Growing Interest in Overseas Radio

Radio Record, Volume II, Issue 14, 19 October 1928, Page 24

Growing Interest in Overseas Radio

Re-broadcasts of Short Wave PCJ.

NHE re-broadeasts by 2YA of the outstanding Ameriean and Duteh short-wave stations that have been made on recent Saturday afternoons have created a great deal of public interest. Recently

P.C.J.

J.

the well-known station in Holland, was broadeasi. ‘ine

following description of this station wili therefore be 01 interest. in a fortheoming issue we will publish details of a simple and efficient short-wave set which ean be made very cheaply.

WOME years ago, it was suddenly realised that radio telephony over long distances on short waves was possible. The question which couid not be solved, Lowever, was whether reliable communication could be obtained. This was doubted by many, who bused their opinion on the fact that contacts which had been established between amateurs in England and U.S.A. were unreliable. The output used for their transmissions, however, was not greater than 1 k.w. Nevertheless, tests on short waves from 25 to 35 metres had given excellent results for radio telegraphy over large distances, with only slight fading (at least not so often as on long

waves). Moreover, 2 wavelength of 30 metres enabled communication over the greater part of the day and night. Tt was assumed that onee regular telegraphic communication had been established. with a power of ; to 4 k.w. it would ultimately be possible to transmit telephony on a power of 10 k.w. In principle, it is sufficient for telephony transmission to cause variations of the amplitude of the emitted frequency oscillations, e.g.. by variation of the grid yoltage or the anode yoltage of the trans.nitting valves" by means of a microphone and a suitable amplifier. In the course of experiments it was proved that the voltage variations in their turn cause fluctuations in frequency, same causing frequency modulation, which can be larger than the real modulation by amplitude variation. Distortion caused thus is so great that on short waves especialy, speech becomes unintelligible. Therefore, exneriments were conducted with a view to maintaining a constant frequency. independent of the voltage. In the U.S:!A. a piezo-electrical crystal was used to obtain a constant frequency, especially on short waves, Such a ‘erystal oscillates with a very high and constant frequency, thus act- ing as a kind of electro-mechanical tuning-fork. In this way. high frequency energy of some watts. but with a very constant frequency, can be obtairied. This energy is amplified by

means of valves, the great difficulty being to avoid reaction between the single stages of the amplifier. This undesired reaction has a very bad influence; it causes unreliable working, and can spoil the transmitter. In many cases, screening of the amplify-

ing stages by surrounding earthed metal screens is insufficient. SATISFACTORY solution of the problem is to transform the frequeney of the stages following one another. The anode circuit of the transmitting valve is tuned to the first or second harmonic of the grid circuit. The oscillations obtained in the anode circuit under these circumstances are very powerful, while the difference in tuning of the two cireuits is large enough to avoid reaction. This method of producing oscillations can be compared to a pendulum kept in motion by a shock after every second or third swing. On June 25, 1926, the first experiments were made at PCJJ with a transmitter having a small output, and operating on a wavelength of 90.56 metres. This transmitter was the first radio-telephonie short-wave transmitter in Europe controlled by a_ piezoelectrical crystal. The transmissions of this station, which had an output of 300 w., were received very well throughout the continent. The purity of the transmission left nothing to be desired. In Mareh, 1927. a new transmitter, operating on a wavelength of 30.2 inetres, was completed and opened on April 28, 1927. by Mr. A. F. Philips, governing-director of Philips Tamps and Radio. It was an immediate success. being heard with phenomenal clarity in most every country in the

world. During May, 1927, London broadcast programmes were heard in Australia and New Zealand through. the medium of the Philips transmitter and re-broadcast by 2BL. Then followed a series of successful test transmissions, including re-broadcasting of famous continental stations. Regular transnuissions «are still maintained three or four times a week, and PCIF has become recognised as the most consistent station at present in operation. HCENTLY the wavelength has been shifted from 30.2 metres to 31.4 metres, and the following description of the equipment used at PCJ on its former wavelength of 30.2 metres is of value. The frequency of the transmitter, which has now been transferred to Hilversum, is kept constant by means of a quartz crystal, which has a resonance frequency of 1.656.000 periods per seeond, which corresponds to a wavelength of approximately 181,06 metres. The transmitting valve, TB 04/10, which is controlled by this crystal, is followed by six amplification stages, one of which is intended to double the frequency, and another to triple this new frequency. The frequency finally obtained in this way corresponds to a wavelength of about 30.2 metres. In total, ‘the amplifier has seven stages. of which the last two are water-cooled transmitting valves of type TA 12/20.000 k. Modulation takes place in the anode circuit of the last water-cooled amplifying valve. The modulator valves used of

type MA 12/15.000 ure also .watercooled, and have a totai output of 30 k.w. Preceding these valves are two modulator valyes type TB 2/250 uaircooled, with a total output of 400 w. When examining the various units of the installation in detail, it can be seen that the frst unit on the left consists of three stages; the crystal tramsmuitter, with valve TB 04/10, and two wnplifiers, equipped with valves type TB 1/50... The middie stage serves at the same time for doubling the frequency. The output obtained amounts to approximately 50 w., the wavelength approximately 90.58 metres.. Anode’ voltage is supplied by an accumulator battery of 500 voits, and the filament is fed by alternating current. The second group shown below consists of three stages, of which the centre is intended to triple the frequency, With the result that the final wavelength is approximately 30.2 metres. Both first stages contay Philips valves of type TB 2/25 the third stage is fitted with a rater. cooled valve type TA = 12 /10.000 The high frequency output amounts to wupproximately 8 kw. with a wavelengthof 30.2 metres. The third group shown contains the last Philips watercooled amplifying valve TA 12/20.000 k. The output of the latter valve amounts to approximately 20-25 k.w. With an efficiency of approximately 70 per cent... the power in the serial is approximately 15 k.w. The aerial current has a value of approximately" S amps. The output of the watercooled transmitting valve in the anode circuit is derived from a rectifying installation, which contains six watercooled rectifying valves, type DA 12/20.000 with a corresponding smoothing system and speech choke. . The nerial consists of a single phosphorbronze wire connected to the top of.a wooden mast in the courtyard of the laboratory. A general view shows the high’ fre. quency, portion of the transmitter. in-

-, a8 ee oe eee a. Cludiue primary and secondary moduwelators on the right-hand side. The rubber tubes used for the water-cool-ing system can be seen. ‘The resistance of the water-column in these tubes is S0. high that the electrical loss can be ‘eémpletely ignored. As the output for the excitation of the first amplifying stage amouits to less than 1 W:, measures: must be taker to avoid any reaction of the following stages, and especially of the last stage on the first. Considerable difficulties can be caused by a reaction of only 1/20.000. Seri-

ous difficulties can also be caused by parasitic oscillations in ultra-high frequencies. These oscillations are mostly due to the fact that certain of the connecting wires form an oscillation circuit with the internal capacity of the valves. This evil is to be feared, especially when several valves are eonnected in parallel. The dimensions of all connecting wires are kept as small as possible, which, of course. leads to a compact installation. This is in contradiction to the electrical requirements to which the apparatus must come up. Condensers especially cause difficulties of this kind.