An Aid to DX.

Radio Record, Volume V, Issue 16, 30 October 1931, Page 17

An Aid to DX.

Constructing an Aerial Tuner

By

Megohm

qz may be taken for ‘granted that d.x. enthusiasts will welcome any scheme that will strengthen the weak Signals they are hunting, The idea of tuning the aerial itself to the frequeney of the incoming wave is not new, but is more or. less lost sight of. because it is assumed that all the necessary tuning is provided for in the first stage of the receiver. It is true that with the efficient multi-valve sets of to-day small losses or gains may be tolerated or neglected, but the: owner of a small or merliumsized receiver and the dxer who is ‘stretching out after the faintest signals will .be interested in the small find inexpensive unit here described. Its‘use does not entajl any alteration to the receiver, although the idea could be built into any set providing sufficient space was. available. Not only will the device make faint signals more readable, but, acting as an additional tuned circuit, there is a conyore gain in selectivity. Constructional Details. N itS most convenient form the tuner consists of a .0005 variable condenser and solenoid, both in series and in series with the aerial. When placed in this way the solenoid has a much lower inductance than when tuned in parallel in the usual manner, and on that account about double the usual number of turns are required. In order to keep down the r.f. resistance, 22 d.c.c wire was used, 100 turns on’ a 8in. former, occupying about 4 inches. Taps may be taken at 20 and 40 turns from the top, but are not essential; they can.be used for low waveleagths. The aerial is connected to the top of the coil, and the bottom of the coil to \. the condenser stator. The rotor, which

is not earthed, connects to the aerial terminal of the receiver, A baseboard about 6} by 43 will take the solenoid standing: upright behind the condenser, and for the panel ebonite or three-ply about 6 by 43 should be used. Method of Use. WHEN searching for a distant station pick up the carrier in the usual way, then bring the tuner into maximum position as hearly as can be judged, then tune receiver, finally adjusting the tuner if necessary. Then, by way of experiment, remove the tuner and see how the volume of signals is altered when the receiver is unaided. , Every aerial has its own natural or fundamental wave-length, at which the tuner would be of no assistance in reception, and on wave-length near to the natural little difference would be noted, but in actual broadcast reception the natural wave-length of the aerial is always much below any wave-lengths to be received. The ordinary inverted "L’"’ type aerial has a wave-length in metres approximately equal to 14 times its length in feet, so that a 50-foot aerial has a

natural wave-length of 75 metres, and an 80-foot 120 metres. Nothing in this article -is intended to refer to the "Beverage" aerial, of which

there appear to be a number in use in more spacious surroundings than the average city back yard.

Alternative Methods. FNSTHAD of the coil and condenser . shown above, a variometer may be used, if one is on hand, though it would be. more trouble to make than the solenoid. The one used for experiment had 40 turns on the stator and 36 turns on-the rotor, 20 enamelled wire, outside (square) dimensions of stator 3% by 2 inches. A variometer gives less range than. the previous method, ‘Another alternative, cheaper and just as effective, though slightly less convenient method, is a coil of 140 turus of 26 enamelled wire on a 2in. former, with a, tap at every 10 turns. At one end a section of ten turns may be tap-ped-every: two turns to provide for fine tuning, so. that any even number of turns may be used. Taps are taken in the usual way by cleaning the wire and twisting into. a small loop. Two clips may be,used to make contact with the taps, or they may be connected to sockets on a strip of ebonite, with two plugs to. engage the. sockets. One plug or clip is attached to the aerial, and the other to aerial terminal of the receiver. All particulars given are to suit the average aerial. of 70 to 80 feet, For longer dimensions a few extra -turns may be required if coverage aboye 600 kilocycles (500 metres) is required, In practice, tuning is less eritical on the higher wave-lengths, so there is little loss if the range of the tuner is a little short, ,