AN/GRD-501 D/F SET

The AN/GRD-501 is a  3 to 30 MHz DF set developed for the Royal Canadian Navy. It is a twin-channel Direction Finding set and being in the AN/500 series,  it was designed and built entirely in Canada. Development of this system appears to have started around 1954 as evidenced in one of the National Research Council documents.

 During the fiscal year 1958-59, funding in the amount of $305,000 was approved for the installation of AN/GRD-501 DF sets at the following locations: Gloucester, Coverdale, Churchill, Masset, Frobisher Bay and Inuvik. By May 1959 , the installations at Gloucester and Coverdale had commenced and  the tendering for Gander having recently closed.

 Each GRD501 site shared some common elements:

* Cleared area.
* Access roards
* Hut that was 8 ft in diameter sitting on a concrete pad of 20 ft diameter.
* Ground mesh  of 100 feet radius.
* Telephone, power and coaxial.

Available Specifications:

Frequency Range: 3 to 30 MHz  in 4 bands. 3 - 5.3 MHz; 5.3 - 9.5 MHz; 9.5 -17 MHz and 17-30 MHz.
Selectivity: 1 kHz, 3 kHz or 9 kHz.
BFO: One of three frequencies could be selected - 174 kHz; 175 kHz or 176 kHz.


 
coverdale_grd501_a.jpg
GRD-501 console. The unit did not have a patterned finish. That's the result of scanning a dithered source image. (Photo courtesy RCN)
A GRD-501 antenna site contained 8 eight monopoles approximately 15 feet in length. These monopoles were installed on top of a circular wire mesh ground plane consisting of wires laid 90 degrees to each other forming approx. 4 to 5 inch squares. Each intersection of the 4 inch  square was bonded to the opposing wire. This created a large circular wire mesh system that was terminated around it's circumference with a ground rod system. The ground mats at the GRD-501 sites in Coverdale and Frobisher were laid on the ground and not buried. Many many ground rods were driven to which the ground mats were fused. The antenna array and portions of the receiver were collocated at some distance from the main console.

Remote equipment was housed in a prefabricated hut inside the antenna array and was connected to the main equipment bay by seven buried cables. To aid in servicing the remote unit, there was an intercom system which provided a two-way link between the operator's console and the remote unit.

One of the key features of the set was the recorder section. There were two recorders, each with a total recording time of 2 minutes. One unit could record while the other was used in playback. On playback, the bearing of the recorded signal and its modulation characteristics could be displayed on two separate CRT's. Because the signals were stored on a drum, whey could be played back as many times as desired. This feature was particularly useful when dealing with short transmissions. In direct mode, the output of the AF amplifier was fed to the speaker. In OFF mode, the speaker was disconnected entirely while in playback, the recorder output was connected to the speaker.

When tuning with the Outer Tuning knob, the operator would automatically activate a motor drive which could sweep an entire band in 4 seconds. The inner tuning knob was used for manual tuning. On coarse tuning, the main dial could only resolve frequency in increments of 100 kHz. Vernier tuning was implement by a 0 to 100 kHz scale which provided further resolution..

Another feature of the set was the dot-lock circuit. This permitted the set to only display that portion of the signal which arrived by the most direct means and excluded any portion of the signal which arrived via other paths.
 

overdale_grd501_b.jpg
A sketch of the GRD-501 operator's controls. (Photo courtesy RCN)

Another feature of the set was the printing clock. The operator would insert a paper card into a slot next to the clock. Any events of interest could be time stamped on paper by manipulating handles which were closely situated to the clock. This also recorded a short burst of 400 Hz tone on the recording drum used at the onset.  During playback, it was also possible for the operator to interact with the playback and determine the precise time interval of a recorded signal on the paper card.

Eric Earl who served at Coverdale, Gloucester, Frobisher and Inuvik, says he never saw the recorder section of the the GRD-501 in working condition at any of these sites. "I remember a P1 working on the recorder at one time in Coverdale, but also remember him saying that they never could get the thing working".

