Satgen 159  Interference to Amateur Satellites.      by GM4IHJ 12 April 92

There have recently been suggestions that the UK Government order
prohibiting all but very weak amateur transmissions on the lower end of
the 70 cm band in North East England, is connected with the repeated
failure to obtain stable uplink command and control of Amateur Radio
Satellite RM-1 . The RM-1 uplink is on 70cms and it appears that false
uplink signals are misdirecting its onboard computer.
The postulated common link in this chain of events is the arrival on
Fylingdale Moor in North Yorks of " Pave Paws ", the replacement for the
old "4 minute warning, BMEWS Ballistic Missile Early warning radar" which
previously occupied the site. BMEWS used relatively straight forward high
power 70 cm txs feeding mechanically rotating antennas. By contrast Pave
Paws is an electronically scanned system sweeping two 120 degree arcs. But
as Scientific American for Feb 85 shows in its discussion paper on the
developement of Pave Paws , the "Within the pulse scanning" of its
transmission pulses is not greatly different in output effect from BMEWS,
in that a very wideband pulse results from frequency sweeping inside each
transmitter pulse in order to achieve good range discrimination between
adjacent targets with the frequency sweep, whilst maximising the pulse
length for optimum echo power.
Stations hundreds of miles from Pave Paws have reported S6 to S9 signal
spread over 300 to 500 Khz or more on each of a triplet of staggered
channels. So it is possible that any satellite coming close is going to
get its paint work scorched.
But before rushing off to condemn Pave Paws I suggest , we consider the
following. Pave Paws has been in use at 4 different land sites for the
last 7 years, and it also operates from mobile installations on some big
converted cargo ships. Equally noteworthy Pave Paws is not alone . The
Russian version , usually referred to as " Pechora" from the site in the
Soviet Arctic where it first appeared, is also a possible offender.
There is however another potential offender which worries IHJ a bit more.
This is the constellation of " Super Over the Horizon UHF Terrestrial TV
stations" which has begun to appear as the advent of very advanced tetrode
and Klystron techniques allows US TV stations to improve their static
coverage by using megawatts to antennas up to 1000m high. Theoretically,
these giant Radio cookers should have a near horizontal polar diagram. But
I suspect that any amateur satellite regularly overflying Georgia USA ( as
RM-1 ) does could have recently begun to experience a lot of RF. Given the
shape and power of the sync pulse of these video monsters, all sorts of
non linear mixing products may be entering our satellites front ends.
As a complete side issue , I also note that these UHF TV monsters might
just perhaps get their signals direct into UK at times ? 73 de GM4IHJ @
GB7SAN