Fading Might be Overcome

Radio Record, Volume III, Issue 45, 23 May 1930, Page 31

Fading Might be Overcome

"Interesting Experiments Meet with — . Success ene

(By

James H.

Waiter

FADING, one of the greatest bugbears in the path of long-distance reception, has long occupied the attention of experimenters, both amateur and professional, but until now the progress that. has been made is negligible. Interesting particulars of extensive experiments just concluded on both sides of the Atlantic, however, definitely reveal that fading can be overcome, and will shortly, it is hoped, be a relic of the past. Data and information harvested during the past few years, formed the basis of the tests which were performed by the Philips organisation. A spe-: cial programme. was broadcast to the United States from shortwave station PHI at Huizen on a wavelength of 16.88 metres.. The N.B.C. network rebroadcast the programme throughout the whole of America, being picked up in Europe by special apparatus at Eindhoven, and then passed on to the broadeasting transmitters at Huizen and Brussels by land-line. Surprisingly good results were obtained at the laboratories, and fading was entirely absent, but the quality of reproduction from the long-wave transmitters at Brussels and Huizen suffered owing to the length of telephone cable connecting them to the laboratories, and the distortion introduced by the number of repeating installations necessary. Nevertheless the relayed programme proved to be by, far the most enjoyable yet received in Europe from America. "THREE directional aerials of the socalled dipole type were erected at the laboratories under the direction of Dr. Balth: vy. d. Pol, who had prede termined their construction to suit exactly the wavelength of the transmitter to be received. The object in using three aerials at a certain . distance from one another was to compensate for the fading effect, for it has been found that fading in various places does not occur simultaneously. That is to say, that whereas fading will be experienced by one aerial, another in close proximity will escape. Each aerial

was connected to an independent receiver, but the output of all three was combined. It was immediately noticeable that there was considerably less fading, than when only one aerial was used. In order to avoid parasitic noises, which might be caused by electrical machinery operating in the neighbourhvod, the receivers were placed in a cottage in an open . space a few miles from the _ laboratories. The receiving apparatus consisted of .ordinary commercial sets which had been altered to function on the super-autodyne principle. Their most noteworthy feature consisted of an automatic anti-fading device, the basic principle of which may be defined as follows :- * THE output of the last intermediate frequency amplifying valve of the receiving set is partly utilised to supply after amplification negative grid bias for the I.I’. amplifying vaives. The value of this negative grid bias, which also determines the LF. aimplification, is dependent upon the output. The circuit has been so designed, that when the output increases, the I.F. amplification decreases. This naturally results in a decrease of output, and consequently of the negative grid bias, with the result that the I.F. amplification increases again; automatically keeping the volume at the same level. In ¢ nutshell the current is moving in a "vicious circle." The great value of the experiment lies in the fact that it has definitely opened up the solution of successfui shortwave internatidénal transmission and reception. * Further experimental work is in hand, and it will be interesting to see whether the solution which will finally enablé the amateur to entirely forget fading. will be the result of discoveries having a bearing upon the transmitter, or the receiver.