RF25B

This unit is a plug-in part of the Gee Navigation System.

 During WW2 a method of assisting pilots in blind landing, and later finding a target for bombing was designed by ARE, at the time located in Swanage, later RSRE to be located in Christchurch and then Malvern. The original Gee system (named after the radio "grids" developed by the transmitters) operated between 20 and 30MHz but the later MkII used a wider range of frequencies, had anti-jamming and internal detonators to destroy the equipment if it became necessary.

The four RF Units including the RF25, were part of Gee MkII.

David Marshall from Brockenhurst kindly donated an RF24B, the lower frequency version and an RF26B, one of the higher frequency versions (click to see these)

Front view showing the 5 preset channels. The RF24 was similar but the RF26 and 27 had slow motion tuning instead of a switch

Below: Low loss ceramic coil formers and beehive trimmers.

 Below: Three VR65/SP61 pentode valves, RF amplifier mixer and oscillator. Although very common in WW2 equipment these Mazda valves were quite advanced in their design.

Like the EF50, the SP61 was intended for use in TV receivers, before WW2 put an end to that idea, or at least postponed it.

They use an 8-pin Mazda octal base which at first sight looks like a standard octal base but has slightly different pin spacings.

Why would there be two different shaped valve envelopes? Did Mazda sub-contract their manufacture? Interestingly the metal coating is perfect on the outer two valves but about to drop off the centre valve perhaps implying a different maker.

 

 Below: Very clear layout with carbon composition resistors and moulded mica capacitors; the IF output coil is mounted on the lower edge of the chassis allowing it to be tuned for maximum output.

IF output coax linked to the Jones plug.

Below:High quality ceramic bandswitch and each of the five preset ranges has a set of three beehive trimmers

 Below: Rear view with label showing no modifications incorporated

Power is supplied from the R1355 chassis via the 6-pin Jones plug. Note the manufacturer's identification stamp "F.R. 5 G. B." The serial number plate also carries leading characters of "F.R." Is this Ferranti? Maybe someone knows?

After WW2 the number of UK Defence Electronics contractors gradually dwindled. In the field of military computers the three largest being Plessey, GEC and Ferranti although there were lots of smaller companies such as Elliott Automation. There were numerous take-overs, mergers and bankruptcies. British Aerospace bought into areas other than aircraft manufacture and absorbed the remnants of Plessey.

 

Below: View showing a Pye plug which connects the RF amplifier coil to an aerial tuning unit.

Views of the label attached to the unit and original packing.

The information indicates 14MU which is RAF Carlisle and presumably the unit whas been removed from an aircraft or another RAF station stores

I think Form 4509B indicates the item is up for disposal.

 

 

 

 

A little about Gee

 There were sets of synchronised transmitters sending trains of pulses received by the aircraft-bourne Gee receiver and the distance between these and the aircraft displayed as a periodic display on a horizontal trace on a Type 62 Unit.

Drive to the 62 Display Unit came from an R1355 Receiver which carries the IF strip, handling the output from the chosen RF unit, video amplifier and power supply. The R1355 has a front slot into which one of the RF units could be inserted.

See the R1355 circuit diagram

The four equipments, RF24, RF25, RF26 and RF27 were capable of reception of a specific band between 10MHz to 20 MHz wide as follows.

RF24: Tunes to one of five switchable frequencies between 20-30 MHz.

1= 22MHz, 2 = 22.9MHz, 3 = 25.3MHz, 4 = 27.3MHz, 5 = 29.7MHz

RF25: Tunes to one of five switchable frequencies between40-50 MHz

1= 43MHz, 2= 44.9MHz, 3= 46.79MHz, 4= 48.75MHz, 5= 50.5MHz

RF26: 50-65 MHz continuously tunable

RF27: 65-85 MHz continuously tunable

The receiver of choice was connected to a simple whip aerial mounted on an aircraft, via a simple tuning unit type LU51, and has an IF output of 7.5MHz which was amplified in the R1355. As the aircraft had to be simultaneously in range of a set of three transmitters it was possible to try reception from 20 to 85 MHz by plugging in one of the receivers listed above. This choice would help the system to work despite the vagaries of skip and propagation.

