The AN/GRD-6 Direction Finding system automatically furnishes azimuth  indications within the frequency range of 2 to 32 MHz for targets of interest. Coverage is split into two parts. One antenna array coves the low portion of the HF band from 2 to 8 MHz while another array covers the high portion of the HF range from 8 to 32 MHz. The system is comprised of two antenna arrays, a radio receiver and a bearing indicator. Antenna elements forming the low HF group are terminated, folded, monopoles forming a U-Adcock configuration of four pairs of monopoles, equally spaced to form a circle. High band HF antennas are broadband sleeve or cage type antennas. Low band elements are 27 feet long and are spaced around the circumference of a circle 100 feet in diameter. The high band elements are 15 feet long, and the circle they form is 25 feet in diameter. In the centre of each circle is a sense antenna.
Buried 2 to 4 inches under each antenna array is a circular mat constructed of copper wire mesh. The low band mat is 300 feet in diameter, the high band mat 100 feet. As can be seen by comparing figures, the ground mat extends well outside the arrays. This ground mat reduces polarization error and provides a stable ground system allowing consistent and reliable direction finding performance, independent of local ground conductivity.
The R665 receiver used in the system covers the entire frequency range in four bands: 2 to 4 MHz; 4 to 8 MHz; 8 to 16 MHz and 16 to 32 MHz.
Two receiving groups are used with the AN/GRD-6, one for the low band and the other for the high band. Each performs similar operations for its corresponding frequency band and is almost identical in function and physical appearance. Each receiving group consists of several units but the operator will be concerned primarily with the superheterodyne receivers. The receiver converts RF signals from the antenna array to suitable deflection current for display on the indicator cathode ray tube. In addition to the deflection current output for the indicator, the receiver also supplies audio output which may be applied to a loudspeaker or headset.
Both the high and low band indicator groups are composed of an azimuth indicator and an indicator control. The prime function of this group is to provide a visual DF display to the operator on the face of the cathode ray tube to indicate the direction of arrival of an incoming signal.
Additional details on the operation of the AN/GRD-6 can be found in NAVSHIPS 10232-A (Communications Technician M 3 & 2) Included are functional and overall block diagrams of the system. Note that this is not a GRD-6 technical manual but an extract from Chapter 18 regarding direction finding techniques using the GRD-6 as an example.
|Developed in 1958 by the US Naval Research Labratory (NRL), the highly
classified AN/GRD-6 system was comprised of two arrays and associated
electronics. The high HF array, depicted here, consisted of eight monopole
sleeve antennas and eight, terminated, folded monopoles for
the Low HF band, arranged in two separate circles. In the middle of each
circle was a sense antenna.
The arrays could also be installed in two concentric rings if space was a premium. Such was the case for Canadian Forces Station Bermuda, where guyed, rod antennas were used. The concentric ring configuration was staged and tested by the United States Navy at Winter Harbor, Maine.
The monopoles were also known as "self-sustaining broadband sleeve antennas". This design greatly reduced the octangular  error of the GRD-6 set. (NRL photo #37851)
|This is a schematic of the terminated, folded monopole antenna which comprises the low HF array. Note the mast diameter of 4 inches. (Drawing courtesy USN)|
|This photo illustrates the high HF antenna array. (Photo by Dave Maberry via the Naval Security Group web site).|
|GRD-6 low HF antenna array. (Photo courtesy USN)|
|A closer view of the lo HF array monopole antenna. (Image courtesy USN)|
|At Terceira Island in the Azores, the area of land used for the antenna pad relative to the cleared area is clearly illustrated. The low HF array sat on a pad 100 feet in diameter while the cleared circle was around 550 feet. For the high HF array, cleared circle was about 190 feet and the pad was 25 feet in diameter. (NSGA web site photo).|
|AN/GRD-6 system electronics. This configuration
only represents one half of the required equipment since the high and low
arrays each had their own dedicated electronics but sharing the bearing
sender and teletypewriter. See the functional block diagram below.
The system consisted of the following components: Azimuth Indicator IP-272, Teletype Coder KY-168, Switch Box SA-430, Receiving Group Control C-1647 and Radio Receiver R-665. All component designators are suffixed with GRD-6
Some web sites *incorrectly* state that the R-278A UHF receiver was used with AN/GRD-6. The R-278A belongs with the AN/CRD-6 UHF/DF system. It appears to be a propagated error among the web sites.
|This functional block diagram suggests that the GRD-6 was comprised of two separate direction finding systems and the only commonality between them was the Bearing Sender for the Teletype circuit. (Diagram courtesy USN)|
The IP-272 azimuth display consists of a CRT using magnetic deflection. Its sweep speed is synchronized with the goniometer rotor speed. Deflection is adjusted in such a manner that for no signal condition the spot is travelling around the edge of the CRT. When the signal is maximum, the spot is at the centre of the screen. With this arrangement, the familiar propellor pattern develops from the Adcock antenna system.
|IP-272 azimuth indicator controls. (Image courtesy USN)|
Bearing data observed by the operator on the CRT is translated into teletype code and automatically transmitted to recording equipment by the Bearing Coder group. The text calls it a Bearing Coder but the device in the equipment photo is labelled Teletypewriter Coder. The KY-168 is not a cryptographic device since it lacks the TSEC (suffix) designator in the model number. Bearing data can only be sent after the operator has determined the sense of the signal . Pushing the quality pushbutton determines strength and readability of the signal then launches some other events which cause the bearing to be read, coded and sent over the teletype circuit. Additional detail on this operation is found in the NAVSHIPS 10232 manual.
