knew any issues with interference weren't in their remit and that was OFCOM's job and his dad used to be a ham too so it was rather a pleasant visit and my antenna got to stay put with a caveat I lowered it once in a while to show willing.