The RACAL RA-17 and RA-117 were notable as the first production communications receivers to implement the Wadley Loop tuning system. The Wadley Loop was a technique to improve tuning precision and stability at higher frequencies in the days before phase-locked-loop systems were feasible economically. These were premium receivers in their day, and the choice of many Naval and commercial services in the 1950s and 60s.
Using these receivers is a unique experience relative to that of their contemporaries. The tuning precision and stability is remarkable for a tube receiver. The tuning dial is graduated with 1 KHz markings and the tracking can be accurate to less than ± 1 KHz. The Collins 51J/R388/R390 series are the only other receivers from the period with comparable tuning capabilities that I am aware of. The Collins receivers took a very different approach to achieving those abilities, employing multiple crystals and complex mechanical linkages. In contrast, the RA-17 and 117, using the Wadley Loop, rely on a single 1MHz crystal reference.
The Wadley loop was named for its inventor, Trevor Lloyd Wadley, a South African engineer also known for inventing the tellurometer, a microwave distance-measuring device used in surveying. Both the Wadley Loop and the Tellurometer were based on sophisticated use of frequency-domain concepts.
The SPARC Radio Museum originally obtained two RA-117s around the late 1990's as part of two racks of RACAL radio equipment that were removed from a decommisioned Canadian destroyer before it was sunk to become an artificial reef. In 2007 a further group of three (!) were donated to the museum. None of the five were functional as received. The first two from the destroyer were substantially intact and unaltered, while all of the group of three were a little beat up and had been hacked over internally: capacitors had been snipped out, some component boards were broken or removed, other bits and pieces missing. In 2008 we set out to get some of them working, starting with an assessment of the five units. Five years later, all five of them have been restored and are functional. In 2009 SPARC received an RA-17.
Pop Culture Trivia: In the 1964 James Bond film "Goldfinger", several RA-17/117s can be seen in the background as electronic filler in the 'laser table' scene (YouTube clip, ~ 0:53 and 2:06).
Racal superseded the RA-17/RA-117 with transistorised Wadley Loop designs through the 1960s.
By the late-1960s, digital synthesized / phase-locked-loop tuning had become the standard in military / high-end radio, eclipsing the Wadley Loop in such markets.
In the 1970s and early-1980s, several consumer-grade solid-state receivers would be produced using the Wadley Loop. The first was the Barlow-Wadley XCR-30, an enterprise in which Trevor Wadley was involved. The XCR-30 was a portable introduced around 1970 - a time when portable multi-band receivers were all the rage in the consumer market. A couple of years after the XCR-30, several desktop models would be introduced, targeted at the amateur/SWL markets. Some of these were OEM'd by one Japanese manufacturer (see OEM column). They appear to be identical or very similar except for the front panel or model label. The design appears to have begun with the Drake SSR-1 and sold under the guise of the other labels a few years later. Similarly, the Yaesu FRG-7 was also sold under other labels.
PLL technology continued to become more economic as solid-state integration levels increased and by the mid-1980s the Wadley Loop was essentially obsolete.
The General Dynamics R-1051, produced starting in 1964, is a unique receiver amongst those listed.
The R-1051 was a very high-end military receiver with a synthesized tuning scheme based on a complex arrangement of comb-generators, oscillators, mixers and multipliers rather than a PLL.
The R-1051 might be charactersied as having 2-and-1/2 Wadley Loops but their use differs significantly from that in other Wadley receivers.
The Wadley Loop principle of differential mixing for drift cancellation is used in the synthesizer to correct the heterodyne mixing frequencies relative to a single high-stability reference oscillator, but most of the drift cancellation takes place before the heterodyne mixing with the received RF signal, outside of the received signal path.
(Thanks to David Wise for bringing the R-1051 to my attention / 2014 Aug.)
Visit the sub-pages linked below for more commentary on the RA-17 and RA-117. The schematic for the RA-117 presented here is a re-organised drawing based on a scan of the manufacturer's schematic.
PDF file (this window)
PDF file (new window)
Unit N0115 Log
Unit N0144 Log
Unit N0714 Log
Racal RA-17 & RA-117