Wireless Broadcast

Star (Christchurch), Issue 17567, 18 June 1925, Page 13

Wireless Broadcast

CONTRIBUTED BY

“ AERIAL.”

STATION SAC. From time to time the above-named station has been mentioned in these columns, but so far no detailed description of the equipment has been given. Since the station is already familiar to large numbers of listeners in throughout the length and breadth of the Dominion, a short description of SAC will perhaps be of interest. When the Radio Societ}' decided to install the set, the first question was: “What power?” Second was: “What circuit?” It was decided that nothing 1 smaller than a fifty-watt Radiotron would be of much use, so an output of fifty watts was decided on. One of the chief things thta should be studied in a broadcasting transmitter is the selection of apparatus which will put out a perfectly steady wave, and its steadiness must not be affected by swinging of the aerial in the wind. Unsteadiness would not matter very much locally, but nothing is more exasperating than trying to tune in a distant station whose carrier wave is swinging about. It is almost impossible to receive a concert under such conditions. Realising all this, the Electrical Committee of the Radio Society decided that a “master oscillator power amplifier” circuit should be used. To put out good telephony, the carrier wave should be in the form of a pure sine wave, and not modulated by A.C. hum or generator ripple as is the case with many stations. To give pure plate supply an Edison battery is used. This unit was designed and constructed by the members. The battery is fool-proof, and it is so made that a person cannot come into contact with any live portion unless he gets right inside it, and doing that is looking for trouble. A full-wave Tungar charger keeps the big battery in good shape. The frequency generator or master-oscillator unit, consists of two five-watt lubes in parallel working in a Hartley circuit. This circuit is tuned by a 43 plate Cardwell condenser, which controls the wave-length of the station. A milliameter in the plate circuit shows the input to the master oscillator tubes, which arc fed with direct current from the battery at 600 volts. A radio frequency thermoammeter is included in the tuned circuit, and serves to show how the oscillator is behaving. The >l.O. inductance is a helix of 25 turns seven inches in diameter. It is wound with No. 10 wire on a slotted ebonite low loss frame. This helix was used on the club’s old five-watt set at 3AF for a long time. The power amplifier is next to be considered. This tube is known as a fifty-wattcr. Its plate is supplied with 1000 volts at 120 to 150 milli-amperes. Instead of being connected up in the usual manner, the grid of the power tube is fed through a variable condenser from the master oscillator helix. The power tube is simply a big radio frequency amplifier. It functions, in a similar manner to a. radio frequency amplifier in a receiver. The output from the' powe’’ tube (known for short as the P.A.) is fed into a largo spiral inductance having sixteen turns with a maximum diameter of about eighteen inches. This helix is also timed by a Cardwell 43 plate transmitting condenser. A good condenser is very necessary here, as a poor one with bad spacing would either spark from plate- to plate „ continuously or elso the insulating ends’ of it would.very soon ho burned The aerial coil has eight turns and is coupled inductively to the P.A. plait© coil. A thermo-ammeter in the aerial load indicates the value of the aerial current, and provides a means of judging when the whole transmitter is most efficiently tuned. A series condenser is also included with a knife switch for shorting i't out when not desired. The series condenser is used when it is desired tr; transmit with a wave-length or 14C metres or less. In parallel with the power amplifier tube is connected the fifty watt modulator valve. This is the control tubo and serves to superimpose tbo speech frequencies on the P.A. tube. A little distance apart- from the transmitting gear is the studio. This is about fifteen feet square or a little more. To deaden echo effects as much

ns possible the studio is draped with heavy curtains on all sides and above, whilo a soft carpet on the floor is used for the same purpose as well as being an article of furniture. The remaining articles in the studio consist of piano, gramophone, and micro phone. When the microphone is spoken into, the currents set in motion are applied to a “ speech-ampli-fier.” This is simply an ordinary frequency amplifier consisting of a valve, transformer, etc. The valve in this case is an old-time U.V. 201Its output is then fed by a transformer to the fifty watt modulator. Up till recently the speech amplifier used three 201-A valves in an audio push-pull circuit. It delivered a- big output, but caused all sorts of noises, and as the tubes themselves were very microphonic, it has been discarded in favour of the one valve amplifier described. It was possible, with the push-pull amplifier and 201-A tubes ro disconnect the microphone entirely ami then speak near the amplifier tubes. Due to their microphonic properties the voice could still be transmitted quite strongly. The normal input to the power tube is around 125 watts, and the antenna current from to to 2£ amperes. If this meets the eye of any Australian reader;?, the Radio Society will be glad to receive reports from anyone hearing 3AC, on about 240 metres. Correspondence should be addressed to the secretary, 158 Manchester Street, Christchurch. To date SAC has been reported from Australia a few times. JOTTINGS. The boisterous weather experienced last week-end very soon picked out the weak points in some of the aerials round the city. Quite a few masts came down altogether, and in many cases aerials were properly wrecked. On Sun da}' morning J found that my own had suffered. Right at the top the down-leads parted company with the top cage, and some trees below were prettily festooned with my cage leadin. Getting the aerial down was no easy matter, as the rope for pulling it down went out of commission some months ago, and the aerial would not drop by its own weight, as a wire clothes-line is used as a halyard, and naturally does not run at all easily through the pulley at the mast-head. So tying a heavy pair of pliers to one end of a length of wire, I finally succeeded in throwing the wire over the aerial, and eventually got the aerial down. A few twists and a blob of solder repaired the break, and the aerial was then hauled skywards again. In goin" so the halyard at the other end went thirty feet into the air, well out of reach of a clothes prop. The timelv arrival of a friend solved the difficulty, as he was dare-devil enough to climb the half-rotten three by two mast in the face of a howling sou’-wester, and grab the halyard, lie took a big risk, but all’s well that ends well. 3AF states that his big valve has arrived, and is now ready to carry out tests with any station on earth or Mars. All crackling noises heard in a pair of ’phones are not caused by static. A dead cell in the B battery is a great noicJe-maker, and a burnout in a transformer or in the 'phones themselves is often the cause of troublesome sounds. A transformer may be tested by connecting the primary winding in series with a battery and milliameter. No indication on the meter means an open circuit. The secondary car: be tested in a similar manner. The windings should be tested for short circuits from primary to secondary, and from windings to the core. To test the phones, plug them into the set, and then short circuit one ear-piece at a time. If cracklings cease when one earpiece is shorted it is a sign there is something wrong with the shorted one. The telephone cords should also be thoroughly tested' also, especially at the tips, and at the other ends. The inside tinsel may easily be broken while the cotton covering is quite O.K. A mistake occurred in last week’s notes when it was stated that Mr C. Russell lectures on Wednesday evenings at the Returned Soldiers’ Rooms. The last three words should have read Radio Society's Rooms, St Asaph Street.