> From: Peter Chadwick <Peter.Chadwick@gpsemi.com>
Hi Peter,
> What is the predominant polarisation of static?
With an antenna near ground, vertical.....but then so are the
desired signals unless they are from a high angle. With an antenna a
large fraction of a wavelength above ground, anything goes.
>Is it different between
> close in static and thunderstorm hundreds/thousands of miles away?
If a signal comes from straight above, there is no vertical to it.
Think about that a bit, and you will see it is true since the
electric field of the wave has to be transverse to the signal path.
The polarization of something coming straight down can be east-west
or some other compass headings but not vertical! So your horizontal
antenna (running east-west or some other direction) would be much
better at responding to it.
If the thunderstorm noise arrives from any direction except straight
up, it can be vertical, horizontal, or something in between any of
this. It's really quite random.
> Is man made noise mainly vertically polarised at 160? If so, what is
> 'mainly'? 50%? 90%?
The noise is RANDOM polarity, but what we observe is affected by our
low antenna heights on 160. HORIZONTAL noise at low angles is
attenuated rapidly by the earth. All we are left with is vertical
polarized trash, once the wave propagates any distance at all along
the earth. Our perception is the noise sources are mostly vertical,
because that's what propagates best along the earth.
> What is the best horizontally polarised receive antenna with good noise
> rejection for local electrical noise? And the best for a small (<100
> foot) plot?
Low horizontal antennas generally have low signal voltages (low
sensitivity) and require great attention to unwanted feedline pick-up
and system balance. The low sensitivity gives us an illusion they are
quieter, but the low angle DX signals are attenuated as much as the
noise. If the wave angle is moderate to high, they can work well
however.
In my comparisons I found vertically polarized antennas to be
better nearly all the time, but not every time, for DX. It varies
from day to day.
> In QST about 2 or 3 years ago, there was a low noise horizontal loop
> antenna, which was, as I remember it, a small Alford loop. I got no joy
> with it, but as Carl, KM1H, said the other day
I think that is a good design, but it would be very sensitive to
common mode signal pick-up, mainly because it would be one of these
"dead-head" antennas if the soil is any good at all. Again consider
the earth is trying to short circuit the electric field (and Faraday
tells us we can NOT have a time varying magnetic field without a
time varying electric field) and that antenna responds with the
electric field parallel to the earth. The earth and the antenna fight
each other.
It might be better to have two, with one vertical.
> "IMO we all need more info on antennas in real world situations."
> And as locations are different, it doesn't always follow that what works
> in one location will work as well in another.
That's true. My phased short verticals worked gang busters in Ohio,
but are useless over Georgia clay (the feedline is too tough to
decouple down here). Same here for EWE's, they are useless without
a large ground screen..and that is too much work.
73, Tom W8JI
-
---
FAQ on WWW: http://www.contesting.com/topband.html
Submissions: topband@contesting.com
Administrative requests: topband-REQUEST@contesting.com
Problems: owner-topband@contesting.com
|