As jamming was prevalent in WW2 four preset options were available in the R1355 MkII to reduce the effect.

The system proved to be so useful it remained in service up to 1970. Whilst at camp with the Air Training Corps at RAF Kinloss in 1956 we were given a task of searching an area of heathland for debris from a crashed aircraft. I remember finding a mangled piece of metal which I recognised as the chassis of an R1355. Other than in adverts in magazines for government surplus that is the only R1355 I've seen, although I've had lots of the RF units which I used for parts.

Surplus RF units in the 1950s were useful for receiving Band I TV sound. In Liverpool this was available on 48.25MHz which fell neatly into the range of the RF25 where judicious twiddling of the beehive trimmers magically gave one TV on headphones. This could be done for 7/6d or about 37p when at the time a TV set cost an astronomic amount equating to at least 2 months average wages.

 

Testing the RF25B

Out of interest I powered up my latest purchase from a power supply delivering 250 volts and 6.3 volts for the valve heaters.

 Above, the circuit showing an RF amplifier V1, Mixer V2, and local oscillator V3.

Results were excellent and are shown below.

The pictures need a little explanation.

The first shows the local oscillator signal on the right and indicated as 52.3MHz. This is only roughly correct as it reflects the accuracy of the marker placement.

Take no notice of the indicated signal levels as I'm using a special attenuator to protect the analyser from high voltages. See this explanation

On the left is a scan of the response of the RF unit from its aerial socket to its IF output connector as seen by the tracking generator which is sweeping from 30MHz to 60MHz.

The peak of the response is set by the tuning of the receiver and, in this particular case indicates the setting of the beehive trimmers for Range 2 as well as the tuning of a couple of IF coils.

Essentially you are looking at the response of the unit to a signal at the local oscillator frequency minus the IF.

Because there is no IF amplifier the response is rather broad, but peaking at 7.5MHz below 52.3MHz or 44.8MHz. As each horizontal division represents 3MHz the response at the -60dB level is from 42MHz to 47MHz, centred at about 44.5MHz.

As the unit is switched to higher ranges the response curve keeps its shape and moves to the right tracking the oscillator by 7.5MHz.

 

 In case you're not familiar with this sort of picture, the centre of the display above is set to 45MHz and the span is 30MHz (meaning that each horizontal division is 3MHz apart) so the vertical divisions are as follows:-

30MHz (left edge)-33MHz-36MHz-39MHz-42MHz-45MHz (centre)-48MHz-51MHz-54MHz-57MHz-60MHz (right edge)

Amplitudes as shown are only relevant in terms of differences in their heights because I'm using an uncalibrated probe. Marker 1 frequency is indicated on the screen.

The next picture has the centre frequency set to 10MHz and the span 10MHz (ie each vertical division is 1MHz apart) giving a sweep of 10MHz plus/minus 5MHz.

The second picture shows the response at the IF without connecting the tracking generator, but intead using a signal generator adjusted to approximately peak the IF signal.

Although this is indicated as 8.33MHz it's true frequency is 7.5MHz and shows the response of Range 2 of the RF25 to an incoming signal of about 44.8MHz originating from my TF2008 signal generator.

The scan setting is 5MHz to 15MHz. I just tuned the generator to peak the curve hence there will be a small error, although the settings of the beehive trimmers and ageing of components will no doubt be responsible for some of the 830KHz difference.

 

Below, offered on Ebay some time ago, a rather tatty R1355 complete with an RF24 

 Below, a picture of a pristine example of an Indicator Unit Type 62A for a Gee setup. This version has a line of VR65 (SP61) valves but newer versions used the VR91 (EF50).

Gee handbook

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