The transmitter unit accepts drive from the master drive unit (MDU) at a master station, and from the receiver unit at a slave station. It boosts this up to sufficient level to feed to a transmitting antenna. It is constructed on a 4U high 19 inch rack chassis, and operates from a 24V DC supply with a maximum consumption of 3 amps. The simplest of the HiFix system units, the RF stages use valves, while the HT is derived from a transistor invertor which produces: one HT rail at 215 to 265 volts for the driver stage and the PA screen grid supply; One HT rail at between 480 and 530 volts for the PA anode supply; One Bias rail at -50V.

The drive from the associated MDU or reciver is a keyed RF signal of 1.5-2 volts into 100 ohms balanced. The drive input is fed to the grid of an EL91 pentode which is biassed so that it draws a very small anode current under no drive conditions, i.e in class B. The anode of this stage is coupled via a broadband RF transformer to the grid of the Power Amplifier (PA) stage. The PA is a 5B-258M tetrode biassed beyond cutoff (the full -50V bias is applied to its grid). Therefore in the absence of drive, no anode current whatsoever flows. The anode is coupled to the output socket via a tuning unit giving anode tuning, coupling, and aerial tuning controls. In the PA anode circuit there is a 'feedback' coupling transformer connected to a FB terminal. this is used for Type B operation.

Under these circumstances each transmitter station uses two transmitters, each working one of two RF frequencies. This is done to enable lane identification by the user receiver.

In order to couple the two transmitters to a single aerial, their RF outputs are paralleled while the FB (feedback) connection is cross-connected from the output of one Tx to the FB terminal of the other. The point of this is that the output of the other transmitter is fed back out-of-phase, this causes it to cancel out in the PA anode circuit. In this way, the other transmitter becomes 'invisible'.

There was considered to be a possibility that the resonant antenna of a station could possibly pick up and re-radiate the transmission from another station, causing distortion in the pattern and therefore positional errors. To eliminate this, the aerial tuning inductance in the transmitter is shorted out by a relay contact when the transmitter is not actually keyed. This destroys the resonance of the aerial making it less likely to re-radiate signals.

Last Modified 16/11/07