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Racal Engineering Ltd.,
GB - Bracknell

RA - 17 L

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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.
This Wadley Loop circuitry with drift cancelling system has been used to built compact commercial shortwave receivers and can be found in some comsumer grade radios later - one example is the Barlow Wadley XCR-30 designed by Dr. T. L. Wadley after his retirement and return to South Africa, this portable radio was a milestone in comparison to other portable shortwave radio in the seventies.

Triple conversion, I.F. 40 MHz, 2,5 MHz, 100 kHz

Analog dial, linear, accuracy ca. 1 kHz

AM, SSB(BFO)

980 kHz - 30 MHz

Sensitiviy< 1 uV SSB

Selectivity -6 dB
RA 17: 8 / 3 / 1.2 / 0.75 / 0.3 / 0.1 kHz
RA 17L: 13 / 6.5 / 3 / 1.2 / 0.3 / 0.1 kHz

RF gain, antenna tuner, crystal calibrator, ATT

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 original version came with I.F. bandwidths 0,1 / 0,3 / 0,75 / 1,2 / 3 and 8 kHz, the slightly later variant RA-17 L which seems to be most popular one comes with I.F. bandwidths 0,1 / 0,3 / 1,2 / 3,0 / 6,5 and 13 kHz.

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.
In the bottom row of controls, You find the MODES switch right in the middle, the switch for the automated R.F. gain control called AVC, the crystal calibrator giving You a signal every 100 kHz. Next to the mains switch, You find the volume control called "A.F.Gain" and hidden under a cover the "A.F.Level" control to regulate the level at the 600 ohms output to meet the demands of an external amplifier, tape recorder or radioteletype converter. In the right lower corner, You find the 5 step attenuator, the RA-17L can be fed from commercial antenna systems with quite high signal levels without overloading.
At the left of the MODES switch in the bottom row of frontpanel controls, You find the switch for the NOISE LIMITER and the BFO, the BFO note control with a reduction gear is located above at the left hand on the frontpanel. In the left lower corner, You find the switches for AGC speed and the bandwidth filter selector. In the 100 and 300 Hz positions, crystal filters are active, in the other positions L/C filters.
A small switchable monitor speaker is located in the left upper corner of the RA-17L frontpanel, the R.F. gain control lust below.

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.
Tha harmonics of the 1 MHz main oscillator ar restricted to 32 MHz by a low pass filter, after being mixed with a 37,5 MHz signal, the VFO 1 / MHz selector let's You tune in signals with 1 MHz spacing from 40,5 to 69,5 MHz, in the first mixer, the first intermediate frequency of 40 ± 0,5 MHz which is fed through a bandpass filter, after substraction of the 37,5 MHz signal, the second I.F. of 2,5 ± 0,5 MHz is generated. The second VFO used for kHz tuning and coupled with the kHz film dial covers the 2,1 - 3,1 MHz frequency range and is very linear. After another substractive mixer, the third intermediate frequency of 100 kHz is generated. After an amplifier stage, this will have to pass the filter bank with the two crystal filters for CW and the L/C filters for the AM bandwidths. With the quite low I.F., the coil filters have quite steep filter skirts.
The signal is fed to a diode detector for AM demodulation, another diode is used to generate the AGC voltage.

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.
After the dial calibration has been optimised with the 100 kHz crystal calibrator, dial accuracy is better then 1 kHz, the sensitivity of the receiver stated as 3 uV for 20 dB S/N ratio in the handbook is similar to the one of my Collins 51-J4 or the Rohde & Schwarz EK 07. Even with the 6,5 kHz filter, adjacent channel signals 5 kHz away can be rejected in most cases, otherwise You have to switch to the 3 kHz filter.

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.

weitere Lektüre:
d: Der Enkel Dr. Wadley's: Racal RA-17, Nils Schiffhauer, Oldie- KW- Empfänger
e: THE RACAL RA-17 COMMUNICATIONS RECEIVER, Bruce Portzer, fine tuning's proceedings

© Martin Bösch 24.11.1999