Technical Materiel Corp. - GPT-10K (AN/FRT-39) Transmitter

In 1966 and 1967, Eric Earl reports that he was tasked to keep  watch on at least two AN/FRT-39B (GPT-10K) transmitters at the Scoudouc site. These were located on the north side of the floor an collocated with Department of Transport Air Traffic Control transmitters . "The main reason for being there was to change frequency or, in the event of failure, to switch to the standby unit. In that era the transmitter fed a folded dipole using 600 ohm ladder line.

Modulation was provided by  VFT (Voice Frequency Telegraphy) modems, and a  Frequency Division Multiplexor (FDM) with inputs from the crypto equipment located at Coverdale. This equipment was part of the wideband radio network for the North Atlantic HFDF net. Eric says " I also remember very vividly that we had nothing but trouble trying to sync our on-line crypto equipment in Coverdale with the outputs from the matching synthesized TMC receivers from this same net".

Crypto traffic was sent over the air (OTA) or by landline depending on type.

1) LANDLINE - Operations/administrative/personnel traffic was sent to Ottawa or other SUPRAD stations via 75 baud landline using the Adonis cryptosystem. This traffic would have been handled exclusively from the COMCEN. None of this traffic was sent by radio with the exception of  Frobisher (via radio backscatter) and possibly Alert.

2) OTR- the HFDF wideband radio Flash network.  The Atlantic HFDF Net migrated to on-line operation in the early 1960s, with the introduction of the KW-37 JASON cryptosystem. Traffic to and from this network would have been encrypted by JASON and transmitted via the GPT-10K. Every outstation in the HFDF network used radio transmission as as integral part of "flash" traffic but if the landline(s) failed, then all the landline traffic was sent over the air.

3) Manual CW link -  Eric Earl distinctly remembers communicating with Frobisher Bay on CW but there may have been others. The only reason for communicating with Frobisher on CW was in the event that the backscatter wideband radio link from Frobisher to the south would have been down for one reason or another. It was maintained and used by the USAF as part of the DEW line. The link from Frobisher was to Cape Dyer then south from there.

Bascially, the flash radio system would have worked like this.


a) EVERY station in the Atlantic HFDF network would have had to have been setup with similar transmitters and VFT  modems.

b) EVERY station would have to transmitting and receiving on exactly the same frequency at any one point in time.

c) Each individual station was assigned a pair of VFT's  to modulate the common transmission frequency as an outstation or, if directed, as a net control station.


Let's suppose that Coverdale was the net control.

Coverdale would be transmitting to the network outstations on a single pair of tones on the common frequency that everyone was tuned to. This transmission, being in a flash format would request that all outstations listen on a particular frequency for a specific target.  The outstations then would act on this flash message and would  transmit their bearing information on their own individual set of tones back to Coverdale (net control) which would be received by one modem per outstation and then fed into crypto equipment and subsequently to a printer where the data could be taken and plotted for that station on a master plot board.  In spite of TMC's phase-locked master oscillator system it was difficult to achieve stability at times.

Eric goes on to say "I can vividly remember these VFT transmissions up until about the  mid 1990's when they all pretty much went dead. I believe that the USAF and AT&T were the last users of this multichannel, very narrow band technology".

GPT photo and specifications courtesy TDP web site, Belgium. <>


Frequency range - 2 to 28 mHz
Power output: 10 kw PEP
Modes of operation: SSB, ISB, AM, CW, FSK, and FAX
Output power : 10,000 watts peak envelope power; Emergency : 1,000 watts
Frequency Control: Synthesized frequency control in 100 (cps) incremental steps
Keying: FSK : 75 baud (100 wpm) maximum 50v, 100v, 20 ma, 60 ma, all neutral, floating, or  either side grounded. 12 to 1,000 cps shift.
CW : 140 baud maximum.

Mains Power Input: 210-250vac, 50-60 cps, three phase,  approximately 50 amps/phase (20kw)
Operation at 390-480vac, 50-60 cps, three phase, approximately 25 amps/phase, was also available.

The RCN procured the GTP10-K as the AN/FRT-39D since TMC made these to military specifications. Besides Coverdale, there were also examples of this transmitter at CFS Massett and CFS Bermuda.

Here is a short chronology of the AN/FRT-39 development:

Radio Transmitting Set, HF, 2-28 MHz, 6 bands, 1 channel. A 5,000 watt
band switched transmitter used for fixed plant, point-to-point,
ground-to-air and shore-to-ship comm. Primarily designed for SSB
operation but may also be used for CW, FS, DSB, AM, MCW and SS
suppressed carrier.

Interchangeable with the AN/FRT-39 except for output power raised to 10
KW and it has a 7.5 kHz audio bandwidth in lieu of a 3 kHz bandwidth.

Synthesized sideband excitation added.

Oscillator-Power Supply group improved.

Oscillator improved.


1) Eric Earl,  KG4OZO Atlanta  <eearle(at)>
2) George Wilton <>
3) Military List Equipment Page
4) TDP web site, Belgium. <>

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Jan 21/10