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Old 6th Sep 2008, 10:28 am   #1
humphrey
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Default Aerial capacitor

On my Defiant M900 the aerial capacitor is 200pf. Is a modern replacement resembling a lentil good enough or should I be looking for something more rugged as I live in an area where lightening strikes are very common.

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Old 6th Sep 2008, 10:48 am   #2
Hermit6345
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Default Re: Aerial capacitor

I think I would be inclined to use a good quality silver mica cap, commonly 500volt working, in that position.

Ian
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Old 6th Sep 2008, 2:21 pm   #3
GrahamN
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Default Re: Aerial capacitor

If the aerial is struck by lightning, would any capacitor actually make much of a difference? I've never actually seen a TV or radio hit directly, but would think that the power there would destroy any capacitor (and quite a lot else!)
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Old 6th Sep 2008, 3:43 pm   #4
OErjan_S
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Default Re: Aerial capacitor

as lightning can bridge few hundred meters of air easily a few mm of plastic is neither here nor there. it is more a way to prevent static to get in (generated from wind...)
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Old 6th Sep 2008, 3:45 pm   #5
Brian R Pateman
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Default Re: Aerial capacitor

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Originally Posted by GrahamN View Post
If the aerial is struck by lightning, would any capacitor actually make much of a difference?
The lightning pulse in a direct hit releases a huge amount of energy very quickly indeed.

Direct strikes on radio aerials are not common though. I've never seen a radio after the aerial has been struck but I have seen several TVs completely destroyed that way - insides completely vapourised and much damage to house wiring.

Of more concern is a nearby ground strike which will produce an electric field for a considerable distance. In that event a suitably rated capacitor of good quality and suitable rating might provide some protection.

The most common form of lightning protection was to use a spark gap on the aerial lead-in with a connection to a good earth. I don't know how effective they were though.

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Old 6th Sep 2008, 3:54 pm   #6
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Default Re: Aerial capacitor

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Originally Posted by Brian R Pateman View Post
The most common form of lightning protection was to use a spark gap on the aerial lead-in with a connection to a good earth. I don't know how effective they were though.

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I remember as an inquisitive child being allowed to remove, from a house in Whitehaven, a little knife-switch on a porcelain base mounted next to the paxolin-tubed aerial lead-in. It had a red and green legend mounted in a celluloid-covered brass double-sided disc: 'Aerial' and 'Earth'. The idea being to throw the blade over to earth the aerial when lightning was around. Since then, I've seen some knife-switches fitted with zig-zag spark gaps, rather like those on a telephone lightning arrestor.

It may even be better (on the HF bands anyway) to use a 'static leak' inductor mounted permanently between aerial and earth, as used on some HF transmitters.
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Old 6th Sep 2008, 4:47 pm   #7
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Default Re: Aerial capacitor

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Originally Posted by Brian R Pateman View Post

The most common form of lightning protection was to use a spark gap on the aerial lead-in with a connection to a good earth. I don't know how effective they were though.
Another use for a neon lamp?
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Old 6th Sep 2008, 5:13 pm   #8
Brian R Pateman
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Default Re: Aerial capacitor

Yes, I've seen neons used in these applications.

The loop aerial I'm building at the moment has one across the input to the loop amplifier.

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