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Re: [TowerTalk] Rain Static

To: "John and Mary Powell" <zl1bhq@paradise.net.nz>,<towertalk@contesting.com>
Subject: Re: [TowerTalk] Rain Static
From: "Tom Rauch" <w8ji@contesting.com>
Reply-to: Tom Rauch <w8ji@contesting.com>
Date: Fri, 2 Sep 2005 22:31:50 -0400
List-post: <mailto:towertalk@contesting.com>
> I have been following the thread with interest. I have a
question regarding rain static, At our contest station,
ZM1A, we have a Force 12 C3S on a 60' tower and 100 metres
away a triband, 2 element Quad at 35'. During approaching
and, with rain storms,. the Yagi attracts a high level of
rain static, while the Quad is largely unaffected. Is there
an explanation for this phenomena ?. However, some years ago
I did recall reading about the comparisons of a simple
dipole setup and those of "closed loop" type of antenna, it
being stated that "closed loop" antenna were immune to rain
static. I believe the author was either Lew McCoy (ex W1CP),
or the late Bill Orr W6SAI.>>

John,

While it is true that quads are less susceptable to
"p-static", a quad being a "closed loop" has very little to
do with the static level. After all, a simple dc choke would
make any antenna a "closed loop" as would a folded dipole or
monopole!

We have to go back to what causes the noise. The noise is
caused by corona discharges. The quad has two distinct
advantages in this arena:

1.) The quad generally does not have a sharp point
protruding out into space. It has four blunt sides with only
four 90-degree corner areas on an element to concentrate
charges. If you think of a dipole element like a yagi uses,
it has a rather abrupt end sticking out into space and it
sticks out *further* than the quad does!

As a matter of fact, the quad came into common use because
of corona problems. Radio station HCJB was plaged with
corona problems. This was caused by atmospheric conditions
(pressure and humidity) at HCJB's location and power level
they used. They would melt down dipole elements from corona.
The solution at HCJB was to form a full-wave element into a
square so there were no HV ends sticking out into the air!
While this was transmitting corona, it stands to reason that
weather induced corona would have similar reduction.

2.) The quad generally has fully insulated parasitic
elements that have a very long leakage path back to the boom
and tower through the insulated spreaders. A yagi has
elements that are only marginally insulated from the boom
and tower, if they are insulated at all.

3.) If you do have corona, it is generally a very high
impedance noise source that is coupled directly to the
element. In a quad the corona would almost never be at a
point where the element hase very high impedance. So if you
would have corona in a quad, the noise source is mismatched
to the "load". In a yagi (or log), the corna is almost
certainly at element tips. That's a very high impedance
point, so the high impedance of the source (the very high
voltage and very little current corna) matches quite nicely
to the load (the element).

It's pretty easy to see we not only have higher voltage
gradients from the yagi being  "pointed" and with element
tips extended a larger distance from the boom, we also have
more elements potentially contributing noise and better
impedance matching from noise source (the corna) to the
receiving system.

This is actually where the old wive's tale that closed loops
are "quieter" than other antennas comes from. In the case of
corna discharges, the quad is better. The advantage is the
physical shape (blunt and retracted element) and
construction (insulation  to parasitic elements), not the
fact it is a  "closed loop".

This all brings up another good point.....

If the noise is from charged drops hitting the element, the
quad would probably be no better at all. It has twice the
area for moisture to hit. If the noise is from corna, the
quad would have an obvious advantage.

The more an antenna looks like a porcupine with sharp quills
extended a long distance into space, the more problematic
corona is for a given earth-to-cloud potential difference.

See how nicely corona fits all the observations people have
made? Fits much better than OJ's bloody glove.

73 Tom

_______________________________________________

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