AERIAL INSULATION

Radio Record, Volume I, Issue 15, 28 October 1927, Unnumbered Page

AERIAL INSULATION

‘he outdoor aerial has to be suitably supported in the open air from. supports mxe masts, buildings, or trees, according to the facilities locally available. These supports being conductors cognected to the earth, we have to interpose an insulator between these supports and the aerial proper of coper wires, so that electrical energy of he aerial may not leak away to earth except through the receiving instruments. Since all insulators are in ractice conductors of high resistance he current lost by leakage through them is expressed by Ohm’s law, which. states that ¥ (electromotive force) .C (current) equals R C (resistance) Hence, whether the aerial be a transjnitting or a receiving one, the percentpge of energy lost through bad insution remains the same, and as such th require as perfect an insulation as

can be given them. In fact, with the receiving aerials where the available energy is so very small, no insulation can be considered too good for the purpose. ; The efficiency of aerial insulators is dependent on the following three principal factors. Firstly, the material of which it is made must have a high specific resistance, so that the current lost by conduction through its body may be negligible. In fact, all the usual materials-ebonite, porcelain and glass-ate suitable from this standpoint, Secondly, the insulator must have a high resistance along its surface, even when exposed to the action of the elements in the open air. This is a very difficult criterion to be satisfied, since the deposits of dust, smoke, and films of water forming over the insulators are highly conducting surfaces, The usual types of ehonite and composition insulators are especially unsatisfactory in this respect, since their comparatively rough surface assists in the formation of such deposits which }do not get washed away in rain, Hyen

in porcelain insulators the deposit is considerable, and the loss of signal strength is easily noticeable after they have been in use for a mouth or two. Thirdly, the insulator must have a minimum of capacity between the copper wire and the (earth connected) rope attached to it, so that the loss of high-frequency energy to earth through this capacity may be negligible. In this. respect the usual type of reel, egg, or shell type of porcelain insudown and kept cleats : lators are very inefficient. In these the aerial wire and the supporting rope pass within a half-inch of each other in the insulator, thus forming a small condenser between the earth and the aerial, The ebonite and composition insulators, too, suffer from this defect, especially in service, when their surface gets coated easily with a conducting deposit or their surface insulation deteriorates by exposure to light, Hence these insulators, if used for an ontdoor aerial, become efficient only when a large number of them are used in series at either end of the nerial, and even then taken

In addition to satisfying the electrical conditions, a satisfactory aerial insulator must have the requisite tensile strength in ability to stand the maximum strains coming over the aerial on which it is to be used. ‘These have been calculated out, and found to be well below 100lb. for the ayerage amateur aerial. All these insulators are capable of standing a stray of well over 5001b., and as such are suitable enough for aerial insulation if they would maintain the requisite aerial insulation under service conditions in the open air, For the latter purpose, which is the chief aim of an aerial insulator, the glass insulator is undoubtedly the best. Standing a strain of over 500lb. without any leakage or capacity losses, it maintains almost perfect insulation even at the very highest potentials | useg in wireless work.

then this same light reflected from the moon will increase it is a difficult problem. It may be that gravitational or electronic lines of force from the earth t othe sun, the earth to the moon, and the sun to the moon set up certain interference bands which improve or lower the resistance of radio transmission. If these same interference bands vary we have a simple explanation of fading. On the other hand the action may not be due to lines of force at all. A possible ‘theory might be evolved in connection with the difference between the direct sunlight, with its ionising properties, against the reflected light which has had these characteristics removed in the process of reflcction.