Sikorsky HO4S-3 Helicopter -
Electronics Suite
The electronics suite shown below was derived from RCN Aircraft General Information Data . Dated May 1, 1963. If  anyone can provide additional information or photos, please contact:
AN/AQS-4C This airborne sonar type, introduced into service in the 1950's, was specifically designed for use by helicopters. It was also known as dipping or dunking sonar. The transducer was enclosed in a small spherical dome that was free flooding. Mounted in the dome was a special type of compass called a flux valve that indicated the true bearing of the transducer on an indicator mounted in the helicopter. The transducer could be manually trained in either direction or automatically clockwise. An operator could select one of three operating frequencies by selecting a screwdriver operated switch on the front panel of the sonar transmitter/receiver. (Need photo and more info).
AN/APX-25 (SIF) IFF (SIF) L-band IFF transponder.  Range: 800-1300 MHz, output 1 KW pulse.
SIF panel (Courtesy Old Cockpit Panels) 
AN/ARC-27A UHF radio set.  Consists of RT-178/ARC-27 UHF aircraft receiver-transmitter. Frequency Range 225-399.9 MHz; Modes: MCW/Phone; Power output: 9 watts; 18 preset frequencies on any one of 1750 frequency channels. Transmitter may be tone modulated at 1020 Hz for emergency or direction finding purposes. One guard channel in the 238- 249 MHz range can be simultaneously monitored. 

AN/ARC-27 exterior view. (Photo courtesy: 
AN/ARC-27 interior view. (Courtesy Fair Radio Sales)

AN/AIC-4 Intercom amplifier and radio input control. 
? HF Radio?? . The photo at the bottom of this document shows a mast for a wire HF antenna. Might be for an AN/URC-13. To confirm,  contact: 
? VHF Radio ??- Not sure if it was part of the Comm suite on the HO4S-3. Photos of RCN HO4S-3's show a VHF mast on the tail boom. (Item 1 in pictorial below)
AN/ARA-41 L.F. Radio Compass (Need photo and info) . It appears from circumstantial evidence, that AN/ARD-7, AN/ARN-41, AN/ARN-42 and AN/ARN-54 
radio compasses comprised from different combinations of about two sets of components.
AN/APN-1 Radio altimeter;  420-460 MHz, Doppler frequency shift, dual scale meter 0-400 and 0-4000 feet. Minimum height that can be measured is 5 feet. 

APN-1 Tx/Rx only. (Photo courtesy Kurrajong Radio Museum)
APN-1 system components - (Courtesy

C-2A Compass 

HO4S-3 antenna placement. This is a modified image of Figure 4-19 from Chapter 4 of HO4S-3 Aircraft Operating Instructions.  The manual is a "Canadianized" version of the US manual so the antennas which do not jive with the Canadian electronics fit have been deleted. Can anyone help to fine-tune this diagram? Is there a port side photo available which shows more details? Contact  (Diagram provided by Ernie Cable) 

1) Seen on the RCN HO4S3's but unable to identify the equipment it was connected to. In the US versions this was the antenna for the AN/ARC-1 VHF transmitter.
2) AN/ARC-27 UHF transmitter/receiver.
3) HF wire antenna seen on some RCN HO4S-3's but unable to identify what equipment was connected to it.
4) AN/APN-1 radio altimeter receiving antenna.
5, 6 and 7) are intentionally omitted.
8)  AN/APN-1 radio altimeter transmitting antenna.
9) AN/APX-25 IFF

It is very rare to see a photo which depicts the dunking sonar deployed and the homing torpedo. This helicopter from HS-50 squadron is demonstrating its search and kill capability in 1960. (DND photo)

Above and Below: The dunking transducer for the AQS-4 sonar. This artifact is on display at the Shearwater Aviation Museum (Both photos by Ian Snow VA3QT)


Credits and References:

1) Leo Pettipas <lpettip(at)>
2) RCN Aircraft General Information Data . Dated May 1, 1963
3) Robert Downs <WA5CAB(at)>
4) Ian Snow <va3qt-4(at)>
5) HO4S-3 Aircraft Operating Instyructions (AOI) from prior to 1964.

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Sept 6/11