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Re: (meteorobs) Terminating/Jim
Hello Philip,
I found your answer to my post to be a bit puzzling, so I apologize if I
was not very clear in what I was trying to demonstrate with the equations.
The original premise to which I was responding were these statements from
your October 24, 1999 post:
"The location of the meteor (relative to me) is irrelevant, so it is as
likely to be overhead as at 45 deg or at the horizon. (This is another
purpose of The Waubaushene
Project--to determine if elevation angle is indeed irrelevant.)"
and in particular:
"I know what you are saying about pointing the antenna at 45 deg or at the
horizon, but the antenna elevation angle is irrelevant for sporadic meteors
since by their nature their direction is unpredictable. In fact, it is
irrelevant for showers as well. The main criterion for maximizing signals
is antenna azimuth. The reason for pointing the antenna at the horizon is
to maximize distance between the transmit and receive sites. It does not
maximize signal amplitude, signal duration or number of bursts."
These statements appear to be quite general in nature, supposedly
applicable to all forward-scatter links and not just your own setup. going
directly to the canonical reference source for forward-scatter theory
(which has also been tested empirically) I sought to show that the above
hypothesis is not correct. However, i did admittedly stray away from my
primary point in that while I had the equations up on the chalk board, I
launched into a discussion about several of the factors which, while
interesting, were not directly applicable to the point at hand.
The primary points are these: (1) the elevation angle at which the beam of
the receiving antenna is pointed is *very* relevant, especially for
receiving antennas having a high degree of directivity, and will have a
direct effect on the number of echoes you receive, echo power levels, and
signal durations; (2) the primary reason for this sensitivity to elevation
angle is the importance of the angle of reflection from the meteor trail
(phi) on received power level and especially on received echo duration.
For example, a "typical" end-point reflection from a meteor located
"nearby" over the receiver will have a reflection angle of about phi = 45
deg, such that sec^2(phi) = 2.0. If the transmitter-receiver link
distance is 600 km, then a typical "hot spot" mid-point reflection would
have a phi of 75 deg, and sec^2(phi) = 14.9, meaning that echoes from this
area will have durations lasting, on average, 7 times longer than those
from over the end-point. For a link distance of 1000 km (near the limit
for end-point reflections), mid-point reflections have a phi of 82 deg, and
a sec^2(phi) of 51.6, meaning that the mid-point reflections will have
durations that are about 26 times longer than those echoes occurring over
the end-point. Where you point your receiving antenna does matter,
because it can make the difference between receiving a 3 second echo verses
a 0.11 sec echo. It will also strongly affect your echo numbers since
those short duration echoes from faint meteors that you might have detected
by utilizing a hot spot region will become undetectable if an end-point
region is utilized.
I think it is also somewhat of a misnomer to claim that link optimization
is only of concern to the "big boys," or as you put it: "military
installations with huge antennas with super small beam widths, universities
with radar facilities, and commercial enterprises where
optimizing the circuit affects how much the shareholders earn." Almost all
radiometeor enthusiasts are concerned with eking as much performance out of
a particular link as possible, even when commercial transmitters are used
as the signal source. It is true that multiple high power (100 kW)
transmitters do make it possible to get away with less gain and beam
directivity than would be required for a point to point meteor burst
communications setup having only a 2000 Watt master station. However, a
noticeable improvement in system performance (higher echo counts and
fainter magnitude meteors) can be achieved if the used transmitters are
limited to the best cluster of hot-spot regions within a 20-30 deg beam
width having link distances of about 600 - 1400 km. The Poplar Springs
station is an example in point. My first meteor echoes utilizing the low
VHF television band were obtained on a very cheap, log-periodic standard TV
antenna on a chimney mounted mast. This was upgraded, with phenomenal
improvement, to a home-built 5-element Yagi which was eventually placed on
a short 35 ft tower. This setup, along with some additional home-built
equipment inside the house, was good enough to obtain scientifically useful
and publishable data for statistical meteor studies.
However, not everyone is looking to seek after such results or put up
towers in their back-yards. For many, just the simple joy of hearing a few
nice meteor "pings" is the only goal, and this can be achieved with a lot
less effort. R.B. Minton, in New Mexico, makes relatively short-range
radiometeor observations using a dipole antenna, cut for the commercial FM
band, and a used automobile receiver. He has even recorded on video
several visual Perseid meteors which simultaneously caused forward-scatter
bursts from stations to his north in Colorado. In your post, you posed the
following rhetorical question:
"If you lived on a small, city lot where you couldn't put up a
horizontally-mounted antenna without overhanging the neighbor's property or
if you lived in a location where you were surrounded by tall buildings,
what would you do? Would you sit on your ass and whine about how unfair
life is because you can't optimize your antenna for an imaginary
propagation angle in some equation? Or would you put up an antenna pointed
at your zenith and make the best of the situation?"
I consider this statement to be somewhat of a red herring to the topic that
I was originally addressing, and won't bother to type in the very obvious
answer. But it does bring to mind the very excellent example of Michael
Boschat in Halifax, Nova Scotia. Michael worked for months to set up a
system which would let him monitor meteors by radio from his downtown
apartment. In the end, and after a lot of frustrating attempts, he finally
wound up with a workable setup which included a dipole antenna strung up
across the interior of the apartment and using a monster cavity filter to
block out most sources of interference. With tenacity, it can be done, and
I always smile at his success. Is this an optimized, directional system?
No. But to Mike it doesn't matter, which is the point you eventually got
around to making. On *that* particular premise, i can at least agree.
Best regards,
Jim
James Richardson
Tallahassee, Florida
richardson@digitalexp.com
Operations Manager / Radiometeor Project Coordinator
American Meteor Society (AMS)
http://www.amsmeteors.org
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