RACAL 6117 Receiver
|KHz are read on the top dial which is a 35
mm plastic film calibrated in one KHz increments. As you can see, the frequency
on the dial is approximately 890 KHz. The small semicircular dial reads "6."
Therefore, the frequency is 6.890 KHz.
Note that there is no bandswitch. The desired MHz is simply dialed in with the MHz knob on the right. The band "snaps" in when the marker is close to the desired MHz marking. Even though all of this magic takes place with tunable oscillator operating at VHF frequencies, the inherit drift-cancelling nature of the Wadley Loop circuitry results in a very stable receiver.
The bandswitch on the right side of the panel is for the preselector. Operation of the preselector is optional.
There are six positions of selectivity from 100 Hz to 13 KHz. This is done by switching L/C circuits in the 100 KHz i.f. strip. The result is not in the class of mechanical filters, but, selectivity is quite adequate and better than most other receiver made in the 1950's. It's too bad the designers didn't include a notch filter.
I particularly like this receiver for its high sensitivity, ease of tuning, calibration accuracy, and good audio. The fact that it isn't your average cookie-cutter design is also appealing. If nothing else, the Wadley Loop is a complex, yet highly effective bandswitching system. It eliminates the need for 30 crystals to cover the HF spectrum. Everything is locked to the accuracy of a single crystal oscillator, so calibration accuracy is good. Since this is an up conversion scheme with an i.f. in the VHF spectrum, images which are so common to other receivers of the time are essentially absent in this design.