the dawn of amateur radio in the u[1].k.(收音机雏形的诞生)-第25节
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transmitter hall by dropping one of the antenna towers onto it； but the
equipment was not damaged。
They were more successful at the Royal Navy transmitting site at
Votanikos。 Here they tried to destroy six 300 foot tubular masts。 One
remained standing and also the lower part of another。 All the test gear in
the lab was thrown out of a second floor window and burnt。 I was acting
as official photographer for my unit at the time。 When I walked into a
small store room I saw all the equipment had been thrown off the shelves
on to the floor； but appeared to be intact。 I spotted a box of brand new
packed German navy morse keys and decided the time had come for me to
acquire a small war trophy of my own。 As I bent down to pick up a key； I
was horrified to see two large sticks of gelignite perched perilously on the
edge of a shelf。 The explosive was tied with white ribbon； with a weight
attached to the other end。 I froze to the spot。 Gingerly I lifted my trophy
out of the box and began to walk slowly backwards； being very careful not
to knock anything over。 I breathed a sigh of relief when I was out of the
room and immediately alerted the engineers who came and defused the
booby trap。 So this book might never have been written thanks to the
German army。
At the Athens broadcasting station transmitter site at Liosia my unit
erected a small temporary 'T' antenna which allowed the station to come
on the air again； but a short time later； when the ELAS guerrillas overran
the area they began using the transmitter to broadcast their own view of
events。 We provided the broadcasting authority with a BC 610 mobile
transmitter installed next to the Parliament building in the centre of town；
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using the same frequency of 610 KHz。 Listeners in Cairo couldn't
understand what was going on when one moment they heard an official
government announcement and a little later a war communique issued by
the Communist guerrillas。
8。 Over…the…horizon or Ionospheric HF Radar … OTHR
As mentioned briefly in Chapter 1； it was in April 1976 that the then
Soviet Union first unleashed a diabolical noise on the HF bands which
caused widespread interference to all broadcasting and telecommunication
services between 6 and 20 MHz。 On the first day the 〃knock…knock…
knock〃 went on continuously for over ten hours。 Radio amateurs； who
were among the services that suffered from the interference； soon came to
call this noise 〃the woodpecker〃。 By rotating their beams when tuned to
the 14 MHz band they established that the transmissions appeared to
originate from the vicinity of the town of Gomel in the U。S。S。R。
The governments of many countries world…wide immediately
protested to Moscow； and all they got in reply was a brief statement that
the U。S。S。R。 was carrying out 〃an experiment〃。
The reason for the very strong on/off pulses was probably because； at
first； the Russians were using existing radar antennas which permit the
transmitting and receiving functions to share the same antenna。 Modern
OTHR installations have different transmitting and receiving sites； often
located many miles apart。
From the early 1950s pulsed oblique ionosphere sounders had shown
that the normal ionosphere is much more stable than had previously been
thought to be。 The physical reason for this is that the incredibly tenuous
ionized gas which does the reflecting has a molasses…like viscosity。 Of
course； there are daily and seasonal changes； but over limited periods of
half an hour or so； the F layer at a given location is actually quite well…
behaved。 It bounces back signals in a nearly constant direction and with
nearly constant amplitude just what is required for good radar
performance。
Over…the…horizon HF radars use the ionosphere as a kind of mirror to
〃see〃 around the curvature of the earth。 They have a variety of uses； both
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military and civilian。 And they have the advantage over line…of…sight
microwave radars of being able to cover enormous areas with much less
power and at a fraction of the cost of the latter。
A 〃relocatable〃 OTHR system can track aircraft targets right down to
ground level。 In an early experiment operators were puzzled by the sudden
disappearance from their screen of an aircraft they had been tracking as it
taxied along the ground。 They found out later that the reason for the
disappearance was that the aircraft had gone into a metal hangar which did
not show on the screen because it was not in motion； as explained below。
In 1979 the United States Air Force began experimenting with an
OTHR system at a site near Bangor； Maine。 Because HF frequencies were
being used the power was kept very low to minimize interference to other
services during the early tests。 At the time of writing （1989） it is believed
that a full…power relocatable OTHR system situated in Virginia is being
used in the anti…drug war。
As can be seen from the map this ROTHR can cover a vast area of
1。6 million nautical miles； straddling the whole Caribbean。 The scan area
stretches from the coast of Colombia in South America up through
Nicaragua and Honduras to Florida （on its west boundary） and then
southwards through Puerto Rico； to Trinidad & Tobago and the northern
coast of Venezuela。
But this vast area is not covered continuously； the system operator
can provide surveillance in a number of sectors known as DIRs （dwell
information regions）。 Each one of the 176 DIRs can be 〃illuminated〃 for
only a few seconds at a time。 Small aircraft and small vessels can be
detected by an ingenious method； only when they move。 This is how it is
done：
At the receiving site of the ROTHR system a very large antenna
stretches out over a distance of 8；400 feet。 It consists of 372 dual…
monopole vertical elements each 19 feet high； backed by a huge reflector
screen which makes the antenna substantially unidirectional。 Each pair of
vertical elements has its own receiver which digitizes the incoming signals。
All the digitized signals are then fed through a fibre…optic link to a master
signal processor。 The main receiver can be programmed to pass on
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〃returns〃 from one particular region while eliminating most of the other
returns as unwanted noise or clutter。 But because the wanted target is
moving； while the clutter is not； a filtering system based on the Doppler
Shift principle （even when the echo is only one or two Hertz different）
will lock on to it and track it as long as it stays in motion。
Futhermore； the ROTHR system has its own built…in automatic
management & assessment function and does not have to depend on
external sounding data。 It measures the ionosphere height continuously
and instantly selects the most appropriate frequency to use to scan the
target area； ideally in one hop。
This automatic function uses a quasi…vertical incidence sounder （QVI）
to measure the height of the ionosphere near the transmitting and receiving
sites； which as mentioned earlier can be miles apart； and a radar
backscatter sounder to measure the height of the ionosphere downrange
500 to 1；800 nautical miles away。 The incoming real…time data from these
soundings are compared with data stored in computer memory。 Once real…
time data are matched to a model of the ionosphere； the model can be used
to operate the system for the best results； based on the prevailing
propagation conditions。 The dat
