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Old 17th Jun 2009, 6:56 pm   #1
richrussell
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Default Mains lead as antenna

On the Invicta AW57 I've just finished, it has no loop or ferrite rod internal aerial. I guess this isn't uncommon for 1930's superhets (despite the case being huge enough to hold a nice bit loop). However it does have a wander plug on the back which can be plugged into the aerial socket - which is connected to the mains live lead through a capacitor.

I was a bit sceptical as to how well it'd work given the amount of noise on modern mains circuits, but am very impressed. OK, it's next to useless on LW and the longer end of MW, but on the shortwave bands it's fantastic. Pulled in stations very strongly and clearly, which is not something the loops and rods in my 1950s sets can manage. No trace of hum, though a lot of switched mode rubbish right across MW, and crackles as the fridge switched in and out.

What I was wondering is how common this design was? If this set is anything to go by, I guess that as MW and LW became more popular it's performance isn't good enough so loops were used instead, and later on ferrite rods taking up less space taking over later still. Wouldn't the performance be very variable depending on the circuit it's plugged in to? I would guess that my house is relatively clean - we're an isolated house with a transformer on a pole 100m away. And we're only 2 miles from Drax power station. But in the middle of a city in a block of flats would it work as well?

Pre-war electronic design is a bit of a mystery to me, but I'm very interested to learn more.
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Old 17th Jun 2009, 7:32 pm   #2
Peter.N.
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Default Re: Mains lead as antenna

I don't recall seeing this on a radio, but the 'Baird Townsman' TV used the mains lead as an aerial and it worked very well in reasonable signal strength areas.
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Old 17th Jun 2009, 7:43 pm   #3
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Default Re: Mains lead as antenna

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Originally Posted by richrussell View Post
However it does have a wander plug on the back which can be plugged into the aerial socket - which is connected to the mains live lead through a capacitor.
A lot of pre war Marconi/H.M.V radios used exactly the same setup. I've never been brave enough to try it though as I trust the old (mica?) capacitor on AC mains

If you imagine how few electrical items were connected to the mains at the time and the lack of earths, they probably worked very well in their day

David
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Old 17th Jun 2009, 8:42 pm   #4
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Default Re: Mains lead as antenna

hi richard , i have a 1940s invicta , i certainly wouldent risk trying to connect the aerial to the mains , i assume you mean the 'neutral 'side and not phase , i doubt if with the modern regulations that it would be coucher , ive seen mains wireing in older houses where neutral was used as an earth return , but this is frowned apon by the authorities, and rightly so , best regards matt
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Old 18th Jun 2009, 8:42 am   #5
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Default Re: Mains lead as antenna

It was I believe very common to use the mains as an aerial, or at least to offer the facility via a plug on the back of the chassis which could be plugged into the aerial socket. Even constructional articles advised the use of a "good quality" condenser to allow the use of an aerial connection to the mains. Now I guess we'd use a class X component.

I have occasionally tried them for comparison with a short indoor aerial, but on long wave especially, there's too much interference from all the modern electronic stuff they didn't have pre-war.

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Old 18th Jun 2009, 9:03 am   #6
richrussell
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Default Re: Mains lead as antenna

I did replace the tired looking capacitor with an X2 rated one, as when not in use as an aerial, the plug goes into a socket that is connected to chassis (ie mains neutral), so in effect it's just a common mains filter.

Matt - no, it's the phase side, between the power switch and transformer. The neutral side is earthed at the meter, which in my case is only about 5m away from the set.

It does work very well, and I'm happy that it's electrically safe. Was just curious as to how common it was. If only it was better on LW so I could listen to Radio 4...
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Old 18th Jun 2009, 9:54 am   #7
John M0GLN
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Default Re: Mains lead as antenna

Attached is the relevant bit of the circuit for a 1936 Marconiphone 556 showing the lead with wander plug for the the mains aerial, it comes from the primary of the mains transformer via C26 a 0.00035 cap', which depending which way the set was plugged in, may or may not be the live side, but as the mains switch is only single pole, even if the lead was from the neutral, when the set was left plugged in and switched off, the wander plug would rise to the live potential through the transformer.

