the dawn of amateur radio in the u[1].k.(收音机雏形的诞生)-第21章
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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 data for the ionospheric models take up more
than 200 megabytes of computer storage space。 Operators thus know when
and where to expect degraded performance。 Of course； strong solar
activity can virtually make over…the…horizon HF radar unusable。
A Spectrum Analyser display shows all the frequencies between 5
and 28 MHz。 In order to avoid possible interference to other services；
those frequencies which are known to be permanently allocated to fixed
broadcasting and telecommunication stations are locked out； as well as
frequencies which happen to be used at any instant so that they can also be
avoided by the OTHR transmitter。
GLOSSARY for non…technical readers。
A。M。 A mode of modulation （amplitude）。 A。R。R。L。 Amateur Radio
Relay League （U。S。A。）。 Beacon Transmitter radiating identification signal。
C。Q。 General call； to any station。 C。R。T。 Cathode ray tube （like TV screen）。
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C。W。 Continuous wave （mode of sending telegraphy）。 Callsign Station
identification （letters & numbers）。 Coherer A device for making radio
frequencies audible。 DE Morse abbreviation for ‘from' （French）。 DX
Communication over a long distance。 Detector Any device for making
radio frequencies audible。 Doppler shift Change in pitch （of sound） or
frequency of a （radio） wave E。D。E。S。 Initials of a war…time Greek guerrilla
organisation。 E。E。R。 Equivalent Greek initials for R。A。A。G。 （q。v。） E。L。A。S。
Initials of a war…time Greek guerrilla organisation。 E。L。F。 Extremely Low
Frequency。 E。M。E。 Earth…moon…earth。 Also Moonbounce q。v。 H。H。M。S。
His Hellenic Majesty's Ship。 Gasfet A type of transistor。 KHz Kilohertz …
international unit for kilocycle。 M。U。F。 Maximum usable frequency。 MHz
Megahertz … international unit for megacycle。 Moonbounce
Communication by reflection from the moon。 OTHR Over…the…horizon
radar。 Q code Abbreviations used when communicating by telegraphy。 Q1
Unreadable。 Q2 Barely readable … only some words。 Q3 Readable with
considerable difficulty。 Q4 Readable with practically no difficulty。 Q5
Perfectly readable。 QRO High power。 QRP Low power。 QRT 〃Stop
sending〃。 Frequently used for 〃shut up〃。 QSO Two…way communication。
QST Call to all stations。 Also title of journal of the A。R。R。L。 QTH
Location or address of a station。 R。A。A。G。 Radio Amateur Association of
Greece。 R。F。 Radio frequency。 R。S。G。B。 Radio Society of Great Britain。
RST System of reporting readability； strength & tone of a signal。 RX
Receiver。 S unit Unit for reporting strength of received signal。 S。I。 unit
International system of definitions。 SSB Single side…band … a mode of
modulation。 SWL Room where radio equipment is set up。 Shack Room
where radio equipment is set up。 Silent key Deceased radio amateur。
Sporadic E。 Propagation via the E layer of the ionosphere。 T。E。P。
Transequatorial propagation。 TX Transmitter。 Troposcatter Propagation via
the troposphere。 U。H。F。 Ultra high frequency。 V。H。F。 Very high frequency。
W。A。C。 Worked （contacted） all continents。 XYL Wife of a radio amateur。
YL Young lady operator。 73 Morse abbreviation for ‘‘best regards''。 Yagi A
type of antenna designed by a Japanese of that name。
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