FITTING A COACH FOR WIRELESS RECEPTION.
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Continued from Our Issue of May 22nd, Further Details are Given of the Equipment of a Motor Coach with the New Means of Adding to the Pleasure of a Party.
IN OUR last two special articles dealing with this subject we have dealt with the possibilities of equipping a coach in a eatisfactiry manner, and we have described how the aerial wues 8hou1d be attached to the spreaders and the spreaders to the insulators that carry them. It now remains to detail how the lead-in wires should be arranged.
The lead-in wires are really among the most important parts of the aerial system, but they are often given scant attention, end an otherwise admirable aerial may, thereby, be more or less spoiled. It cannot be too strongly emphasized that the lead-in wires themselves have nearly as much to do with the picking up of the radio energy from the ether as does the aerial proper, and in the ease of transport, where a satisfactory antenna system is so difficult to devise, we must take every possible care with this important detail.
First of all, we must avoid, so far as possible, any joints in the wire between the aerial and the instrument. Even if these are carefully soldered, they are subjected to internal corrosien, which insidiously increases their resistance, with a consequent and, what is worse, an almost imperceptible, although gradual, falling off of signal strength. Especially should soldered joints be obviated between the lead-in wires and the horizontal aerial wires, as at this point
corrosion will be most rapid. It is, therefore, necessary. to take the actual wire composing the aerial straight on down as a lead-in wire without a brake.
To do this, using the bobbin insulators recommended, the aerial wire should be passed round the groove in the insulator for a quarter of its circumference only (see Fig. I), and should then be-secured to the insulator by means of a piece of 18-gauge copper wire, which is twisted round each of the two limbs of the aerial wire as shown. Before cutting off the aerial wire be sure that you leave enough to reach right down to the point at which the lead-in must enter the vehicle. Each of the four wires composing the aerial should be similarly treated, and they should then be brought together and carefully soldered exactly at the place where they must enter the bodywork. In soldering them. together they should at the same time be soldered to a length of 14-gauge copper wire, and this latter should form the conductor which passes through the bodywork. In soldering this connection nothing but resin should be used as a flux, since even the best of the admirable soldering pastes on the market are liable to set U p. corrosion in the course of time.
In arranging the lead-in wires, care should be taken to keep them .as far away from, the bodywork of the coach as possible. Short struts about 18 ins. or so long may be used for this purpose, an insulator being attached to the end of these where the lead-in wire comes in contact with them: (see Fig. 8).
Fig. 9 shows the method of arranging the lead-in. The .14-gauge copper wire should be passed through a piece of ebonite tube abod 1-in. internal diameter as shown. The outer end of the tube shouid be blocked up with au ebonite plug and shellaced in place. A small hole should be drilled on the under side of the tube to take the copper wire, which should then be bent so as to clear the sides of the tube and be held central in the tube at the inside end by means of a thin ebonite plug.
The reason for bringing in the lead-in in this manner is, firstly, to prevent water from running down the wires into the tube, and, secondly, to avoid as much as possible the bad effects of capacity between the lead-in and the earthed body of the vehicle. This latter,, by shunting off aerial current to earth, tends to destroy signal strength. A solid insulator would, of course, aggravate this, so that is why we use the tube. Before leaving this part of the installation, it should be mentioned that the soldered joint should be-very carefully wrapped with insulating tape and varnished with best shellac varnish.
There are on the market a number of ready-made lead-in tubes. Those in porcelain, with the end bent down like an inverted tobacco pipe, are excellent on a stationary installation, but too fragile for successful use on a moving vehicle, the bodywork of which is always more or less subjected to strains. The • ebonite ones, which are complete with a central rod and binding nuts at each end, should not be used unless one is prepared to solder nut and all up solid— which is hardly worth doing. The actual arrangement of the whole lead-in will, of course, vary very greatly with the design and arrangement of the coach body, also with the exact location of the instrument on the vehicle. One can, therefore, -give only a very general outline, and this is done in Fig. 10, leaving the devising of the exact layout to the reader who is in possession of all the data in the case. In passing, however, the writer would like to mention that, should any reader have any difficulty in devising a suitable arrangement, he (the writer) will be very pleased to help on receipt of details as regards the shape and size of body, the place that it is desired to place the set, and, if possible, a sketch or photograph of the coach. These should be addressed care of the Editor of The Commercial Motor.
This completes the aerial installation, and we may now pass on to the instrument and its associated apparatus. If the foregoing instructions are carefully carried out, one can at least be sure that the results obtained are the very best that ca-n be achieved in the circumstances.