MILITARY COLLECTOR GROUP POST, Jan.8/98 INDEX: RADIO COMPASS AN/ARN-6; By John Mackesy More On The BC-611 Canvas Bag; From John Kid WHATSIT; HARP?, Now We Know! URC-10; Manual Available DON NEEDS; HUMOR; ********************************* RADIO COMPASS AN/ARN-6; By John Mackesy Dennis, I can just hear you saying "Shit! Aircraft crap!" Oh well, it's a heavy habit, why not share it around? ------------------------------------------------------------------ Back in the '60s - does anybody remember that far back? - my colleagues and I spent many happy hours flying low and slow above the North Atlantic. These were the days of the Cold War, a chapter of our history which today seems rather tinged with absurdity. My role in this madness was servicing Anti-Submarine Warfare (ASW) aircraft. These aircraft (sadly, now all gone to their reward) patrolled 'the Pond', maintaining the peace. Like so many of our doings of the past, it all seemed to make a certain amount of sense at the time. Our rather large, comfortable and well-equipped aircraft bore a wide range of electronic equipment, used for offensive, defensive, navigation and - well, other purposes. You wouldn't want to get lost over the 'Pond' (particularly when pursuing 'other purposes'), so the navigational equipment included things like LORAN and a couple of AN/ARN-6 Radio Compasses (obsolescent even then). It could be rather boring droning along in the darkness above the (curiously remote) sea. One of my occasional diversions (apart from reading, drinking coffee and making toasted turkey sandwiches) was roaming up and down the broadcast band on the Radio Compass. What a great performer! I could hear stations on both sides of the Atlantic, all through the Caribbean and beyond. Three decades further on, I have my own AN/ARN-6 Radio Compass, and it's still a great performer. As a military collectible, the ARN-6 has a lot going for it. Let me quote the opening paragraph of the ARN-6 manual, USAF T.O. 12R5-2ARN6-2, issued 4 August 1950. "1. GENERAL PURPOSE.- Radio Compass AN/ARN-6 is an airborne navigational instrument. It is designed smaller and lighter than other automatic radio compass equipments for the purpose of using it in small aircraft." As a restoration project, especially for the beginning military collector, the ARN-6 is definitely worth considering. 1. Parts are readily available. 2. It was widely used. 3. Needs only 26.5 VDC. 4. Covers the broadcast band. 5. Reliable and easy to work on. 6. Relatively compact and lightweight. Now that I have your attention, we'll look at the interesting aspects of the ARN-6. First the name... 'A' stands for 'Airborne', 'R' for radio, 'N' for 'Navigational. -6? That's a number suggesting that it was the 6th of this type of equipment accepted into this series. As the name would imply, the device uses the directional properties of radio signals to aid in navigation. It does this by exploiting the directional characteristics of a LOOP aerial. As you may recall, such an aerial will exhibit maximum signal strength (PEAK) and minimum signal strength (NULL) characteristics dependent on its orientation to the signal source. This allows the operator to home on a signal, or to derive a position by triangulation. Back in the old days (before about 1941), the directional loop aerial was driven by hand, indicating its bearing on a mechanical device looking much like a compass. This was a fairly slow and complicated business, requiring a fair degree of operator skill for best results. One of the traps of this system was the dreaded 'Reciprocal Bearing Anomaly' - is the station ahead of or behind the aircraft? 'Radio Compass' was an appropriate title for these devices. Time and technology marched on. It was realised that it was quite possible, even desirable, to automate the process of orienting the loop towards the transmission of interest. Electrical means of indicating loop position also allowed much greater freedom in equipment layout, with the additional benefits of reduced weight and maintenance. In this way, the simple 'Radio Compass' became an 'Automatic Radio Compass' - somewhat more complicated, but much more useful. Best of all (through use of a non-directional "sense" aerial and a bit of extra circuitry), it didn't suffer from the reciprocal bearing problem. In more recent times the term 'Radio Compass' has been largely replaced by 'Auto Direction Finder', commonly contracted to 'ADF' RECEIVER So much for ancient history. Let's talk about the ARN-6, more specifically, about 'Receiver R-101/ARN-6'. Like most aircraft electronic equipment, it presents as a compact, somewhat featureless 'black box' - all the interesting stuff is inside. Frequency coverage is 100 Khz to 1750 Khz (in 4 bands), it uses 16 octal tubes (2 of which are thyratrons), has 26.5 DC on its plates, electric motor-driven bandchange, and much, much more. As a receiver, conceptually it isn't very different to the more familiar domestic radio - it's just that there's more of it, and it's built to much higher standards, from both the hardware and performance angles. A single conversion superheterodyne, it has 2 RF stages (12SK7s), a separately excited mixer (12SY7), oscillator (1/2 12SX7), 2 IF stages (12SK7s) and a detector/1st audio stage (12SW7). Audio output is provided by a pair of 26A7GTs in push-pull parallel, driven by 1/2 of a 12SX7GT. The other half of this 12SX7 is used as a tuning meter amplifier. Half of another 12SX7 is used as a BFO. The tuning capacitor is a 5-section type, driven