GB3CAM Cambridge 3cm  beacon

 

This page has been completely re-written. To see the original proposal document, written a year ago (15.11.06), click here.

Latest news:
28.11.07 Beacon frequency on application to be moved 5 KHz high to 10 368.755 KHz (ie, 5 KHz up on revised bottom edge of beacon band).

01.12.07 G4AKD/B has moved 1 KHz low. Believed to be caused by slightly noisy/intermittant coarse trimmer on OCXO.

05.12.07 Mike, G8VCN shows me a satellite LNB fed scanner and demonstrates that the (unlocked) puck LO is stable enough to copy the beacon call-sign. I go through my stock of LNBs and find that some are ok, some are not. A Lidl unit that is pretty good is recorded here:      wav file     There is a full write-up here.

30.01.08 Ofcom releases beacon NOV.

01.02.08 Now have NOV - it covers both the 10 GHz beacon and one at 24 048.870 GHz that John, G4BAO is building, also to be located on the same tower at Madingley.

08.02.08 Now have 37m of LCF12-50 J cable to link the beacon and antenna. After fitting connectors, loss is measured at 10dB, which is 3dB less than expected. We will only need to feed in 5W from the beacon to achieve the licenced erp.

20.03.08 Beacon at Ian's QTH changed over to standby unit - this transmits 6 KHz higher (ie, on the correct frequency!).
Main beacon now being moved to correct frequency and tested with a 5W external PA. Nearly there now - just need to arrange a date for the rigger(s) to instal the antenna (Thanks to Rob at the CRG for sorting this out).

The beacon ident will be simplified to just repeating the call-sign three times after each 45 second break, though the keying speed will be increased from 10 to 13 wpm.

Frequency is currently 40 Hz low. When the beacon is finally installed, a weekly log will be kept and published here.

26.05.08 Major problem! The CRG are being quoted very un amateur-friendly quotes from the site owner to put up the beacon antennas. Over the last 5 years or so, this has been an increasing problem. We have been looking at the water tower at Madingley (thanks to Ian for this lead), and John has obtained permision to use the tower at RAF Wyton.

28.05.08  John arranges a site visit to the tower at RAF Wyton. The take-off is excellent in all directions and access is very good - we could do all the installation work ourselves. Also, the site owner is beacon friendly.

30.05.08 Having discussed the Maddingley situation with Mike (G8VCN) over a couple of club meets, there seems to be no reason to delay the processing of the Wyton site paperwork (if we find that a Cambridge beacon is still a useful thing to have, we can put something today at a later state), so a revised NoV is sent to Murray, the RSGB beacon coordinator.

01.06.08 Ofcom pass the new NoV!

06.06.08 G4AKD/B has now been turned off and dismantled, prior to the move to RAF Wyton, the new home for GB3CAM. I would like record my thanks to Ian, who's generousity in offering his excellent Dry Drayton home for testing of the beacon (as well as homing the milliwatt beacon of a couple of years ago) has been very much appreciated.

27.06.08 The contract is now signed with the site owner - we should be able to start fitting the beacons next week.

10/07/08 AT LAST! THE BEACON IS OPERATIONAL (coming on air at 12.00).

14.07.08 We have had reports from Wakefield, Martlesham and Andover, to name a few - so it's getting out. For up to date reports, visit the beaconspot website (  http://www.beaconspot.eu  )

NEW SINGLE PAGE WEB SITE ADDED:  http://www.earf.co.uk/GB3CAM.htm


History

About two years ago, knowing that there was 10 GHz activity within the Cambridge and District ARC membership, the Cambridge Repeater Group asked if the CDARC would be interested in producing and owning a beacon that the repeater group would host at either their Madingley or Barkway sites. There had been a WB beacon at the club premises at the Coleridge Community College for many years, and this had only recently been removed because of repair work to the college building. The radio club was enthusiastic about the offer, which it saw it as an opportunity to rekindle interest in the band and also provide a frequency reference, given that operation to date had been WB, using cavity or puck controlled frequency sources. At about this time, Sam (G4DDK) was working in Cambridge and was quite independently generating interest in a beacon located on the company premises on the St Johnís Innovation centre. Roger (G4BEL), John (G4BAO) and myself were already active on narrowband, and most weeks had pub meals with Sam when he stayed over, so knew about both plans. When all this came out at a club visit some time later, I was asked if I was interested in being involved in the CDARC project, which I certainly was. With such good sites on offer from the CRG, the Innovation centre idea was not taken any further.  

Site

The Cambridge Repeater Group have two sites, Barkway and Madingley. From a local point of view, Madingley would provide a very strong signal - strong enough for testing quite simple WBFM as well as narrow-band equipment. It would be a good site for propagation further afield also, though not perhaps as good as Barkway. There is also the MOD exclusion zone centred on Charing Cross that might make Barkway a bit 'iffy', so Madingley was chosen.

Testing is currently (15.11.07) underway with the beacon located at Dry Drayton and signing G4AKD/B. The frequency is 10368.750 MHz.

Antenna

beacon antenna

The waveguide slot antenna used on the defunct wideband beacon, located at Coleridge Road site, was re-used, complete with its shrapnel marked radome. A new base plate was produced, and due to lack of radome height (because a waveguide to sma transition had to be added), the assembly had to partially extend  through the plate. Hence the lower weather-proof housing.

Antenna feed

A 37m length of Cellflex LCF12-50 has been obtained and N connectors fitted. Measured loss at 3cm is 10.1 dB. For full licenced output (5W erp), a drive power of 0.5W at the antenna is requred. Thus, 5W drive is required into the LCF12-50 feed cable, and a single stage amplifier has been produced to increase the 1W beacon unit to this level.

Transmitter

beacons

Two complete beacons have been built to make any servicing less stressful.

block_diagram

Links to modules:    864MHz exciter    PLL    3456 MHz multiplier     10368 MHz multiplier   1W PA    Keying logic      PSU

Station keying is 400 Hz FSK, and consists of the callsign repeated three times followed by the site locator, all at 10 wpm. There is then a 30 second period of unmodulated carrier.

5W RF PA

This single stage add on unit is required to overcome the 10 dB feeder loss. With a gain 7 - 8 dB, only 0.5W drive is required, so 3 dB of interconnection cable loss between beacon and amplifier is acceptable. It could be that we need a reasonable length interconnect cable, because the antenna feeder cable may not be long enough to quite reach to the cabin!. In this case, the RF PA module will have to be placed outside (below the feeder tray between mast and cabin).

A Toshiba TIM0910-4 device is used in the amplifier, and this runs from a 9v rail. Both voltage and current limit regulators are used to avoid unnecessary device failure...

3cm PA RF_PA_inner

 

10th March 2008

 Simple narrow-band converter (for 1296 or 144 or 28,8 MHz IF)