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Observations on the RCAF AT1/AR2 - General Purpose Radio By Dave Lawrence, VA3ORP
Introduction 1. The AT1/AR2 is a transmitter/receiver combination made for the RCAF by Canadian industry during early years of WW II. Intended for airborne use, the set has facilities for voice, cw and mcw. The set also provides an intercom function for the crew members and could be used for radio navigation. The AT1/AR2 was made for 12 volt use while a similar combination, the AT7/AR6, is configured for 24 volt operation. Frequency range is 1.5 to 20 MHz (plus 375 and 500 KHz) for the transmitter and 140 - 400 KHz plus 480 KHz to 21 Mhz for the receiver. Transmitter power output is approximately 15 watts. Technical details for the receiver can be found in RCAF CAP 63-2, June 1944 (C&E Museum Archive box 91). Data on the transmitter is found in RCAF CAP 63-1, June 1944 (C&E Museum Archive box 167). Discussion 2. Power. The set in use consisted of AT1 serial number 2802 (1943) by Canadian Marconi and AR2 serial 3559 (1943) by RCA Canada. Power to the receiver dynamotor was from a large, regulated 12 volt supply while the transmitter was connected to a 12 volt storage battery which was floating across a 12 volt battery charger. This was deemed necessary as the transmitter dynamotor draws about 15 amps key down. When first putting the transmitter into service, its main fuse blew. This was found to be a 20 amp item instead of the required 30 amp fuse. When this was replaced with the proper fuse, the dynamotor started normally. The high voltage fuse holder had an intermittent connection which was most troublesome to identify. 3. Antenna. The set was fed to an 80/40 M trapped dipole through a modern antenna tuner. This was found necessary in order to get the transmitter properly loaded. The set is designed to work into a load of approximately 10 ohms with 100 to 500 picoFarads in series. A dummy load was constructed with 10 ohms in series with 100 picoFarads. With this it was possible to get approxialtely 1.2 Amps of RF current. Without the tuner, proper plate and output readings could not be obtained when feeding a 50 ohm load. Even with the tuner, the set would not load on 80 M. To work into normal 50 ohm, coax-fed loads, some additional antenna tuning elements must be added. 4. Receiver. The receiver worked well on AM signals however the AVC did not seem to work well on CW and SSB. Much adjustment of the RF Gain Control was required to prevent the set from overloading. It was noticed that adjustment of this control also seemed to cause some shift in tuning. This was especially noticeable when trying to zero beat to the transmitter (the strong signal from the tx made it necessary to reduce the rf gain, when it was turned back up, the set shifted frequency slightly). In the end it was found best to zero-beat with the rf gain turned up. This required the AF Gain to be reduced and seemed to produce a less precise zero-beat than would have be desirable. After the set had been on for about two hours, there were some unexplained shifts in tuning (approximately 200 Hz). The cause of this was not identified. The connection to the antenna is through the transmitter's break-in relay. Although this is a good feature, the operation of the relay results in a very loud crash in the headphones. AF or RF gain must be significantly reduced while transmitting to prevent this. Some backlash was noted in the front panel drive mechanism, however the dial did seem to be easily "re-settable" (Note: the manual does outline a procedure for reducing this backlash). The main tuning control has a high ratio gear reduction making it slow to get from one end of the band to the other. At 7 MHz, one rotation of the dial shifts the tuning approximately 100 KHz. The BFO adjustment is very generous (approximately +/- 7 KHz). This makes for fairly critical tuning. The IF bandwidth of the set was not measured however it seems to be very wide. Also, it was not determined which way the control actually shifted the BFO. This should be determined so that the operator can be "listening" on the same side as everyone else, thereby eliminating some of the QRM experienced. 5. Transmitter. The set was used exclusively on Channel B, Master Oscillator. It was tried with crystals and gave appropriate output power however there seemed to be a delay in the power coming to full strength. This produced an unusual, slightly chirpy signal. The cause for this was not determined. On Master Oscillator the set seemed to be very stable and easily adjustable in accordance with the settings in the manual. While the grid drive was normal, the plate current and output indication were only about one-half of that expected. The reason for this was not determined however it is suspected that the unusual output load was the main reason. Power output was measured at 10 watts into 50 ohms (no SWR). The operation of the break-in relay must be examined more closely. It was noticed that when the transmitter was not turned on, there was little signal fed through to the receiver terminal. This condition disappeared as soon as the transmitter was powered up (even though not keyed). It may be that this relay has a neutral position in which the antenna is not connected to either the transmitter or the receiver. 6. On-Air Operations. On 24 January 1940, the first AT1/AR2 was used in a Noorseman aircraft flying between Trenton and Montreal. It seemed fitting that the contacts made for the Vintage Operators Award be made on the 60th anniversary of that flight. A total of six contacts were made that day at distances out to 649 Km. Signal reports were good (RST 579 to 599). Four of the contacts were to modern equipment (VE3RIH, NN8R, K1BXZ & WA2BQC) while one was to a Viking Ranger/Hallicrafters SX-111 combination (WA2QQF). Of particular note was a second contact with VE3RIH who was using a Wireless Set No. 19 Mk III (serial C88384) at a distance of 276 Km. Throughout the almost three hour operating period, no modern receivers were used for monitoring or back-up. A digital frequency meter was used to ensure that operations remained in-band. On 23 January 2000, this set was used to pass a commerative message to retired AVM Ralph McBurney in Ottawa Ontario. AVM McBurney was the RCAF's Director of Signals at the time this set was developed. Conclusions 7. In spite of the problems with the RF/AF gain when going to transmit, the break-in function of this set is a very nice operating feature. Once a sequence was worked out, rapid exchanges were possible. Having a VFO instead of crystal control is also excellent as it allows the operator complete freedom when looking for stations calling CQ. It does however demand that a rapid drill be developed for getting the zero-beat accomplished quickly and accurately. Once this skill is mastered, it becomes possible to rapidly scan the bands and quickly make contacts. Overall, the AT1/AR2 was found to be a very pleasant set to operate with a number of innovative features and good stability. QSOs Completed by VA3ORP (Kingston, Ontario) for Vintage Operators Award 24 2001Z Jan 00 with NN8R, Ray in Fremont, Ohio 7.020 MHz. Signal RST 559 24 2035Z Jan 00 with VE3RIH, Alan in Mississauga 7.020 MHz. Signal "good" 24 2059Z Jan 00 with K1BXZ, Dave in Nashua, NH 7.139 MHz. Signal RST 579 24 2215Z Jan 00 with WA2QQF, John in Norwich, NY 7.026 MHz. Signal RST 579 24 2229Z Jan 00 with WA2BQC, Ron in Plattsburg, NY 7.026 MHz. Signal RST 599+10

RCAF AT1/AR2 General Purpose Radio Photo #1

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