RADIO NOTES

Rotorua Morning Post, Volume 1, Issue 14, 8 September 1931, Page 5

RADIO NOTES

"Kathode." v

By

A properly constructed aerial is a great help to suceessful radio reception. The antenna acts as a feeder of radio frequeney to tbe receiver, and as there is a relation between the transmitting and reeeiving aerial a knowledge of both is desirable before installing a reeeiving set. In erecting an antenna, the main information required is that concerning the length, size, material and insulation of the wire used, together with the.ground conneetion and lightI ning protection. : In previous notes Kathode has dis- : eussed suitable wire and earths for- | use in Rotorua ; theref ore, it is" not proposed to deal with .them here. There is a definite relation between the total length of the antenna system and the natural period — or frequeney — to which the system will respond. In caleulating the length of an aerial, the lead-in and ground wire j must be included. The natural pe- | riod of the system may be ealculated I by the f ollowing simple rule, . Adcl together the length of the aerial, Iead-in and earth wire, and multiply the total by 1.5. For example, if the horizontal span is 100 feet, the lead-in 50 feet and the ground wire 20 feet long, the total is 170 feet. Multiplying by 1.5, the result is 255, the natural period of the system. As the desirable dimensions of an aerial should have a natural period near the centre of the broadcast band, the above measurements are alrnost ideal. Of course, variations may be made in the different lengths. The horizontal span may be 120 feet, the lead-in 30 feet, and the ground wire 20 feet. An approved lightning arrestor is essential as a safeguard against fire and shock. Porcelain insulators are good, but those made of glass are to be preferred, especially in damp weather. Useful Service. A wave trap is a useful device for eliminating certain kinds of electrieal interference and sharpening broadly tuned receivers. A suitable form of wave trap is easily constructed and the one to be described is particularly efficient. On a coil former baving a diameter of four inches wind 45 turns of 28 d.c.c. wire, labelling the two ends of the resulting coil'A and B. Over the centre of this coil wind a second one containing 12 turns of No. 22 d. c.c. wire, labelling the two ends C and D, taking care that each winding is in the same direction. Secure a suitable baseboard for the coil and a piece of three-ply for a panel. Mount a .0005 variable condenser and dial on the panel and secure the coil to the baseboard. Join coil end A to the stator plates of the condenser and the end of B to the rotors. Disconnect aerial from the receiver and join to coil end C. Connect coil end D to aerial terminal' of receiver. In using the trap tune bo.th it and the receiver until the interference is at the loudest point. The trap may now be neglected and the receiver used in the normal manner. When so adjusted the filter is absorbing a great deal of the interference to which it is tuned. It is bad practice to operate a screen grid receiver with the metal cans removed from the valves. "Kathode has known several cases where this has been done to increase the sensitivity of the receiver by causing the radio frequeney tubes to oscillate. When in such a condition the set is

radiating and causing interference to other listeners. Tone quality is also impaired when a tube- is oscillating. Owners of loop type superhetrodyne receivers should be careful never to use such sets connected to ~ an outside areial. By so doing they are breaking the law and causing no end of interference to other listeners. The howling valve type of interference is becoming very bad in some localities. It often happens that owners of oscillating receivers are ignorant of the fact that they are spoiling the pleasure of others. If you possess a receiver of the neutrodyne type, do not tamper with the neutralising system of the set unless you have expert knowledge, otherwise the receiver will become deneutralised and wilb cause interference. It frequently happens that a new set of valves will cause a receiver to become deneutralised. If your receiver 1 squeals on the carriers of stations I ; and you are unable to prevent it from jj j doing so, have the set attended to. | { "Kathode" inspected ZLlGM's new I rtransmitter during the week-end. It | | is a particularly well built and wired 1 ! set and should radiate an excellent | I signal. A good microphone is a fea- | I ture in the transmitter.