Communications (Air) Ltd - CAL - UK Vintage Radio Repair and Restoration Discussion Forum

Restoration73 · 21st Jun 2015, 5:21 pm

Does anyone have any information on this firm - I have a Model FT11B
Marker/VHF Ramp Checker, MOA Type M41, and would like some info.
I could not find any in Graces guide or FlightGlobal archive. Thanks

Restoration73 · 26th Jun 2015, 7:55 pm

Obviously not - I have found out this is a VOR/DME simulator for checking equipment
installed in the aircraft. It's a useful box to recycle.

Radio Wrangler · 26th Jun 2015, 11:57 pm

It will be a collection of signal sources. Marker beacons are at 75MHz with three different AM modulation formats, each sounding more insistent as a plane approaches the runway threshold. The RX also lights different colour lamps. It'll be adjustable in output level to check receiver sensitivity.

VOR is at 112-118MHz and has a compound AM and FM content. The phase between the demod AM and the demod FM gives the pilot his bearing from a simulated moving antenna. A subcarrier is used at 9950Hz.

DME is on a much higher frequency,using separated channel pairs near 1GHz. The plane initiates things by sending patterns of pulse pairs on the input channel of the ground gear. The gear on the ground replies on the matching channel and the plane moves its puls pattern around to be sure it's locked onto replies to it's pulses, measures the round trip time and displays the distance from the beacon. To test these the ramp tester has to become a transponder with variable delay.

Ramp testers for these equipments have to be good, because aircraft navigation relies on things set to agree with them.

David

Restoration73 · 27th Jun 2015, 9:34 am

Thanks, David - it is a basic 1960's unit with germanium transistors, although I can
see a 75MHz crystal. It is always interesting to find equipment made by smaller
firms which have disappeared without trace. Although it's unlikely the 100-160MHz
oscillator would cause QRM to an aircraft, I won't be using it in its original application.

Radio Wrangler · 27th Jun 2015, 11:01 am

Aye, it's the one part of the spectrum that still is heavily policed for any interlopers or interference, despite the tales of Ofcom not being much bothered elsewhere.

Unfortunately the most critical bit - used for autolanding or just ILS is right next door to the high power broadcasters, some only 200kHz away 8-( and the LOs of standard VHF FM entertainment radios tune right across the NAV band. I really do want a new word in the English language "Pessimised"

David

majoconz · 28th Jun 2015, 1:39 am

In the 60's the DME portion may have been on 200MHz - a holdover from the old IFF - Murphy (not the Irish bloke!) made the aircraft boxes - ISTR "Rebecca" on the internals or was it in the maintenance manual?
200MHz DME was still in use in NZ when I arrived in 1974.

21st Jun 2015, 5:21 pm	#1
Restoration73 Nonode Join Date: Jun 2013 Location: Surbiton, SW London, UK. Posts: 2,801	Communications (Air) Ltd - CAL Does anyone have any information on this firm - I have a Model FT11B Marker/VHF Ramp Checker, MOA Type M41, and would like some info. I could not find any in Graces guide or FlightGlobal archive. Thanks

26th Jun 2015, 7:55 pm	#2
Restoration73 Nonode Join Date: Jun 2013 Location: Surbiton, SW London, UK. Posts: 2,801	Re: Communications (Air) Ltd - CAL Obviously not - I have found out this is a VOR/DME simulator for checking equipment installed in the aircraft. It's a useful box to recycle.

26th Jun 2015, 11:57 pm	#3
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,858	Re: Communications (Air) Ltd - CAL It will be a collection of signal sources. Marker beacons are at 75MHz with three different AM modulation formats, each sounding more insistent as a plane approaches the runway threshold. The RX also lights different colour lamps. It'll be adjustable in output level to check receiver sensitivity. VOR is at 112-118MHz and has a compound AM and FM content. The phase between the demod AM and the demod FM gives the pilot his bearing from a simulated moving antenna. A subcarrier is used at 9950Hz. DME is on a much higher frequency,using separated channel pairs near 1GHz. The plane initiates things by sending patterns of pulse pairs on the input channel of the ground gear. The gear on the ground replies on the matching channel and the plane moves its puls pattern around to be sure it's locked onto replies to it's pulses, measures the round trip time and displays the distance from the beacon. To test these the ramp tester has to become a transponder with variable delay. Ramp testers for these equipments have to be good, because aircraft navigation relies on things set to agree with them. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

27th Jun 2015, 9:34 am	#4
Restoration73 Nonode Join Date: Jun 2013 Location: Surbiton, SW London, UK. Posts: 2,801	Re: Communications (Air) Ltd - CAL Thanks, David - it is a basic 1960's unit with germanium transistors, although I can see a 75MHz crystal. It is always interesting to find equipment made by smaller firms which have disappeared without trace. Although it's unlikely the 100-160MHz oscillator would cause QRM to an aircraft, I won't be using it in its original application.

27th Jun 2015, 11:01 am	#5
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,858	Re: Communications (Air) Ltd - CAL Aye, it's the one part of the spectrum that still is heavily policed for any interlopers or interference, despite the tales of Ofcom not being much bothered elsewhere. Unfortunately the most critical bit - used for autolanding or just ILS is right next door to the high power broadcasters, some only 200kHz away 8-( and the LOs of standard VHF FM entertainment radios tune right across the NAV band. I really do want a new word in the English language "Pessimised" David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

28th Jun 2015, 1:39 am	#6
majoconz Heptode Join Date: Jan 2008 Location: Ashhurst, Manawatu, New Zealand Posts: 570	Re: Communications (Air) Ltd - CAL In the 60's the DME portion may have been on 200MHz - a holdover from the old IFF - Murphy (not the Irish bloke!) made the aircraft boxes - ISTR "Rebecca" on the internals or was it in the maintenance manual? 200MHz DME was still in use in NZ when I arrived in 1974. __________________ Cheers - Martin ZL2MC