GB3CAM Cambridge 3cm beacon
page has been completely re-written. To see the original proposal
document, written a year ago (15.11.06), click here.
G4AKD/B has moved 1 KHz low. Believed to be caused by slightly
coarse trimmer on OCXO.
Mike, G8VCN shows me a satellite LNB fed scanner and demonstrates that
(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:
file There is a full write-up here.
30.01.08 Ofcom releases
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.
connectors, loss is measured at 10dB, which is 3dB less than
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
The beacon ident will
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
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.
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!
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
27.06.08 The contract is now signed
with the site owner - we should be able to start fitting the beacons
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 (
NEW SINGLE PAGE WEB SITE
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
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
'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
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.
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
Two complete beacons have been built to make any servicing
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...
10th March 2008