On March 19, 1932 the then Premier of N.S.W. Mr. Jack Lang officially opened the Sydney Harbour Bridge. This was a "big day" of celebrations which was closely followed by listeners of commercial Radio Station 2UW and a chain of 23 other stations throughout Australia. This was made possible by the use of a "Roving Mike".
With only a week to go before the opening of the bridge construction of the transmitter had only just began, the following is a description, published in "Radio Monthly" April 21st 1932, by Don B Knock, a former technician from the B.B.C. who was familiar with "stunt" broadcasting and the designer of the transmitter.
As many of the amateur transmitter fraternity will be interested in a few details of the transmitter these are given, but for commercial reasons exact specifications are withheld. The transmitter used two Osram P625 valves in the pure and simple push-pull "T.N.T." circuit. The radiating system is the oscillator "tank" circuit extended in the form of a loop aerial made to the correct dimensions for the frequency used, and the Telefunken method of modulation is employed. The loop aerial (tank) consisted of Lewcos special silk covered flex wire with the familiar green covering, and as a radiating system for loop transmission we can speak in the highest terms of this particular wire.
The push-pull P625 oscillators derived their 6-volt filament supply from a Diamond type No. 36 starter and ignition battery. This battery, along with all other batteries excepting the plate supply, was housed in the frame of the transmitter itself and operated by a control switch on the transmitter. For mobile use we consider these Diamond starter batteries ideal and the sort of power unit that instills a feeling of complete confidence; for the test we put the one in the "Walking Transmitter" to on this memorable day involved 10 hours practically continuous working. At the end of the day, which was 11.30pm when we finally switched off, this battery was only showing the slightest voltage drop after the heavy load, and the next day it seemed all the better for use. Two Osram P625's with the filaments in shunt pull just half an ampere, which is a hefty load for continually working a primary cell.
The modulator is an Osram L410 valve, with grid bias provided from a Diamond 9-volt cell, and the 4-volt filament operated by two Diamond 4.5 volt C batteries in parallel. The microphone is energised by another 4.5 volt C battery. It was essential that there should be no chance of grid leak breakdown, and against this contingency a Renrade 5-watt non-inductive 25,000 ohm resistor was provided. This was found to keep perfectly cool the whole time, and we can heartily recommend them to the amateur transmitter as a thoroughly reliable job for such a purpose. A Radiokes volume control of the new wire-wound type provided for gain control on the mike, and if any "ham" doesn't think this necessary with a hand mike and no speech amplifier when using Telefunken modulation on a T.N.T. oscillator, then let him try without and note the results.
"The mike used was a Stromberg-Carlson solid back Post Office type with a little doctoring. This doctoring was purely external, in the form of doing away with the mouthpiece and wrapping the shell of the mike in layers of sheet rubber taken from an old car inner tube. This was necessary to damp the mike enough to stop resonance."
The Author also described other "stunt" broadcasts he was either involved in or knew of, all of which occurred during the 1920's. These included broadcasting from the famous "Flying Scotsman" express train in Great Britain, a Dance Band playing their music in an air-liner, put their music over the air, from the air! And a parachutist commented on what he saw and felt as he jumped out of an aircraft.