Theory and Construction of Frame Antennae

Radio Record, Volume III, Issue 21, 6 December 1929, Unnumbered Page

Theory and Construction of Frame Antennae

Ideal for Summer Conditions

"oe = WING to the character of the atmosphere during X' summer, reception is -more or less uncertain. . There is always a great deal of static which is picked up by the aerial and.. passed into the set much to the annoyance of all who listen-in. Static, as has been _ explained before, is the result of charges of electricity iS * %} present in the air in greater or in smaller quantities ‘according to the degree of ionization of the air. The longer the aerial the more static is picked up, and it is apparent that if reception during the summer is to be enjoyed, the aerial must be reduced in length. :

If we imagine this reduction to go on until the aerial is so small that both static and signal strength have been reduced considerably, we get the first function of the loop antenna. In any aerial system the portion which connects: the greatest degree of signal strength is the _ vertical part. Suppose we have a loop of wire arranged as shown in Figure 1, the vertical portion A-B will pick up a small amount of energy from the transmitting station. The horizontal position A-C and B-D will likewise pick up energy, but not to the same extent. If the antenna takes a square form as inAieated in thea diacram. there will he

presented to the oncoming waves ‘two receptive surfaces, A-B and C-D. Oncoming waves will then strike first one edge of the loop and then ‘pass across to the other side. As a wave strikes the nearest side of the loop, a voltage is generated. The wave then travels across to the other side and causes an equal voltage on that side. The two voltages oppose each other in phase. Both voltages tend to force a current up, or both tend to force it down on both sides of the loop. If.the currents reach the points E and F at the same time they will cancel out one another and the loop will be ineffective. The only reason the loop delivers any signal is: because the two voltages generated on the opposite sides are not generated at the same time. The voltage on the side of the loop towards the transmitting station will rise to its maximum before the one on the side distant from the station. There is, then, a difference of phase between both sides when they are in the position indicated in the diagram. Because of this difference in the voltage produced a.-

certain amount of signal strength is available. Obviously, therefore, the voltage, even in the maximum position, will be extremely small. Were it possible to build a loop with its sides so far apart that they would be separated by one -half the length of the wave we would have ideal conditions, because the rise of the wave to positive voltage would then act on one side of the loop: while the increase of the wave in negative voltage acted on the other side. This, however, is not practicable, and some compromise is necessary. , The higher the loop the greater will be

the length of the vertical side. The greater the length of wire exposed to the radio wave the greater will be the voltage gnerated. Therefore, the higher the loop, or the longer its vertical sides, the greater will be the signal strength received. The signal energy received by the loop increases with the increase in the number of turns or the increase in the inductance of the loop. The signal energy increases with the decrease of the resistance of the loop. The resistance of a loop increases rapidly as its natural frequency is approached. : The natural frequency is the resonance produced by the inductance of the loop

and its distributing capacity. For best results the frequency of a signal must be equal to at least one-third the natural frequency: of the loop. For frequencies closer to the natural frequencies the results will be unsatisfactory. Therefore, for high frequency (or short waves) the operation of a small loop would.be better than a large one. The capacity in between parts of the loops cause it to act as an ordinary antenna as well as a loop for it is generally understood that an ordinary aerial and earth system obtaining its energy by virtue of its being a species of huge condenser. For this reason,.the actual strength of signals received by: the loop is always greater than a calculated signal. The aerial effect is one reason why the signal from the loop can never be reduced to zero, no matter how the loop is turned with > reference to the direction of the incoming signal. -Continued on page 28.

DUE in no small measure to the prevalence of static, conditions for radio reception are adverse during, the summer months. This to avery large extent can be overcome by the use of a loop antenna. The accompanying article :tells how a loop works, how tt. differs from an aerial, and how to build one.

Loop Antenna --

(Concluded from page 1.) The approximate ratio between. the signal strength from ‘a loop and -from an outdoor aerial is shown by , the following formula :- Signal strength. = loop area X No. of turns Antenna height x wave length in metres X.15938. os Thus assume a loop with an area of 8 sq. feet and with 15 turns to be compared with an aerial 80ft. high both operating at a waye length of 300 metres. The upper part of the ratio will = 3 xX 15 = 45 as representing the strength of the loop signal. The lower part of the ratio = 30 X 300 X 0.15938 = 1333.7 as representing the strength of the uerial signal. The ratio of loop signal to aerial signal is then 45/1333.7 or approxim-

ately 1/18th in strength. This divergence into simple mathematics shows conclusively that a loop antenna can-be nowhere as eflicient as good aerial, Types of Loop Antenna. "THERE are mainly two types of loop antenna-the box and the spiral. The box loop is built up of a number of turns wound as though inside a box. They are all wpon the same plane and each loop is of the same length. This is the type that is usually employed when wound roywnd the frame of portable receivers. The other type is the spiral, which in appearance is flat, each turn being less in diameter than the preceding one. This type is shown in the diagram. Hither of these loops may be operated with or without a ground connection. As a rule the loop alone is insufficient to furnish signal energy to a receiver, but very frequently the introduction of the aerial and the earth will strengthen sigiials. The serial may be brought in at a tap in the loop and the ground taken off from its filament end. It is thus possible to use a loop and an aerial at the same time. Used in this manner the portable set or loop-using receiver is equal to an ordinary set. Another method is to wind ‘two or three additional turns on the loop frame, passing them near to the end. , or side. of the loop that connects with the low potential end of the receiver. The extra coils can now be regarded as an aerial coil loosely coupled to the grid coil. This, however, while increasing the sensitivity of the receiver, reduced both selectivity and directional effect. There are other methods of increasing the signal strength resulting from a loop aerial, and these, with the eonstructional details of a loop, will be described in next week’s "Radio Record."