The Dawn of Amateur Radio in the U.K. and Greece Part 10

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 b.o.o.by 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, 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 woodp.e.c.k.e.r". 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 mola.s.ses-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 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 pa.s.s on "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.

Furthermore, the ROTHR system has its own built-in automatic management & a.s.sessment 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 s.p.a.ce. 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 a.n.a.lyser 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).

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 h.e.l.lenic 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 t.i.tle of journal of the A.R.R.L.

QTH Location or address of a station.

R.A.A.G. Radio Amateur a.s.sociation 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.