AT THE HEART OF THE ROAD TRANSPORT INDUSTRY.

Call our Sales Team on 0208 912 2120

EQUIPPING A COACH FOR WIRELESS RECEPTION.

16th October 1923
Page 20
Page 21
Page 20, 16th October 1923 — EQUIPPING A COACH FOR WIRELESS RECEPTION.
Close
Noticed an error?
If you've noticed an error in this article please click here to report it so we can fix it.

Which of the following most accurately describes the problem?

Capacity Has Been Shown to be of Great Importance in Radio Receivers. Desirable and Undesirable Capacity are Discussed Here.

IN THE last two articles of this series we have discussed the effects of capacity and inductance, and shown how they are mutually affected by one another, in so far as the ultimate wave-length of the set depends. Thus, for instance, we have seen that a reduction in capacity calls for an increase of inductance in order to rnafntain the same wave-length and vice versa. But it has also been shown that, if a tuning coil, for instance, has a large amount of distributed self-capacity, the effect is to increase the wave-length of such a coil, and, consequently, to reduce the amount of useful inductance that could otherwise be used, with a consequent reduction in

signal strength. .

Capacity Is a necessary thing in wireless work, because -without it we could not tune radio circuits at all. But, it is very necessary that the capacity used be in the form of definite condenser units and not distributed indiscriminately about the set. This is an extremely important point, and one that, through neglect, is responsible kr more poor behaviour on the part of radio receivers than nearly any other cause one can name. It is, therefore, fitting that we devote some little space to showing where losses

• due to capacity can and do occur. So far as radio inductances are concerned, we have already pointed out that special methods of winding have been devised in order to reduce the, selfrapacity of these to the lowest possible figure. The first point, therefore, is to see that the design of inductance used complies with this

requirement. If bought, the inductances should be by a reputable maker ; and by reputable we mean a maker who has already acquired repute as a manufacturer of radio equipment—the design and manufacture of good inductances require more experience than can

• have been gained in the shoat time since broadcasting became a popular craze. If the inductances are home-made they should be very carefully constructed in accordance with initructions from a reliable source (a fortnight ago. The Commercial 'Motor published particular of inductances for the broadcasting wave-lengths.)' The next thing to remember is that loss through distributed capacity can occur nearly anywhere, both inside a radio receiver as well as outside. In one f the earlier articles, when we treated of aerials and leads-in, the danger of loss through capacity to B36

earth was pointed out, and the instructions for the avoidance of this need not be repeated here. Suffice to say that no earthed conducting or semi-conducting body should be allowed to come in contact with the lead-in, even if the latter is well insulated, as-it is surprising the amount of energy that can be shunted away thus. (Those already in possession of a radio receiver should try catching hold of the lead-in whilst receiving is in progress. It will be found that, despite the fact that the average human body has a resistance of over five million ohms, more than 60 per cent. of the received energy will he shunted away.) Inside the set, the problem of the avoidance of stray capacity is much more complicated. There are many leads and it is difficultto avoid crossing one over the other. Yet, unless matters are very carefully arranged, each crossing may be the cause of a small but quite definite lops, and this loss, multiplied by the large number of crossings in the set, may amount to a very considerable total. The first thing to do is so to arrange the components in the set that there shall be a minimum of crossings, and the next is to see that where crossings are absolutely unavoidable as great a distance as possible is allowed between the leads. Fig. 26 (A and B) 8110w9 the right and the wrong way of crossing two leads. In no circumstances should they be allowed to touch, even when they are well insulated. If all the leads are arranged as shown in Fig. 26B the use of insulated wire is not necessary. After the crossings the next thing to pay attention to is to avoid, so far as possible, parallelism between leads, especially between leads carrying currents at greatly different potentials, as, for instance, plate and grid leads. The effect of running wires parallel is greatly to increase their capacity to one another. Thus, two leads crossing at an angle of, say, 45 degrees, may be sufficiently separated when they are in. apart, whilst the same two leads may seriously interact when running parallel to one another even if separated by three times that distance.

The only exceptions to this dictum are the leads that carry the filament lighting current. This current being direct and steady the wires carrying it do not act as the plates of a conslenser and, therefore, no bad effects are to be feared by running them close together. In fact, for reasons that are perhaps a little difficult to follow, but which will be easily grasped when it is remembered that the filament leads are the path to earth for all the various highfrequency circuits in the receiver, it is far better to bunch all these latter together as much as possible, so that there shall be the minimum of.difference.of high-frequency potential between them. In this case it will, of course, be necessary to make use of insulated wire for these leads, and its use is doubly desirable here, since the filament wires are always carrying pretty heavy currents and because, in the event of a short circuit, serious damage, both to the set and the accumulators, may result.

The effect of capacity between the grid and plate leads of the valves is to tend to bring them nearer to the oscillating point than they otherwise would be and thus to render the set unstable. In fact, when using high-frequency amplification enough reaction can often be obtained by this means without any reaction coil at all. Since it is difficult to control, such capacity reaction should be avoided at all costs. It is corrected in many badly designed sets by excessive grid damping, at the expense of, efficiency. Better to avoid than cure.

Tags


comments powered by Disqus