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Old 18th Jun 2009, 11:12 am   #8
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Default Re: Mains lead as antenna

John - that's very similar to this 1936 AW57. Capacitor is a 0.001uF, but otherwise the same setup. I have got into the habit of unplugging any of my older electrical items when not in use, particularly important when there's only a single pole switch like this one has.
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Old 18th Jun 2009, 12:33 pm   #9
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Default Re: Mains lead as antenna

We have some posts in the forum for aerial eliminators (capacitors connected to the live mains). I also remember reading in another post that a restorer had found that the primary winding of his aerial coil had become open circuit due to a break caused by what looked like an electrical short but he was at a loss to see how high voltage had been applied to the coil. It might have been due to a faulty eliminator?
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Old 18th Jun 2009, 2:11 pm   #10
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Default Re: Mains lead as antenna

You want a class Y capacitor in any situation where capacitor failure might expose a danger of electric shock -- i.e., between mains and anything touchable (live-to-earth, neutral-to-earth, in series with aerial input on non-isolated sets, &c.) Class Y capacitors are designed never, ever to fail low-impedance. Class X is meant for wiring directly between live and neutral -- it just means that the capacitor shouldn't explode or catch on fire under fault conditions. If there is a metal-film resistor in series with the capacitor, then it doesn't even need to be class X: if the capacitor fails short-circuit, the resistor will fail open-circuit shortly afterwards.
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Old 18th Jun 2009, 2:16 pm   #11
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Default Re: Mains lead as antenna

.... only if the value is low enough.
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Old 18th Jun 2009, 2:45 pm   #12
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Default Re: Mains lead as antenna

I might replace it with a Class Y then- though I don't have any of the right value. Maybe I'll try it what I've got and see if it affects the reception.

The chassis is connected to the secondary side of the mains transformer, the only thing on the primary side is this aerial link capacitor and the switch.

I guess that if the capacitor were to fail short circuit, you'd end up with 230Vac on the tuning coil of whatever waveband was selected, the other end of which is connected to chassis. I suspect that would rapidly burn out the fine wire in the inductor!
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Old 18th Jun 2009, 3:35 pm   #13
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Default Re: Mains lead as antenna

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Originally Posted by richrussell View Post
I might replace it with a Class Y then
Or do as I do, leave the original cap and wander plug (for authenticity) but remove the link between the cap and the mains.

.... use a few yards of wire as a 'conventional' aerial, it's ultimately much safer and works better

David
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Old 18th Jun 2009, 4:06 pm   #14
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Default Re: Mains lead as antenna

I've seen these and although it may not be original, I wouldn't do this. A cap between Mains and the aerial circuitry. Scary! Imagine if it went short circuit....

Also, any noise from the mains would go straight in at the aerial circuit. Switch Mode power supplies, for example. Imagine the interference.

Cheers,

Steve P.
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Old 18th Jun 2009, 4:25 pm   #15
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Default Re: Mains lead as antenna

Steve - the noise issue is why I was sceptical. But it actually works superbly well on shortwave. Not so good on MW and LW though.

I think I'll keep it there until I get round to sorting out some sort of outside aerial from that room. Hopefully a suitable X2 (or Y) rated capacitor will provide reasonable protection from damage due to a fault until then.
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Old 18th Jun 2009, 6:28 pm   #16
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Default Re: Mains lead as antenna

A Marconiphone 264 I did a few years ago had provision for a mains aerial,(again isolated by a bakelite encased mica cap) but I bottled out and left it disconnected. It was going to live in a very non technical house.
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Old 19th Jun 2009, 12:56 pm   #17
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Default Re: Mains lead as antenna

Is it possible to inductively couple the aerial to the mains live? I read that aerials were inductivley coupled to telephone lines by sitting the phone on a metal tray which had the aerial connected to it.
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