by spring-loaded anti-lash reduction (120:1) gearing. Tuning drive is by the usual flex drive cable, cables used on previous Bendix equipment also fit ARN-6. Electric servo tuning has been used on a few installations (C130A, RA3B are two I'm aware of), typically when the receiver is located in an unpressurised area of the aircraft, or where very long control runs are involved. The IF signal path is somewhat unusual in that there are 2 IF channels, IF channel switching being integrated into the bandswitch system. Band 1 (100 to 200 Khz) uses a 142.5 Khz IF, bands 2-3-4 a 455 Khz IF. An "IF Trap" is fitted to prevent feedthrough on the IF frequencies. The bandswitch motor drive mechanism is a work of art (all ARN-6 gears are stainless steel!), and has to be seen to be believed. Despite this, it's relatively simple to both comprehend and remove. COMPASS OPERATION In 'compass' mode yet more circuitry is brought into play. There is a 12SK7 loop amplifier stage (for the directional loop aerial), a 12SX7 modulator, a 12SK7 compass amplifier, 1/2 of a 12SX7 used as a tone oscillator, and finally a pair of 2050 thyratron tubes associated with the loop aerial servomotor system. Thyratrons need a substantial AC plate voltage to operate; this is supplied by a vibrator/transformer arrangement, which also produces AC for the autosyn loop position transmission and indication system. OTHER PARTS OF THE SYSTEM As is usually the case with aircraft radio equipment, the ARN-6 is remotely controlled. There are several different types of controller used with ARN-6, but all are conceptually similar. Tuning is via a flexible drive cable, bandswitching is electric; all controls (and the tuning meter) are located at the control box. The wiring loom from the control box contains 20 wires. LOOP AERIAL AS-313/ARN-6 This is a compact servo-operated directional loop aerial sealed into a nitrogen-filled glass-topped housing. There are 2 connecting cables, containing 7 & 3 wires respectively. It should be noted that the loop to receiver wiring forms part of the tuned circuit of the loop input stage. This means that the L & C of the connecting cable should be to the required spec. for best performance, although in practice this doesn't seem to be too critical. Just use at least 10' of shielded wire. LOOP INDICATORS 2 types are used, the ID91*/ARN-6 (3 1/4" dia.) and the ID92*/ARN-6 (5" dia). 4 wires are required for the position indicator input; several indicators may be wired in parallel.20 MOUNT MT-273*/ARN-6 This is where everything comes together. The receiver slides into this mount (also known as a "rack") and is retained by catches at the front of the mount. Connection to the receiver is accomplished via a 22-pin female plug on the mount engaging matching male pins on the rear of the receiver. In the bottom of the mount, accessible via a removable cover plate, is a forest of screw terminals. These connect the mount to the control box, loop, indicators, and DC power source. Audio output is also available. It is possible for one ARN-6 receiver to be controlled from two positions, typically pilot and navigator. Mounts intended for dual-control installations incorporate rotary solenoid switch to make the necessary connections. A complete ARN-6 system weighs approximately 60 lb, and requires 22 - 30 VDC at about 4.0A. BUILDING AN ARN-6 SYSTEM Probably the most daunting aspect of assembling an ARN-6 systems is the wiring to/from/inside the mount. The wiring loom to the controller contains the bulk of this, but there is still a considerable amount of wiring to do. Cables to the loop, indicators (and certain types of control box) are terminated with MS 3000-series (Cannon plug) connectors. Although tedious, the wiring is not particularly complicated. The secret is to have a detailed plan to work to, and to check off details as you go. All wiring should be checked, double checked, then checked as a complete system. Once this has been done you'll be able to find out if the thing works or not! Assuming your wiring contains no serious blunders, the system should function as a receiver - receiver-mode problems are rare. Audio quality from the push-pull parallel 26A7GT varies from tolerable to awful, due to audio filtering in this stage. Taking the AF output from the 1st audio stage and using an outboard amp is recommended. Loop and compass-mode problems are more common, and may require some hair-tearing to solve - but that's a story for another day. Notes: Autosyn 3-phase AC position transmission and indication system. 12SK7 Metal Octal-based remote-cutoff RF pentode 12SX7 " " " twin triode (similar to 12SN7) 12SY7 " " " pentagrid converter (sim. 12SA7) 12SW7 " " " dual-diode-triode (sim. 12SQ7) 26A7GT Glass " " twin beam power pentode 2050 Glass " " thyratron ARN-6 (1950 onward) replaced the earlier ARN-7, which entered service in 1945. This was the successor to the very similar SCR-269. SCR-269 & ARN-7 were heavier, bulkier and required 115V 400 Hz AC power. A Bendix design, ARN-6s were also built by Magnavox. There is also the ARN-44 version of the ARN-6, which tunes HF frequencies with 4 bands. JOHN MACKESY VK3XAO mack@melbpc.org.au ********************************* More On The BC-611 Canvas Bag; From John Kid Dennis, In Shelby Stanton's book "US Army Uniforms of WWII" there is a photo on the front cover, of a paratrooper with a BC-611C, in a rigger made webbing cradle. This photo is I think from operation "Torch". There is a mating photo with a better view of the webbing taken from