Gord Walker recalls some of the maintenance chores with the GRD-501.

"The GRD-501 itself was a multi-band receiver. A rotatable turret assembly consisting of coils and capacitors came in contact with stationary contacts mounted on this chassis. These components on the turret formed the tuned circuits for each band.  The tuned circuits were similar to small metal cans with a series of button contacts along one side and although they were supposed to be pre-tuned, there was an adjustment to calibrate the coil and/or capacitor in each "can" for resonance.  Button contacts on the turret assembly met fixed leaf contacts, and because it was a mechanical action, the contacts required regular cleaning and in the case of the leaf contacts, occasional replacement, as they would wear through in time.

The antenna array was a circular configuration with a small shack adjacent to it. This was known as the "remote site" because some of the control electronics for the array were situated there. When the operator changed bands on the set, a similar band-change system was operated by a servo motor at the remote site.   The intercom from the remote site was always on  so the operator could hear the bands change at the remote site and know the set was functioning properly.  If the operator wanted to speak to a tech at the remote site, he had to press and hold the TALK button.

A tech faced a potential danger when working at the remote site. It was possible that the operator could change bands without warning thus catch the technician's hands or fingers in the revolving machinery.  If the band was changed while the tech was working on it, he might utter some involuntary and totally uncomplimentary words about the person who was responsible for the near loss of his fingers. These  words which blared into the Operations room for all to hear, were okay as long as it didn't happen to be the Operations Officer who changed the band.

The array itself required periodic accuracy checks. This consisted of having a person carry a portable signal generator tuned to a specific frequency and place it immediately behind wooden stakes positioned at 15 degree intervals. Once each radial was checked, the signal generator was moved to the next stake until the entire check was completed ".

Stuart Morrison, K4BOV, adds this errata to the GRD-501 story from an e-mail dated 2001. .

"Through the years we evolved to the CDAA; however, we had to go through some steps. between the GRD-6 and FRD-10. One interesting system was never seen by the majority of USN DF'ers - the GRD-501. It was ordered by the USN in the late 1950's; but, by the time it was completed, the USN elected to press on to the FRD-10. Since the Royal Canadian Navy was in liaison with our HFDF efforts and in order to upgrade the "north of the border" CNF-4 system, the GRD-501 was sold to the Canadian Navy for $1 per unit - they took 4 of them, and coincidentally, I ended up assisting the with installation and calibration team for that system in 1962".

There is additional technical intormation about the GRD-501 at this link.
This link points to four others:

* Proposed modifications for improved performance of AN/GRD-501
* Dot-lock and blanking unit for twin-channel H/F D/F equipment AN/GRD/501
* Operating and maintenance instructions, recording equipment of AN/GRD-501 direction finder: provisional
* Development of a twin-channel HF radio direction finder

Schematics for the GRD501 are contained in document "Operating and Maintenance Instructions".

OTHER PHOTOS

coverdale_grd501_adcock_array.jpg
This was the Adcock array used with the GRD-501. The antenna site contained 8 eight monopoles approximately 15 feet in length. 
coverdale_grd501_unit_with_drawers_out.jpg
Overall view of the GRD-501.
coverdale_grd501_rcvr_block_diagram.jpg
Block diagram of the GRD-501 receiver only. 
coverdale_grd501_recorder.jpg
Physical view of the GRD-501recorder. The recorder was required to store for 2 minutes, the bearing (to within one degree) and the modulation of transient or steady signals which lasted for at least 1/10 of a second. 
All photos in this table courtesy DND or National Research Council

Contributors:

1) Doug Stewart dougjoy(a)ns.sympatico.ca
2) Eric Earl,  KG4OZO, Atlanta Ga.  <eearle(at)adelphia.net>
3) Gord Walker   <walker6(at)sympatico.ca>
4) Library and Archives Canada . Document  NS 7400-189/165
 
 

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Mar  13/17