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Old 6th Mar 2018, 2:57 pm   #1
ukcol
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Default Polarity of static build-up on an aerial.

This may or may not be a silly question. What is the polarity of static build-up on an aerial?

Reading about lightning it seems that although most lightning is negative with respect to earth, a minority of lightning is positive. The top of a thunder cloud is positive and high winds can tilt a cloud enough to cause a positive discharge to earth.

So perhaps there can be a build-up of either polarity of charge on an aerial. Thinking about it, the most common polarity of charge on an aerial may well be positive because any charge on a cloud is negative at the bottom. An aerial below the cloud would become positive as electrons move away from the negatively charged cloud. (See note below).

I realize that the polarity of any charge on an aerial is of no interest in practice. If there is no DC path from the aerial to earth, a high value resistor can be added to bleed any charge away. All this is only of academic interest of course but I would be interested to hear the thoughts of others.

Note: Is this the mechanism of static build-up on an aerial?
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Old 6th Mar 2018, 3:52 pm   #2
G8HQP Dave
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Default Re: Polarity of static build-up on an aerial.

I seem to recall reading somewhere that the normal potential gradient in the atmosphere is the opposite to what you get during a 'normal' thunderstorm. Presumably the mechanism for gradual build-up of voltage on an antenna is the very small but non-zero conductance of air. Then there is rain static. Finally thunderstorms.
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Old 6th Mar 2018, 3:53 pm   #3
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Default Re: Polarity of static build-up on an aerial.

I don’t have an answer to your question but may I suggest you take a look at the lectures given by Walter Lewin at MIT, you can see them on tube.
They are not specifically on Your topic but they are truly inspirational to watch and I have spent many hours absorbed by his wonderful lectures.
There are many topics covered, try and find the one we’re he uses balloons etc to demonstrate static charge and their polarity etc.
Cheers
John
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Old 6th Mar 2018, 6:11 pm   #4
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Default Re: Polarity of static build-up on an aerial.

Connect a neon across the antenna and earth terms. Then note which electrode glows. This will then indicate polarity, in theory at least.
Alan.
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Old 6th Mar 2018, 6:17 pm   #5
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Default Re: Polarity of static build-up on an aerial.

I wonder if it depends on how the mechanism by which the static is delivered to the antenna? My experience of antenna-static has been from static-charged raindrops but I also know of what I'd describe as "frictional static" due to very dry winds blowing across the antenna (this being in a semi-desert environment).
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Old 6th Mar 2018, 6:28 pm   #6
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Default Re: Polarity of static build-up on an aerial.

Quote:
Originally Posted by ukcol View Post
Reading about lightning it seems that although most lightning is negative with respect to earth, a minority of lightning is positive.
Interesting question. Before the digital switchover on TV, my parents' TV was once taken out by a huge current to earth from the aerial in the loft. There was no direct strike and so the aerial was only capacitatively connected to the charge contained in a nearby storm-cloud.

The TV exploded into flames from inside as a nearby lightning strike took place and on inspection, entire sections of the circuits in the chassis were instantaneously destroyed.

I had previously heard that a negatively charged earth would discharge to a relatively positive charge in a storm cloud, so that lightning 'goes up', contrary to the impression given to us during lightning strikes.
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Old 6th Mar 2018, 6:32 pm   #7
Biggles
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Default Re: Polarity of static build-up on an aerial.

Rain static was a bit of a problem on occasion with low band AM radio networks. The control room received an annoying clatter in their headphones till the shower passed. All before the days of CTCSS on main receivers of course. Not sure how sensible it is to do this but I can't resist connecting a neon across my long wire if there is thunder about. I normally isolate all my antennas from equipment when not in use. It's fascinating to see the lamp glowing on and off. Maybe I shouldn't do it.
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Old 7th Mar 2018, 10:01 am   #8
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Default Re: Polarity of static build-up on an aerial.

It seems there is a lot I don't know about the mechanisms of static build-up on an aerial. The potential gradient in the atmosphere, rain static and friction static are things I need to know more about before I can understand aerial static.

John, thanks for the pointer to the Walter Lewin lectures. Is he in the same league as Richard Feynman? if so I shall enjoy them.

On the point about lightning raised by astral highway another question occurs to me. Does the direction of lightning always follow the electrons? Or put another way, does lightning always travel from negative towards positive?
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Old 7th Mar 2018, 11:28 am   #9
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Default Re: Polarity of static build-up on an aerial.

My, quite possibly faulty, understanding is that an ionised 'leader' is established fom sky to ground, which then acts as a pathway for the main discharge from ground to sky.
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Old 7th Mar 2018, 1:00 pm   #10
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Default Re: Polarity of static build-up on an aerial.

Lightning can run in either direction.

The most common (as dseymo1 indicated) starts as a leader from sky to ground.

The other type starts from tall metal structures (amongst other things) and goes up.

This site has some useful info : http://stormhighway.com/does_lightni...r_downward.php

Google ' thunderstorm strike direction ' for more sites and info.
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Old 7th Mar 2018, 1:57 pm   #11
G8HQP Dave
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Default Re: Polarity of static build-up on an aerial.

I think the lightning always consists of electrons flowing, but this can happen in either direction. Ions would take too long to get going; a big potential would be discharged by electrons while the ions were still getting their boots on. However, ions may help establish the path which the electrons then take.
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Old 9th Mar 2018, 7:58 pm   #12
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Default Re: Polarity of static build-up on an aerial.

Hi Ukol, yes Richard Feynman a great scientist and lecturer, he also went to MIT. I didn’t view as many lectures as I did for Walter Lewin, probably because the recording were not so readily available in the early days as Walter Lewin.
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