Hyperfix Antenna Tuning Unit
The Racal Model 90516 ATU contains a muliple tapped auto-transformer inductor with a switch to select taps, two vacuum-variable capacitors and a relay, and an RF ammeter ('antenna current' meter). The relay is required because the ATU settings will be different for the f1 and f2 frequencies. When the control unit is generating the f1 frequency, the relay is energised and connects an additional variable capacitor in circuit. By this means, the antenna is correctly tuned on both frequencies.
The unit also contains transmit/receive changeover switching, and a receiver preamplifier. This enables the station main antenna to be used for both transmission and reception.
The front panel controls consist of the knobs for the two variable coapacitors, a dual-concentric knob operating two rotary switches for selection of taps on the inductor, and switches for selecting whether the current meter measures the antenna current due to the f1 or f2 frequencies, and the current meter range (1x or 6x).
Simplified schematic of the RF section
The ATU circuit employed is that of a parallel tuned circuit consisting of a tapped inductor tuned by either one variable capacitor (f1) or two in parallel (f2). A high-voltage RF relay switches the extra capacitor into circuit when the station is about to transmit on the lower chain frequency. The input from the PA unit is applied to one of 11 taps on the inductor selected by the inner knob of the tap selector switch. The antenna is connected to one of 11 (different) taps a rotary switch operated by the outer knob. A twelfth position of both switches bypasses the tuned circuit and connects the input directly to the output. In this way, the input an doutput connections are connected to points on the tuned circuit of appropriate impedance.
The unit is extremely well made employing the best quality components. The ATU components live in a metal 'coil box' within the outer case. A seperate box houses the small amount of electronics involved in power metering and relay operation. The tuning capacitors are vacuum variable types rated at 5kV. When you consider that the maximum power handled by the unit was 50 watts, this seems a lot of overkill. Until you consider the effect of that 30m tower taking a lightning strike...
The instructions for tuning up the ATU are straightforward enough:
- Get the controller to radiate an f1 signal
- Adjust the input tap (S5) for maximum antenna current reading
- Adjust the f1 tuning control for maximum antenna current reading
- Adjust the output tap (S6) for maximum antenna current reading
- Get the controller to radiate an f2 signal
- Adjust the f2 tuning control for maximum antenna current reading
- Return controller to normal station operation
Real radio stuff, don't you think?!
The pictures below, and that at the top of this page, were contributed by Paul Tuton G0UBV. Paul's 90516 is absolutely mint and belonged to the UK Ministry of Defence, probably from a site like Bass Point that was decommissioned in the year 2000.
View inside the coil box showing the vacuum variable capacitors and the tapped coil
Rear panel view
Electronics enclosure
Here you are looking at the receive preamplifier section of the ATU. During receive periods, the Hamlin relay switches the antenna connection onto the primary of the toroidal transformer via a 3dB pad. The two ZTX651 transistors operate as push-pull emitter followers, with a substantial bias current (hence the heatsinking) to give the best strong-signal handling performance. Their outputs are fed as balanced drive to the 90515 receiver.
View behind the front panel, as it were, showing the turns-counter mechanisms for the VVC's and the switch mechanics.
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Last Update 10/11/05 prev. 21/5/05 14/5/05, 4/5/05 AC