Any signal within the receiver's frequency range received by the antenna system causes a bearing pattern to appear on the indicator screen. The alidade  ring on the indicator is then rotated by the operator until the arrowheaded cursor line bisects the tips of the propeller pattern illuminated on the face of the cathode ray tube. The direction of the pattern is next determined by depressing the sense switch on the indicator panel This operation replaces the propeller pattern with a cardioid  pattern. Figure 12-4A illustrates the propeller pattern and 12-4B the cardioid pattern.
|Figure 12-4A top and 12-4B bottom. (Image courtesy USN)|
If the cardioid or sense pattern is in the direction of the tail of the arrow (leaving the arrowhead clear), the bearing is direct; if the sense pattern is in the direction of the arrowhead, the bearing is a reciprocal. The appropriate pushbutton on the switch box is then depressed. If the sense of the bearing cannot be determined, the "no sense" pushbutton on the switch box is depressed. The indicator lamp lights after its respective pushbutton has been depressed. One of the quality indicator pushbuttons is next depressed, depending on the quality of the bearing to be transmitted. If a bearing pattern is not present on the indicator screen when attempting to obtain a bearing, the "no bearing" pushbutton is depressed. Figure 12-5 shows the location of the various pushbuttons and controls on the switch box.
|Figure 12-5. Switch Box SA-430 controls (Image courtesy USN)|
The need for accurate bearings cannot be overemphasized. One negligent or careless operator can sabotage the efficiency of the entire HFDF net. The equipment itself has certain features which, if used conscientiously, will help eliminate operator error. One example of this is the arrow etched on either side of the alidade, which, if used properly (as you would use the sights of a rifle) will eliminate parallax. Another feature, although not part of the original AN/GRD-6, is the azimuth mask. The mask is inserted between the alidade and the amber light filter on the indicator and is etched showing the spread of bearings for each classification of bearing, thus providing an aid to the operator in classifying his bearings. With practice, accurate bearings may be obtained without using the azimuth mask, but remember, accurate and objective bearings and bearing classifications are always desired.
|This is the antenna plot plan for the AN/GRD-6 system. Click on image to enlarge. This plan also takes into account, the placement of transmitting/receiving antennas for internal communications between HFDF sites when a direct landline is not available. (Diagram courtesy USN)|
 The terms azimuth and bearing are used interchangeably throughout this document. USN DF manuals specifically define Bearing as degrees clockwise from true North. (and relative bearing from own ship heading). This is also true for many USN manuals. For example a paragraph headed "Bearing Indicating Equipment" says "...azimuth indicator and indicator control". Another says - "...the two bearings are 180 degrees apart in azimuth".
 Octangular - [ok-tang-gyuh-ler] having eight angles
 Alidade - [al-i-deyd] is a straightedge having a telescopic sight or other means of sighting parallel to the straightedge.
 Cardioid - [kahr-dee-oid] resembles heart-shaped curve, being the path of a point on a circle that rolls externally, without slipping, on another equal circle.
If you can answer any of these questions, please contact: firstname.lastname@example.org
1) What is the purpose and usage of the C-1647/GRD-6 Receiving Group Control?.
2) Apparently there is a technical film report which describes the installation of a model AN/GRD-6 DF Central. It shows the preparation of the site, installation of the low-band and high- band antenna arrays, and the handling of cables along with accuracy of plans and installation. Does anyone know how to get access to a copy of this film?
References and Contributors:
1) GRD-6 antenna array photo submitted by Will, W0EOM <W0EOM(at)aol.com>. Source book is titled "Evolution of Naval Radio-Electronics and Contributions of the Naval Research Labratory. NRL Report 7300 by Louis Gebhard. (1979)".
2) Naval Security Group site http://www.navycthistory.com/skaggsisland_introduction.html
3) Bill Robinson <newman-robinson(at)rogers.com>
4) Terceira Island http://www.navycthistory.com/images/azores19_2.jpg
5) Nick England <navy.radio(at)gmail.com> http://www.navy-radio.com/grd6.htm
6) 1964 USN manual .
7) Bearing vs Azimuth http://answers.yahoo.com/question/index?qid=20080614075648AAL4K1X
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