E-655 resp. Communications Receiver RA-1217; manufactured by Racal Engineering Ltd., GB - Bracknell.
The Racal RA-17L, the first shortwave receiver based on the famous Wadley Loop design developed by Dr. T. Wadley, has been in service at the Royal Navy for years. The RA-1217 was the solid state successor of this famous set with an „odometer type“ mechanical frequency display.
The compact receiver in its metal cabinet is with 48,3 x 8,9 x 40,6 cm quite unique in its dimension, its weight is 11,3 kg. The Swiss Army variant E-655 came in a wooden transport crate, together with the complete manual, cables and accessories, you end up with 44 kg.
Use switch D not only to turn on the receiver, but also to select the AGC speed - the position “AGC med” as shown works well to get started.
To avoid bad results due to bad control settings, select the internal VFO (switch F), the attenuator (switch C) should be set to minimum, the R.F. gain (control O) to maximum and the A.F. gain control (control N) should be turneed up until a slight hiss is audible in the headphones.
Use the left tuning knob (E) to select the MHz digits of the desired frequency and the right 0 - 999 tuning knob (J) to select the other digits of the desired frequency. As found in other receivers based on the Wadley Loop design, the correct use of the preselector is very important for good results. Select the correct frequency range and tweak to maximum signal in the headphones and the signal strength meter. For reception of a standard AM signal of an international shortwave broadcaster, the modes switch (switch K) should be in the „am“ position.
Use the modes switch to set the receiver to single sideband reception (usb or lsb), with the second „if bw“ (L) switch, you can select a narrow SSB bandwidth of 3 or 1,2 kHz. For CW reception of morse code transmissions, use the „det bfo“ position of mode switch K and the BFO beat note control M. In special reception conditions, you can vary the AGC speed with switch D, to just scan a band, sh(ort) is recommended, for SSB / CW reception, l(ong) might be your choice.
The standard RA-1217 has an output power of only 10 mV, just enough to drive some headphones. For reception of the VLF band below 0,98 MHz, a converter is used to shift the frequency in the 2 - 3 MHz I.F. range. There has been an optional panoramic display to visualize the R.F. signals in a certain band segment.
The signal path of this receiver based on the “Drift cancelling system” respectively the “Wadley Loop” circuitry, developed by Dr. T. Wadley for Racal, is a bit complex:
In contrast to standard superhets, the receiver comes without a standard bandswitch: the MHz tuning knob selects one MHz segment of the shortwave spectrum, the kHz tuning knob is used to tune to the desired frequency within the 1 MHz frequency segment.
The circuitry is based on two loops controlled by the first VFO, the MHz tuning system. One loop is the “Signal Loop”, the other one the “Harmonic Mixer Loop”. In the “Signal Loop”, the output of the first VFO is mixed with the R.F. signal which results in a spectrum with a bandwidth of around 1 MHz. In the “Harmonic Mixer Loop”, the second portion of the output of the first VFO is mixed with the harmonics of a 1 MHz reference quartz oscillator.
Both mixed products are subtracted in the second mixer stage, a frequency drift of the main VFO will be substracted in the two parallel signal precessing paths and will disappear, that's why the output signal from the second mixer is very stable, a special high stability reference quartz oscillator can be used to further improve stability.
In the case of the Racal RA-1217, the radio frequency signal has to pass the preselector and is mixed with the output of the first VFO (running from 41,5 MHz (1 MHz range) to 69,5 MHz (29 MHz range) in the „signal loop“ to result in an intermediate frequency of 40 +/- 0,65 MHz, only this signal will pass a 40 MHz bandpass filter.
In the “Harmonic Mixer Loop”, the output of the first VFO will be mixed with the harmonics of a 1 MHz quartz oscillator, all exact multiples of 1 MHz, in the „Harmonic Mixer“ and the product will have to pass a narrow 37,5 MHz bandpass filter.
In the second mixer, the output from the “Signal Loop”, the signal in the 40 MHz range representing one Megahertz of the shortwave spectrum, is mixed with the 37,5 MHz signal from the “Harmonic Mixer Loop” - subjected to the same VFO drift - to result in an intermediate frequency of 2 - 3 MHz (exactly 1 MHz wide).
This signal is mixed with the signal of the second VFO used for kHz tuning: From the 2 - 3 MHz spectrum and the linear VFO signal from 3,6 - 4,6 MHz, the result is a third intermediate frequency of 1,6 MHz.
After four amplifier stages and after having passed the bank of quartz i.f. filter, the signal is demodulated in a diode for AM signals and handed over to a product detector for SSB mode signals. For LSB and USB modes, the set has a fixed frequency BFO, for CW reception, the BFO's frequency can be tuned with the BFO note control.
The set is completely solid state.
The E-655 was used as communication receiver to monitor international shortwave brodcasters to provide news for the Swiss emergency broadcasting system of the APF, which was planned to maintain a national emergency radio and television service alive, in case of the Swiss Broadcasting Corporation or its facilities would be taken over by hostile forces.