RADIO CLUB

Timaru Herald, Volume CXXXVIII, Issue 19959, 17 November 1934, Page 14

RADIO CLUB

TECHNICAL ARTICLE (By G.H.S.) Antenna and Earth At present many listeners are finding reception falling off to such an extent that they are giving their aerial and earth systems a good check-over. Also as a result of inferior reception, interference in the air is received well, so all prcautions to ensure a good aerial and earth will be well repaid. The following explanations and suggestions may help. Very often it will be found, on removing the earth wire from an electric set, that there is a decided increase in signal strength. One would think the earth wire unnecessary in such a case, and many do leave it off. However. this is one indication of a faulty aerial system. It is not generally realized that when the earth wire is disconnected from an A.C. set the mains themselves become an aerial, more noticeably so when an inferior aerial system is used, thus giving an increase in signal strength. This does not mean that the mains increase the efficiency of the aerial, but that they are the aerial, and the aerial proper is an earth connection—working in the same manner as a counterpoise through the capacity or condenser effect created between the aerial and ground. This is quite readily understood if we follow the path likely to be taken by any signal, whether broadcast or noise present in the mains. This signal is fed into the power transformer on the set, transferred to the chassis, and then, if there is no earth wire it travels along the chassis to the aerial coil, through the aerial coil to the aerial terminal, and then to the aerial and through the aerial capacity effect to earth. (Thus, if the aerial is a good high one this amount of capacity is reduced and less feed back occurs, so that taking off the earth wire on a set with a good aerial has no effect on signal strength but does increase noise strength). If the earth wire is connected to the aerial terminal instead of the aerial proper, the signal supplied from the mains will be still louder owing to the better coupling to earth through the earth wire. It tvill be noticed that the aerial coil is considered in all these explanations. This being the most sensitive part of the radio set to signal frequencies it is the most important one to consider. The fact that signal energy is fed through the coil backwards makes no difference. The signal is an alternating current. From these facts it is obvious that all broadcast and noise signals present in the mains are directly coupled to the most sensitive part of the set, from which they are amplified many thousands of times, and then heard on the speaker. If the set is provided with a good earth the noise signal coming in through the mains is immediately by-passed by the various filtering condensers to the chassis and then to earth, and does not get anywhere near the aerial coil, hence does not appear in the speaker. A certain amount of mains interference may be radiated and picked up. by the aerial. This will be heard of course. Also should a transformer become faulty, and the A.C. leak into the chassis, a good earth wire will carry this to earth and thus prevent any serious injury due to anyone touching a “live” set. If no earth wire is fitted and an aerial used, this A.C. current will feed back to the aerial, and to touch the aerial in this condition would be the same as sticking a wire in your iron plug and hanging on to it, so it will be seen that, apart from the fact that the regulations demand a good earth wire, the other advantages are that it by-passes all signal energy picked up by the mains thus making for quieter operation; renders the set “dead” in case of an A.C. leak and thus protects the family; and connects the set to the ground wave as it is technically known. With regard to the aerial, every radio set owner has his own private ideas on this. For general use we have found a 100 ft. aerial long and not less than 30ft. high gives best all-round results. By 30ft. high we mean 30 ft. above ground or articles at ground potential. The mains can radiate noise which will be picked up by the aerial and transferred to the set. We have mentioned in these columns the transposition aerial. Since that article appeared much experimenting has been carried out, and a very simple type of noisereducing aerial developed. It should be understood that noise is greater in intensity near ground. The transposition aerial is a high clear aerial with a special down lead so designed that it can traverse the noise area without picking up any of the signals. The simplified style, and one well worth trying, consists of the 80 to 100 ft. top span of wire cut in the middle and insulated and having two separate and independent down leads, but instead of using the transposing blocks the two down leads are kept spaced evenly above five inches apart. This is done by tying small cross strips o. wood every foot or so of the down leads, and using the two inputs—one to aerial and one to earth.