The "good" old days

The British magazine 'Experimental Wireless' published April 1924 a description of the Danish amateur transmitting station 7ZM owned by Gunnar Bramslev, one of the later founders of the Danish radio amateur organization EDR. But in 1924 it was illegal to own a private transmitting station, so Gunnar Bramslev was running a great risk in publishing this article under his own name.

Experimental Station 7ZM

BY GUNNAR BRAMSLEV.

Most British experimenters are familiar with many of the Continental amateur stations, but it is thought that little is known of Danish methods. Below will be found details of 7ZM, which was one of the pioneer stations.

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quite recently nothing was heard from Danish amateurs, and although radio experimenters exist in Denmark, as in every country, they are very few, and not many of them possess transmitting apparatus. The Government restriction upon amateur transmitting (half a year ago, also receiving) has not yet been removed, and this is the probable cause of so few experimenters. A description of 7ZM will possibly be of interest, as this station was the first that was heard in England. The writer started wireless experimenting in 1920, when the station was equipped for crystal reception only. Since then the receiver has undergone many alterations, and the first valve transmitter was started in May, 1923.

Fig.1. - A near view of the transmitter.

The antenna system is of the inverted L type, and consists of three wires 135 ft long supported on 3-ft. spreaders, 3-wire lead-in about 20 ft. long. The mast at the lead-in end being 33 ft. high. The distant end is supported to the chimneys of a house. Unfortunately a nine-wire counterpoise cannot be installed, but a one and two-wire which is used for transmission has given very satisfactory results, having reduced the aerial resistance considerably. The surroundings are far from ideal for transmission, as the house is situated at the foot of a hill and almost completely screened by trees and other houses. The aerial seems rather long for 200-metre transmissions, but difficulty only arises when trying to get under 180 metres.
A diagram of the transmitter is seen in Fig. 2, and a photo in Fig. 1. The valve is a Telefunken type RS.5.C.II.a with filament consumption 10-12 volts and 3 amperes. At present it is only supplied with 10 volts. The tuning inductance consists of 18 turns of 1.5 mm. bare copper wire tapped at each turn. Both this and the grid coil, which is aperiodic, can be seen in the photo wound on old gramophone records. The relay, which is controlled by the key on the receiving table, is placed in the plate circuit. It has been very difficult to obtain a good high tension supply, and I now use 220 volts D.C. from the mains in series with a home-made accumulator battery with about 80 small cells. This gives 380-400 volts, but a higher value was desirable. The set is not used much for telephony transmissions, but ordinary grid modulation is employed, giving very good speech at distances up to 20 miles with an aerial current of 0.2.
With this transmitter a radiation of 0.75 amps, is obtained when the input is 10 watts, but I hope to increase this in the near future with a better H.T. supply.

Fig. 2. - The transmission circuit showing method of keying.

The receiver portion of 7ZM will be seen in Fig. 3. On waves above 300 metres home-made honeycomb coils are used for tuning, and on the short waves 120-300 metres special single-layer, large diameter cylindrical coils are used. Two valves are used, the amplifying valve either being H.F. or L.F. If large amplification is required all the four valves can be put together as one H.F. (tuned anode), detector, and two L.F. The various parts, including two home-made inter-valve transformers and four filament rheostats, can easily be identified in the photo.

Fig. 3. - A general view of the transmitter.

With this receiver all the British broadcasting stations can be heard at any time when it is dark, and it may possibly interest English amateurs to know that nearly all Danish broadcast listeners get their entertainment from the splendid programmes of the B.B.C. stations; their own broadcasting station, Lyngby, being far from perfect. English amateurs come in very well over here on two valves in spite of the distance being in the neighbourhood of 6oo miles. Over 140 foreign experimental stations have been logged, including four American. Sometimes the distance between England and Denmark is covered with very low aerial currents and small power. For instance, in August last year I heard 5RZ, when he was using 3 watts input and 0.12 in the antenna, and a few days ago 2IJ, who stated that he was radiating 0.15.
The first long distance transmission from 7ZM was done on September 8, 1923, when my "test" call was answered by 2KW; owing to QRM, however, communication could not be established. The next Saturday greater success was obtained, and the first two-way communication established with 2JF who reported the signs QRK on one valve. Since then I have communicated with 2DF, 5KO, 2NA, and many others. The longest transmitting range has been 1,400 kilometres to 8CT.
In conclusion, I may say that the station is situated in Roskilde, about 20 miles west of Copenhagen, and I shall always be pleased to arrange transmission or receiving tests with British amateurs.