Why Bother with Resistance?

The "dark ages" of radio are passing rapidly, and even the newest of fans are beginning to talk about the elements of their hobby rationally. There is no definite proof that they know what they are talking about, but at least their terminology is improving.

We cannot blame the fans, though, for their past, for during many years previous even good engineers allowed themselves to muddle aimlessly without tying themselves down to strict fundamentals. It was particularly discouraging to see them put a mechanic's and an artist's skill into apparatus that sorely needed attention paid to the basic electrical design.

High frequency resistance was one of the factors that was all too frequently ignored — to some extent through misunderstanding of its relative importance. Coils were wound with any size wire, with any kind of insulation that was obtainable. Often very fine wire was used so that a great number of turns could be crammed into a small space. I have seen loading inductances that had been wound on a flat stick, layer upon layer, much in the manner one would wind up a fishing cord. Enameled wire was used frequently, because it had a good appearance, took up less room and was cheaper than other wires of equal insulation.

Seeking Durability.

Some of the manufacturers of higher grade apparatus wanted to make a more durable product, so they imbedded the windings of their coils in recesses turned or molded in the heavy form upon which they were wound.

But recently there has been a widespread reversion to the principles that have been known since Tesla was granted patents for his electrical oscillators, a number of years before the opening of this century. More attention is being paid to the reduction of resistance and distributed capacity because people are beginning to realize that these factors are the true corner stones of efficient set design.

The fan has been led to believe that this latter statement is true, but it is quite safe to doubt that he knows exactly why. Of course, to reduce resistance means to reduce a certain loss, and thus enable the feeble currents to flow more freely; but it also means more, and to understand what this more is is to get a satisfying conception of one of nature's most marvelous laws.

Pendulum Supplies Analogy.

Let us first take a mechanical analogy as a primary illustration. We have a common pendulum, suspended so that it will swing freely. If we tap it it will swing over to the other side, then back again, and will continue this oscillating motion, each oscillation shorter than the preceding one, until at last it again comes to rest at dead center. Regardless of its length, each swing will be found to be of precisely the same duration. The decay of the length of succeeding oscillations is caused simply by resistance: the weight loses energy by having to plow its way through the air and by friction at the bearings. Exhaust the air from around it and the weight would swing much longer.

Now after we have started the pendulum swinging let us assume that we tap it again, with the intent of adding power to it and increasing the length of swing. If we strike it as it is approaching us we miss our guess, for much energy will be dissipated in the collision of the two opposing forces, but if we strike it immediately it has reached its maximum and is ready to start on the return journey the amplitude of the swing will be increased, for we add energy to what it already has. By tapping it continuously, timing each tap to the natural swing of the pendulum, the energy will build up to almost anything within its power.

If we suspend the pendulum from an infinitely elastic cord within a vacuum, the energy from the taps would be added at a tremendous rate, for the resistance would be almost nothing.

In the radio receiving set we have practically the same phenomenon and the same problem to overcome. The circuit is the pendulum that must receive its impulses from the incoming waves. The amplitude to which the waves can build up within the circuit depends entirely upon the resistance which the circuit offers to them. If it is not in tune with them the waves will deliver their blows wildly, sometimes fighting and sometimes coinciding with the natural swing of the circuit. If, however, it is perfectly tuned, one wave is added to the remainder of those that preceded it, and blow by blow the total builds up tremendously. It is only by this action that radio waves can be recognized at all over more than a few miles. Distributed capacity in a coil makes this tuning less sharp, and besides decreasing the selectivity also reduces the possible amplitude of the waves.

Resistance Lowers Selectivity.

The ohmic resistance of a coil also lowers the selectivity, and besides prevents the waves from reaching their maximum heights, just as the frictional resistance retards the swing of the pendulum. As much energy is extracted from each impulse, the succeeding oscillations cannot compound themselves to any great value.

Such resistance must be cut down through the choice of proper conductors. Unlike when dealing with ordinary direct currents, or alternating currents of low frequency, a wire of sufficient current carrying capacity cannot be chosen by a simple calculation of the cross section — for radio frequency currents barely penetrate the surface. The higher the frequency the further the current must travel from the center of the wire and the greater, therefore, must be the surface area to properly handle it.

Radio Currents Are Small.

There are no large currents to be handled in a receiving set, but every effort should be put forth to save what little there is. It should never be necessary to use wire smaller than 22 in any oscillating circuit, and wire from 20 to 14 would be better. Litzendraht is made up of a number of insulated strands of very fine wire — so fine that it is practically all surface. This is excellent to cut down resistance to the utmost. Litz is made in many sizes, with different numbers of strands, and one should take this in consideration when comparing prices, for often a cheaper wire is cheaper because it has fewer strands.

There is no doubt but that an experimenter, by securing the lowest resistance and distributed capacity that the most modern methods allow him can make one tube do the work of two or two do the work of three. I have seen it done. By taking care of these factors, even a crystal set can be made to give remarkable service. It is really not the quantity of apparatus, but the efficiency that one gets out of each part that counts.