Racal Engineering Ltd.,
überarbeitet am 23.10.2010
In the late fifties, the English company Racal Engineering Ltd. brought out their first receiver based on the Wadley Loop circuitry developed by Dr. T. L. Wadley. In contrast to earlier receivers which used a separate crystal for each oscillator frequency, the oscillator frequencies are extracted from the harmonics of a single high stability 1 MHz oscillator. Thanks to a special drift stabilisation circuitry, these oscillator signals on every MHz are very stable and any drift after warming up is compensated automatically. In connection with a linear 1 MHz VFO, this receiver design gave a high stability and high frequency accuracy over all 30 MHz bands in the complete shortwave range.
The basic version of the RA-17 appeared in the market in 1958, there has been a U.S. variant RA-17 UA (UC with product detektor) which can be recognized from a directly connected mains cable, no mains power selector, a signal strength meter calibrated in S units and a S-meter calibrator potentiometer located just below the signal strength meter.
The receiver front panel is painted in the characteristic "light battleship grey" fount in English marine receivers, a strange light blue - grey - cyan colour. In the 19" rack variant with the dimensions of 48 x 26,5 x 51 cm (the set is much deeper then wide) it comes to 30,5 kg and with the matching steel cabinet a mere 44 kg. After You have taken awav the covers, You have access to the shielded modules of the receiver circuitry. As the oscillator is active on frequencies within the shortwave spectrum, careful shielding has been very important for good suppression of spurs and birdies caused by the receivers oscillators.
The window of the frequency dial is located in the middle of the front panel, in the dial window which is 15 cm wide, You see a 100 kHz segment of the shortwave spectrum, a complete 1 MHz band will be covered by a 1,4 m film dial. The 1 kHz dial marks have a distance of 1,5 mm so frequency accuracy is around 500 Hz. You can use the crystal calibrator and move the dial pointer to the correct 0 position. The kHz dial is activated by the KILOCYCLES tuning knob, use the MEGACYCLES tuning knob to select the corresponding 1 MHz band, the MHz marks are visible in the small dial window. Between the tuning knobs, You find a mechanical tuning lock to be used when the receiver is used for fixed frequency operation in pont-to-point communications.
In the right upper corner of the frontpanel, You find the signal strength meter calibrated in microampères, it can be set to indicate the antenna signal strength or the A.F. level. Below, You find the controls for the preselector, the preselector band switch below and the preselctor tuning control above, in position WIDE BAND, the preselector is bypassed.
On the rear face of the receiver, You find a row of connectors with all important outputs included one for the AGC voltage which is used in diversity operation to activate the receiver with the higher signal level when two RA-17 sets are switched in diversity mode. There is a voltage selector and mains power socket, an antenna connector and an output for the 100 kHz intermediate frequency to feed an external panoramic adaptor.
The Wadley Loop circuitra is not too hard to understand when You have a look at the block diagram: The signal from the antenna connector has to pass a low pass filter and an amplifier stage, the manually tuned preselector and after a second low pass filter, it will be fed to the first mixer.
In practical use, the operation scheme is a bit more complicated then the one found in modern PLL synthesized receivers. Use the MEGACYCLES knob to tune in to the desired 1 MHz band, select the corresponding band range with the preselector range switch and tune the preselector for maximum noise in the speaker. The tune the set to the desired frequency with the kHz tuning knob, You should be able to hear the station in the speaker. Carefully tweak the preselector for maximum signal readability and signal strength meter reading. Sometimes, You also have to tweak the MHz tuning a bit for maximum S-meter reading, the RA-17 does not provide an automatic lock of the MHz oscillator.
The Racal RA-17 L is a boatanchor receiver with a very interesting receiver design found in many later receivers. It still offers a quite accurate dial accuracy - at least far above average what can be found in amateur grade receivers from that era - and has a good useful receiver sensitivity and selectivity. Of course, it cannot compete with modern receivers as far as electronic frequency memories and rapid channel switching is concerned, but for bandscanning in one shortwave band, it still gives very nice results to the technically interested DXer.
© Martin Bösch 24.11.1999