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Observations on the AN/GRC-109 Transmitter/Receiver
By Dave Lawrence, VA3ORP Introduction 1. Identification. The set used consisted of receiver R-1004A/GRC-109 (serial 496A), transmitter T-784A/GRC-109 (serial 37A), power supply PP-2684A/GRC-109 (serial 588A) and the complete spares kit with most item still sealed. The set was obtained in trade from another ham radio operator/collector. 2. History. This set was developed in the post WWII period and was initially used by the CIA as the RS-1 (1950-1964). With only minor changes from the RS-1 design, the GRC-109 was produced from approximately 1961 - 1969) with the GRC-109A being in production between 1969 - 1973. The RS-1 was widely used by American Special Forces teams in SE Asia until the early 1960s. Later, as the GRC-109 the set became the standard issue radio to all combat units in forward areas. Special Forces units continued to use it as a long-range set into the mid 1970s. An interesting accessory for this set is the GRA-71 which allowed Morse code burst transmissions to be made at speeds of about 300 WPM. This would have been a highly desireable feature in a conflict where the enemy had only basic radio direction finding equipment. Discussion 3. Power. Throughout the testing, the set was powered from a commercial 120 VAC source. The power supply with this unit is interesting in that it can use a variety of AC voltages as well as 6 volts DC. The power plug is also unique in that it is somewhat flexable allowing it to be used with outlets that have different spacing between the terminals. 4. Antenna. The manual for the set suggests that a 100 foot length of wire makes a suitable antenna. Attempt to load such an antenna were inconsistent. Rather than have a tuning meter, the set has three bulbs which illuminate to give an indication of proper tuning. On some frequencies there was not much indication from the output bulb. This may have indicate an unmatched condition or simply that when feeding a high impedance, there is not much current (even though power is being effectively transferred to the antenna). It was found to be more satisfactory to use a dipole or a vertical, both of which were close to 50 ohms. This gave an unambiguous tuning indication and seemed to function effectively. 5. Receiver. The receiver is a conventional superhetrodyne with a minimum of controls. Internally, the set is very well made with solid construction and an effective anti-backlash gear arrangement. Overall, the set was quite stable and pleasant to use. There were however, two significant tuning problems experienced. The first was that strong signals overload the receiver and causes it to shift frequency. When going from transmit to receive, it takes about 20 seconds for the set to recover. This can cause confusion when working a weak signal, especially if the operator starts making tuning adjustments trying to keep up with the drift as the receiver settles down. The second problem is that the gain control causes the set to change frequency. At 14 MHz this caused the received frequency to decrease by 2.3 KHz as the control was rotated from its minimum position to approximately the 9 o'clock position. This makes it difficult to have confidence that the zero-beating procedure has been performed satisfactorily Finally, the BFO adjustment is fairly course with one division on the scale being equal to about 3 KHz shift. 6. Zero-Beating. All of the above deficiencies made zero-beating a confusing procedure. The best system seemed to be to set the gain to minimum and the BFO to "0". Then key the transmitter and zero-beat as close as possible using the main tuning control. Fine adjustments were then made with the BFO control. Next, advance the gain control to about the 7:30 position and move the BFO about 1/8 of a division CCW. This sets the receiver fairly close to the transmit signal with the BFO being "below" the received signal. It is very important to understand the relationship between the incoming and BFO signals to prevent the operator from "listening" to the wrong side of the BFO. When the above procedure was followed, it was easy to confirm which side of the BFO an incoming signal was located. A slight CCW rotation of the BFO control caused the received tone to decrease, confirming that the incoming signal was on the same side of the BFO as the signal being transmitted. 7. Transmitter. The transmitter is a simple crystal controlled set producing between 10 and 15 watts output (into 50 ohms). It can be keyed using the built in key, by an external key or by a high speed keyer associated with a crypto unit (AN/GRA-71, not available at this location). The transmitter has a table stenciled on its panel showing the most likely settings for the tuning dials over its frequency range. This was very helpful in getting the set properly tuned. Light bulbs are used for tuning indicators and this arrangement is fully satisfactory. The only criticism of the transmitter is the manner in which the antenna connection is routed to the receiver. This circuit seems to have significant loss and it may be better, especially at lower frequencies, to use an external antenna relay. 8. On-Air Operations. Dozens of QSOs have been made with this set and, aside from the zero-beating problem and the slow AGC recovery, the set works very well. It tolerates a wide range of antenna impedances and has more than enough sensitivity. As with all sets of this era, the bandwidth of the receiver is greater than would be desireable on today's crowded amateur bands. It is necessary to have a rapid and accurate drill for zero-beating. Conclusions 9. This is an enjoyable set to operate. The receiver has a wide frequency range making it suitable for both transceiver operation and as a general purpose shortwave receiver. Being crystal controlled, the transmit frequencies are somewhat restricted, however this fact does make it simple and very stable. A great deal of thought has gone into the design of the power supply as it is able to operate on almost any power source encountered. The set is very well made with extremely strong cases. It is clearly intended for hard use. For field operations, the GRC-109 would be a very good choice. QSOs Completed by VA3ORP (Kingston, Ontario) for Vintage Operators Award 24 2101Z Oct 99 with N1OJE, Doug in Stockton, ME 7.110 MHz with RST 579 26 2345Z Oct 99 with N1KTL, Jay in Derry, NH 3.585 MHz with RST 529 27 0018Z Oct 99 with W3DOY, Tom in Warrington, PA 7.030 MHz with RST 599 28 2326Z Oct 99 with VE3RIH, Alan in Mississauga 3.670 MHz with RST 579 10 1929Z Jan 00 with N1BLY, Mark in Bridgton, ME 7.031 MHz with RST 559

AN/GRC-109 Transmitter/Receiver Photo #1

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