Tom Rauch wrote:
>
>
> The mast does reduce the field gradient below and around the mast
> without the need for an "ionized cloud".
i'll agree with this, but more later.
> There are two ways to reduce field gradient. One is by neutralizing
> the charge with ions, the other is by distorting or moving the field.
>
> Spraying ions generates noise, and the result is really just that you
> create a smooth ball made out of an ion cloud...and noise from audio
> to light.... in the process of making the little cloud.
true, but exactly what is the spectrum of this noise. if you know
of a good reference or paper on this i would like to get it.
>
> You can find practical examples all over the world that support what
> I say. One is on high tension lines, where a round very smooth ring
> or ball is used to reduce corona from bolts and other hardware in the
> same general area of the line.
the key words are 'the same general area'. last year i spent many
weeks modeling those corona rings using finite element software.
even at hundreds of thousands of volts the effects of those rings is
measured in inches to a couple feet, not the dozens of feet from a mast
to the tips of elements of decent size yagis. if i get a chance i'll
try to put in a simple model to see how far out from a vertical mast
the field is significantly affected, but i expect it is a fairly short
distance.
another example of the limited range of effects of objects is the
problem of where to put 'static' or 'shield' wires above transmission
lines. even with the shield wires directly above another conductor
there is still a finite probability of lightning striking the lower
wire. it is of course more expensive for utilities to add shield
wires and getting them far enough out to be effective is not easy.
much study of this has been done and is still going on in order to
improve the understanding of how electric fields behave around towers
and wires under charged clouds. and even more is going on about how
lightning is affected by them (but this is not quite on the topic).
>
> Another is in HV systems in TV sets and such. Unavoidable sharp
> points are eliminated as corona sources by having blunt points that
> extend further towards an opposite charged area, like the chassis.
> If you have a sharp point launching a streamer, and add a blunt
> conductor that extends past the sharp one and near it, all discharge
> will stop....and the noise goes away.
agreed here, but this would mean getting the blunt object beyond
the sharp one. as in putting a ring around or over the antenna to
be protected. maybe possible on vhf/uhf but not much use at hf.
the telrex static balls are an attempt to do this, but as i know
very well they don't stop it. this is the same method described
above where the corona ring or ball is placed around the object you
are trying to protect from corona. or on the side of a piece of
hardware away from the voltage source to reduce the field seen by
the object being protected.
>
> The field distorts and bends away from the antenna quite nicely
> without the cloud, and without the electrical noise that could
> certainly be made worse by the addition of an intention noise
> generator.
not necessarily, and i may get a chance to study that a bit this
year. what i don't know is what the noise spectrum would look
like when generated by short sharp spikes vs the spectrum from
a large blunt object that starts going into corona at a higher
voltage. my initial thoughts as to why the porcupine MAY be a
possible source of relief are:
1. the sharp points would have lower energy discharges, the
objects they are exciting are much shorter than a resonant antenna
would be (assuming a porcupine vs an hf yagi, it may be different
at uhf), and they are some distance away from the yagi.
2. allowing the yagi itself to go into corona would probably lead to
higher energy discharges to start with, the pulses generated are
directly exciting a resonant structure, and they are of course much
closer to the antenna.
I am definately not convinced yet, but i am beginning to understand
the idea behind intentional corona sources.
--
David Robbins K1TTT (ex KY1H)
k1ttt@berkshire.net or robbins@berkshire.net
http://www.berkshire.net/~robbins/k1ttt.html
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