AERIALS—INTENTIONAL AND OTHERWISE

Sun (Auckland), Volume II, Issue 326, 11 April 1928, Page 12

AERIALS—INTENTIONAL AND OTHERWISE

The observant experimenter will Dften notice quite remarkable effects due to the proximity of an aerial not actually connected to the receiver. At one time it was usual to boast that Dne’s set would receive certain stations without an areial, and possibly without even an earth. Such a test carried out under home conditions probably involved disconnecting the aerial wire from the set and leaving it lying on the table near by, or at the most disconnecting the wire from the inside end of the lead-in terminal. In both these cases more ■areful experiments will show that the aerial is still influencing reception, and l has even been noticed that an aerial is not quite inert if connected to an earth separate from that used by the set. Receivers are frequently tested nowadays to ensure that they will not receive even a powerful local station without an aerial. This serves as a test of the efficiency of screening boxes. To be quite fair to the set, however, it is important that there should be no aerial, either of the frame or open type, near by. Other sources of unwanted reception are to be found in long H.T. and loud-speaker leads, particularly the former if a mains unit situated at some distance from the set is being used. The blocking condenser within the set beween each H.T. tapping and earth should not be omitted even though there is a full set of condensers in the eliminator: and again, if long loud-speaker leads are to be used, it is desirable that an output transformer the core and secondary of which are earthed, should be employed in the plate circuit of the last stage in the set. The use of a frame aerial will endow most receivers with a marked degree of selectivity, which is in no small degree due to the directional property of the frame; but here again care should be taken to ensure that there is not a lead-in from an open areial close by, or persistent jamming by the local station may be experienced. It is a good plan to mount coils with their axes vertical wherever possible, as, if otherwise placed, they may be acting as miniature frame aerials within the set. Certain special arrangements of windings, such as the toroids and the binocular coils, tend to prevent trouble from this source.