Radio interference investigation receiver
GPO Receiver WT No. 11, was a portable radio direction finder,
made around 1937 1 by McMichael Radio Ltd. in Slough (UK), 2
for the British General Post Office (GPO).
The device was intended for
finding sources of radio interference on the Long Wave (LW) and Medium Wave (MW)
bands, such as clandestine radio stations and interference caused by
domestic equipment and power lines .
The device is housed in a black-painted wooden enclosure that measures
300 x 300 x 150 mm and weights 5160 grams (without the batteries).
It can be carried by the leather grip at the top.
Volume and frequency adjustments are at the top,
at both sides of the frequency tuning scale.
At the front is a signal strength indicator
and a speaker, with the letters GPO carved out of the wood.
At the rear are the sockets for ground,
external antenna and two pairs of headphones.
The antenna sensitivity can be adjusted freely.
It is powered by internal LT and HT batteries.
Although the device can be used with an external antenna, it was commonly
used with the internal directional window antenna, that is part of the inner
wooden frame. It allows the device to be rotated in order to obtain a
maximum (or minimum) signal strength reading on the meter.
During WWII, it was mainly used for monitoring the LW and MW
bands for clandestine stations.
In the 1950s, the receiver was used for locating sources of interference on
public broadcasts in the LW and MW bands. A nice account of the latter
was given by David Rudram in the June/July 1990 issue of Radio Bygones .
According to a label at the side,
the device shown here was last calibrated on 10 July 1962.
The device is also known by its circuit diagram number WL 28600.
In 1939, it was succeeded by GPO Tester WL 53400,
that was also suitable for the Short Wave (SW).
The images below give an overview of the controls and connections of the
WT-11. At the front is a large speaker, protected by a metal grid, with the
letters GPO (General Post Office) carved out in a wooden oval. To the right
of the speaker is the signal strength meter,
which can also be used to check the LT and HT voltages by altering the
position of the function switch situated above it.
At the right side is the power switch,
which also acts as the band selector. At the top is a
with a large tuning scale at the centre.
It has a separate scale for LW and MW.
At either side of the scale is a large bakelite knob for adjusting the audio
volume and for tuning the frequency.
At the rear are two 6 mm sockets for connection of the headphones.
Also at the rear is a panel with sockets
for connection of an external antenna
and ground. A rotary switch allows the user to select between the internal
frame antenna or an external one. To its right is the sensitivity knob.
The interior of the WT-11 can be accessed by removing the panel at the rear.
It is held in place by two U-shaped metal clips at the bottom and a shift-lock
at the top. By shifting the lock to the right the panel can be tilted
towards the rear, after which it can be lifted from the U-shaped clips.
Inside the wooden enclosure is a wooden frame to which the electronic circuits
are mounted. The inner frame also acts as a directional antenna. It
can be used to narrow down the position of
an illegal transmitter or a source of interference.
The actual receiver is mounted at the top right, as shown in the image on
the right. It consists of four miniature valves –
held in place by 2 spring-loaded metal brackets – and several filters.
Below the circuit is a large speaker that is mounted to the front panel.
To the left of the speaker are the terminals of the signal-strength/voltage
The antenna selector and the headphones sockets are located on separate
panels, mounted in the upper corners of the inner wooden frame.
At the bottom is space for the LT and HT batteries.
A wooden support bar – stamped with the text WT 11 – is present to keep
the batteries in place.
The black & white image on the right was kindly provided by Walter Sanger
in the UK , and was probably part of the original documentation of the
device. It shows the interior, seen from the rear, with the two battery
At the left is an Ever Ready Type B117 battery pack, that provides the
+90V HT voltage for the anodes of the valves. At the right is the
smaller Every Ready All Dry (AD) high capacity battery that provides the
+1.5V LT voltage for the valve filaments. Reproductions of these
batteries are still available today .
At present, no further information about this receiver is available.
We are particularly interested in finding the original circuit diagrams
and stories or anecdotes from former users.
➤ Contact us
DeviceRadio interference investigation receiver
IdentificationWT No. 11, WL 28600, MRL 50/1
ManufacturerMcMichael, Slough (UK)
UserGeneral Post Office (GPO)
LW150 - 300 kHz
MW300 - 1510 kHz
Valves4 (see below)
AntennaInternal (frame), external (wire)
PowerLT 1.5V DC, HT 90V DC
- Cor Moerman, GPO Receiver WT-11 - THANKS !
Received November 2020.
- David Rudram, Radio and Television Interference Work in the 1950s
Radio Bygones, Issue 6, page 9.
- Philips M. Moss, McMichael Company History
Slough, 1979/ Retrieved November 2020.
- Wikipedia, Radio & Allied Industries
Retrieved November 2020.
- Walter Sanger, B/W image of open WT11 with batteries
Personal correspondence, December 2020.
- Classic Radio Shop, More Radio Batteries
Retrieved December 2020.
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© Crypto Museum. Created: Sunday 29 November 2020. Last changed: Sunday, 01 January 2023 - 11:22 CET.