This unit is a plug-in part of the Gee Navigation
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
They use an 8-pin Mazda octal base which at first
sight looks like a standard octal base but has slightly different
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
Below:High quality ceramic
bandswitch and each of the five preset ranges has a set of three
Below: Rear view with label showing no modifications
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
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
Below: View showing a
Pye plug which connects the RF amplifier coil to an aerial tuning
Views of the label attached to the unit and original
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
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.
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
RF27: 65-85 MHz continuously
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 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
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).