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Re: (meteorobs) radio meteor polarisation diversity
At 14:16 09/23/2000 +1000, you wrote:
>Maybe crossed yagi's are the way to go. What would be a good physical
>arrangement for that? I'm currently using a FM 3 element yagi.
Another concern is the gain of the antenna. Compare, for example, what can
be heard with a dipole compared to a small, moderate and large yagi. But as
the array gain increases, the beamwidth, or field of view, decreases, just
as with a telescope.
Even sporadic meteors are swept into the atmosphere favoring certain path
directions. As Shelby said, a bistatic radio meteor system may well show
noticeably higher reflection counts for some paths over others at different
times of the day.
The large radio meteor radar systems are monostatic: the receiver antenna
is the same as the transmitting antenna so there is no sensitivity to path
length (although there may still be some sensitivity to the angle of
incidence between the antenna and reflection source which might vary,
again, by time of the day).
It's not easy to do monostatic work using the usual fixed-location FM/TV
transmitters since the antennas are designed to lay the radiated energy on
the horizon (and sometimes BELOW the horizon, as in the case of an antenna
on a mountaintop illuminating a metropolis valley thousands of feet below).
Best results are likely to be experienced when the system is designed and
optimized for the desired path. The antenna array for long-distance amateur
radio meteor scatter, for example, is looking for reflections from a much
smaller portion of the sky lying as close to the horizon as possible (where
the reflector may be as distant as 750 km), and so the desired array
beamwidths can be much smaller than a bistatic system which searches for
reflections from a large area of sky directly overhead (where the
reflector's height may be only as far distant as 120 km). Thus, antenna
gain can be much higher for a bistatic radio meteor system as for a
monostatic system.
As a result of these facts, there can be a great discrepancy in the number
of radio meteor reflections actually heard by the two although the diurnal
variation is likely to remain much the same.
When attempting to compare the number of reflections reported by FM/TV
radio meteor receivers with amateur radio meteor scatterers operating at
144 MHz, consideration should also be given to the difference in frequency
since meteor reflection strength is frequency-dependent.
All of these "considerations" are discussed in the literature.
SteveH
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