the rear quarter, and he is not posing as if he's talking on the set, as in the Stanton book. The cradle would have been devised "inhouse" by necessity as you couldn't jump with a BC-611 slung over your shoulder with it's strap. The webbing obviously allowed use without "unpacking" the set. Necessity is the mother of invention. John A. Kidd Collector: Military radio. Tullamarine, 3043. Interests: Military aircraft Victoria. AUSTRALIA & vehicles, Chrysler cars. ------------------------------------ John, While jumping with a BC-611 over the shoulder would indeed be a trick, how about with a BC-745? A vintage photo exist showing an airborne troop holding a Pogo Stick out in front of him, two handed, at almost arms lenght, he is demostrating his jump/landing posture. Another photo shows cramed troops aboard a C-47 ready to take off, the upper part of a Pogo Stick is bearly visable. Dennis ********************************* WHATSIT; HARP?, Now We Know! Dennis, I recently had several long conversations with Bob Graham and he brought up the subject of "did I know anything about HARP?" I was thinking this was the High Altitude Rocket Project of the 1950's but he said no, it was some semi secret program to bombard the upper atmosphere over the North pole with RF waves which will "burn up" any incomping missile. Have you ever heard of this? Bill Howard THE WILLIAM L. HOWARD ORDNANCE TECHNICAL INTELLIGENCE MUSEUM e-mail wlhoward@gte.net Telephone AC 813 585-7756 --------------------------------------------------------------------------------- Bill, Dennis, HARP was a code name for a Rad Lab project started in Dec.1941. The objective was to create "material with artificially constructed dielectric constant and permeability" to be used to manufacture coatings that would reflect UHF poorly. Yes, to the best of my knowledge, that was the beginning of stealth technology. The details can be found in "Summary Technical Report of the National Defense Research Committee Division 14: Volume 1, Radar: Summary Report and Harp Project." The report was classified "SECRET." The work resulted in a product designated MX-410/AP for X-band and MX-355/AP for S-band. The absorbent film was 20 to 50 mils thick and a reflectivity of less than 5%. The Germans used the equivalent of HARP to coat the Schnorkels of their U-boats near the end of the war, making them impossible to detect with our S- and X-band sets. We and the Brits experimented with the coatings on night fighters as well as our subs. After the war, the research was transferred to NRL. I have a copy of the report if you are interested. Regards, Mike Blom --------------------------------------------------------------------- Dennis, HAARP is a research project on the ionosphere which includes using a very high power transmitter to actually excite an area of the ionosphere so it can be observed. I think it's pretty fascinating, but of course it's got some of the "Greenies" all worked up cuz we're monkeying around with mother nature again...You can read more than you want to know about it at: http://www.haarp.alaska.edu/haarp/index1.html 73, Bob W9RAN ********************************************* URC-10; Manual Available I just received a fax from Roland Maruska. He has found a copy of the a manual for the URC-10. He is offering copies for $5.00. It's T.O. 12R2-2URC10-2. dated 1 Feb. 1969 You can contact him at WACHSTOCKJ@juno.com ed) ********************************************** DON NEEDS; Dennis, could you include this on the next wanted post? Wanted: PRC-47 manual. 3-5KW generator set like PE-95 Don dr@cei.net ********************************************* HUMOR; What's the difference between a girlfriend and a wife? 25 lbs. What's the difference between a boyfriend and a husband? 25 minutes. What is it when a man talks nasty to a woman? Sexual harassment. What is it when a woman talks nasty to a man? $3.99 a minute. What's the difference between a fox and a dog? Five drinks. Did you hear about the new blonde paint? It's not real bright, but it's cheap, and spreads easy. How can you tell if your wife is dead? The sex is the same but the dishes pile up. How can you tell if your husband is dead? The sex is the same but you get the remote. ----------------------------------------------------------------- Did you hear that Loreena Bobbitt was killed in a car accident yesterday? Yeah, some prick cut her off. ------------------------------------------------------------------ A country preacher decided to skip services one Sunday and head to the hills to do some bear hunting. As he rounded the corner on a perilous twist in the trail, he and a bear collided, sending him and his rifle tumbling down the mountainside. Before he knew it, his rifle went one way and he went the other, landing on a rock and breaking both legs. That was the good news. The bad news was the ferocious bear was charging at him from a distance, and he couldn't move. "Oh, Lord," the preacher prayed, "I'm so sorry for skipping services today to come out here and hunt. Please forgive me and grant me just one last wish: Please make a Christian out of that bear that's coming at me, Please Lord!" That very moment the bear skidded to a halt, fell to its knees clasped its paws together and began to pray aloud right at the preacher's feet. "Dear God, bless this food I am about to receive...." ********************************************* EDITOR; Dennis Starks; MILITARY RADIO COLLECTOR/HISTORIAN military-radio-guy@juno.com --------- End forwarded message ----------