Satgen288  Satellite Problems No1 Diffraction   by GM4IHJ       1st Oct 94
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Modern living tends to encourage people to buy the latest gadget, and
(hopefully) , simply plug and play. In practice however it is often much
more difficult, or , worse still, sends adults crazy , whilst being childs
play to their offspring. In amateur satellite work, a number of the
"simple" situation that face the would be operator , are in fact, far from
clear and rarely simple. So perhaps a few bulletins on "what the books
don't tell you", may be in order.

One classic situation often encountered by users of mode A satellites , is
DIFFRACTION. This can be at its worst with RS10, whereby stations sited in
hilly or mountainous country get very curious results. They hear the 10m
downlink quite well on say almost all of six or seven orbit passes each
day. But , their 2m uplink totally fails to get up to the satellite when
all the interesting far away DX is being plainly heard on the downlink.
Worse still, to add insult to injury. The 2m uplink suddenly comes good,
as the satellite flashes overhead for a couple of orbits relaying only the
local stations, and, difficult to tune because of high doppler rates.

GM4IHJ lives in a village called SALINE. Which does not, as most folks
suppose, mean that it is very salty ( the Spanish with their word SALINAS
have never penetrated here). The name is in fact Gaelic and means "the
village under the hill". An old volcano, which last erupted 250 million
years ago, leaving a huge pile of very hard basalt. As a consequence of
which the station horizon from north east to south reaches an elevation of
9 degrees.

The problem this causes on the downlink at 10m, is minor. Because the
knife edge diffraction of the satellite signal , down from the hiltop to
the nearby station, is good at this low frequency and short range. But at
2m the story is quite different. Firstly the higher frequency diffracts
less well from the top of the old volcano less than 1 km east of the
station , and, the long range thereafter, to the satellite from the point
of diffraction, means that the satellite gets little or no signal input.

Results on RS12 at 15m are much better. But Fuji AO20 2m up and 70cm down
simply does not get over the hill in either direction with any great
strength . However the Fuji user is not fooled into thinking his uplink is
defective . Unlike the RS10 user he does not get a good downlink.

So please take care. If you live in a mountainous area, you can plot the
smaller footprints at the higher elevations demanded by your location, if
you modify the footprint circles on your software. Just don't expect to
get coverage over the full footprint which is all most software shows you.

73 de John GM4IHJ @ GB7SAN.#78.GBR.EU