AT5/AR8

Built by AWA to a Royal Australian Air Force spec in WW2, this radio was so versatile that it was used by all three services as well as the US forces.

Read the Military Collectors Group Post about the AT5/AR8.

Below: AR8 receiver.
 

Below: AT5 transmitter.
 

Below: Aerial Coupling Unit for AT5.
 

Below: Type G power supply.
 

COMMENTARY

The AT5/AR8 is a MF/HF transmitter receiver set. Designed originally for the Royal Australian Airforce, it saw extensive use in aircraft, in the ground mobile and ground fixed applications, and marine usage.

It consists of the AR8 receiver, the AT5 transmitter, Aerial Tuning Unit for  the AT5, and a 12, 24, or 240V power supply. There is also a junction box and a remote control unit.

The AR8 receiver consists of two independently tuneable front ends alongside one another, one for HF, the other for MF. There is a MF/HF switch which selects the frequency range.  The receiver can operate from the main antenna fed via the change over relay in the Aerial Coupling Unit, or a free aerial may be used. The RF stages are protected during transmission as the keying circuit disconnects the cathode for the tubes in the RF stages. The intertuning function allows this function to be over-ridden. A winding on the modulation transformer routes sidetone into the receiver during transmission. There is a facility for a loop connection to allow direction finding bearings to be made. 

Oak switches are used to select the frequency range. Two large vernier dials select the receiver frequency, one dial is for MF the other for HF. There is a mode switch to select AM or CW/DF. There is a volume control, a tone control and a BFO note control. There are additional controls for taking DF bearings.

The receiver is a dual conversion superheterodyne type. The aerial input is protected by a 6X5 diode. The audio output stage also doubles as an intercom amplifier. Microphone and headphone positions are connected into the junction box.

The transmitter consists of two seperate master oscillators, one for MF and one for HF. On MF a VFO is used. HF allows the use of either the VFO or a crystal oscillator. In HF mode the 6V6 oscillator drives an 807 buffer amplifer, which drives the two 807 finals. In MF mode the 807 buffer serves as the oscillator, directly driving the finals. A 6V6 serves as a tone oscillator or voice amplifier which modulates the finals on MCW and RT modes. The oscillator uses an overtone system at the higher end of the transmission frequency mode, thus output power is reduced at higher frequencies.

Controls allow the selection of bands for the Master Oscillator, Buffer Amplifier and PA stages. There are separate tuning controls for MF and HF operation. The line switch disconnects the transmitter from the Aerial Coupling Unit and switches a resistor in series with the screens of the finals. This allows the transmitter to be tuned without emitting an on-air signal. The mode switch turns on the transmitter HT supply (normally a motor generator set) and selects CW, MCW or RT. There was an optional external pulse unit which could be used with the transmitter.

The aerial coupling unit connects the transmitter and receiver to the type of aerial likely to be found in an aircraft. The unit actually consists of two independent couplers in the one case. One unit is used for  tuning trailing aerials on MF, and the other tunes a fixed MF aerial. Current transformers are used to give an indication of aerial current. The MF stage used the massive switched coil/variometer arrangement one would expect of an MF tuner. A variable capacitor is also used. The HF tuner uses a roller inductor and a variable capacitor. Capacitance can be switched in as series or parallel. The antenna coupling unit also contains the keying relay and the MF/HF relay.

The Type G power supply is the 24Volt version. It consists of two motor generator sets, one is the receiver machine, the other for the HT. There are various control circuits in the power supply. The 12Volt version is very similar but it uses 12Volt MG sets. The 240V version was used primarily for fixed applications. It is mounted in the same case as the DC power supplies, however the MG sets are replaced with 866 mercury vapour rectifiers and a suitable 240V transformer. This is contained in a cage above the base of the power supply.

The AT5/AR8 is a classic Australian Designed example of a World War 2 vintage transmitter receiver. It saw extensive use in most roles. In the ground role it was used as a beacon transmitter, RADAR relay stations used the AT5. It was used in radio trucks, mounted on the back of jeeps, as emergency radio equipment on RAN ships, and as primary radio equipment on light boats. In the air role it was used in aircraft such as Beauforts and Catalinas.

This set is quite fun for CW or AM operation on the ham bands. I found that the VFO chirps like crazy on CW, I have not used the set on AM. The receiver is quite ordinary, although I have not aligned it, but it isn't really a top line comms receiver. The transmitter is good fun, although you need to use crystals to avoid chirp. I think you have to expect this with most sets of this era. The ATU can be a handful, it depends on the type of antenna you have. Generally it is difficult to tune on 20 metres, although I have had good success on 40 and 80 metres. 

TECHNICAL SPECIFICATIONS

Frequency Range:
    Receiver    140-740Kc, 765Kc-2Mc, 2Mc-20Mc
    Transmitter    140-500Kc, 2-20Mc
Modes:    AM, CW, MCW, Pulse. D/F on receiver.
Power Output:    50W on fundamental frequency

CONSTRUCTION

Construction of the AT5/AR8 is quite interesting. The general layout is a typical WW2 design, but the sections of each unit have been designed in a modular fashion. For example, the HF RF stage of the receiver can be removed as one unit by desoldering a small amount of connections. This allows the entire module to be removed an replaced in the event of a fault. 

Below: AR8 receiver top view.
 

Below: AT5 transmitter rear view. The two finals are at left of shot.
 

Below: Aerial Tuning Unit for AT5. The HF tuning section can be seen at bottom right of shot. At rear left is the massive MF tuning coil.
 

OVERALL RATINGS

    Engineering rating (taking into time of design).

            A lightweight and solid design. An early example of modular design. 9/10

    Fitness for purpose rating

             This radio was so good that it was used in many roles it had not been designed for. 11/10.

    Usefulness as ham equipment rating

            AM or CW only. You really need to use crystals for stability. Receiver is not really good enough for
                 ham use. 7/10

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