About the CRG

The Cambridgeshire Repeater Group (CRG) owns and operates amateur radio analogue voice repeaters on 6m (GB3PX), 2m (GB3PI) and 23cm (GB3PS), a D-Star repeater on 70cm (GB7PI) and a high speed packet node on 70cm (GB7PT) from a site at Barkway near Royston plus an analogue voice repeater on 70cm (GB3PY), a 2m APRS digipeater (MB7UM) and an amateur television repeater on 23cm (GB3PV) from a site at Madingley west of Cambridge. Both Barkway and Madingley are former Pye/Philips towers however both are now owned and maintained by Arqiva. These repeaters serve users in the Cambridgeshire, Bedfordshire, North East Hertfordshire, North West Essex, West Suffolk and South West Norfolk areas. To the north of the county we operate the Peterborough 2m FM voice repeater (GB3PE) from a site at Thorney Toll.

The CRG hold the callsigns G3PYE and G6PYE which are used to help promote the group on-air. More information, including the history of the CRG can be found on the About the CRG page.

CRG Subscriptions

CRG Membership subscriptions run annually from the AGM, see the CRG calendarfor this years date. Membership rates for 2011/12 are: ordinary membership £10 per annum; family membership £15 per annum.  Please help the group by paying your subscription promptly. As always, your subscription will get you into the CRG Rally for free. For those who haven’t yet paid (and those who’d like to join), you will find the CRG membership form on the CRG Membership Page. or pay online by going to the Products Page.

Repeater Status

Please email technical if you have any reports regarding any repeater below.

Call sign Mode Location Output Status
GB3PE 2m Voice Thorney Toll, Peterborough 145.675 MHz Operational
GB3PI 2m Voice Barkway 145.750 MHz Operational
GB3PY 70cm Voice Madingley 433.200 MHz Operational
GB3PX 6m Voice Barkway 50.780 MHz Operational
Back on 08/06/14


GB3PV 23cm D/ATV Madingley 1316.000 MHz Operational
GB3PS 23cm Voice Barkway 1297.075 MHz Operational
GB7PI 70cm D-STAR Barkway 439.9125 MHz Operational
GB7PT 70cm Packet Barkway 439.7125 MHz Operational
MB7UM 2m APRS Digi Madingley 144.800 MHz Operational
MB7PI 2m APRS Digi + iGate Barkway 144.800 MHz Operational
MB7IPE 2m APRS Digi Thorney Toll, Peterborough 144.800 MHz Operational


2010-02-27 11.25.52

Flossie is the groups Amateur Radio Demonstration Vehicle.  For more info about Flossie please visit the Camb-Hams page on her.

CRG Events

Please see the CRG calendar for more dates.

CRG News

MB7UM Mk2 Installed – 29/05/14

With the help of Joe M0ZRN and Rob M0VFC, our MB7UM APRS Digipeater at Madingley has been updated.  The new  MB7UM Mk2 is a new design for the CRG with a Raspberry Pi (thanks to Bob G1SAA for donating it) and a TNC-Pi at its heart, http://tnc-x.com/TNCPi.htm. The radio is a Kenwood TK-759E (752 variant) and it was all installed onsite on Thursday 29/05/2014.

2014-05-28 22.36.35

2014-05-28 22.37.03

Testing MB7UM Mk2 after installation shows very promising results with APRS beacons, both to and from MB7UM, being decoded in places they were not on the old hardware, quite a few stations have reported improvements in decoding and being decoded by MB7UM Mk2 over the original hardware.

2014-05-29 20.44.56

In the short term we have configured MB7UM Mk2’s callsign as ‘MB7UM-2’, this will allow a separate set of stats to be generated in APRS.fi which we can then compare to the original data (callsign) and see the changes within the stats APRS.fi generates http://aprs.fi/info/a/MB7UM-2

The original MB7UM (Mk1) had an add-on developed by Gavin M1BXF which beaconed telemetry data including temperature, 12v & 24v voltages and if the mains has failed.  The new MB7UM Mk2 will do the same but with an extra temperature sensor mounted outside the cabin to give addition temperature data. of the weather outside.  The new sensor suite has been the main development in the Mk2 system and comprises a PICAXE-20X2 chip reading the sensors and then communicating with the Pi over i2c. The hope is to document this and roll it out and make it available to anyone wishing to do something similar, so watch this space…


We also have a 3G modem connected to the Pi, in the short term it can run as an I-Gate but this might not always be configured, MB7PI at Barkway is the CRGs main APRS I-Gate for the moment, the 3G modem is there mainly for remote admin as we commission the setup so don’t worry if you notice MB7UM-2 stops gating APRS data.

2014-05-28 22.37.412014-05-28 22.39.27

Comments about any change in coverage, decode success, or lack of, would be very much welcome.


Gavin Nesbitt, M1BXF.

CRG Technical Coordinator

Chairman’s Note – 20/02/2013

I have some good news for the CRG.

A while ago, we were presented with a bill for business rates for the use of two sites we use by the site owners Arqiva. It was a considerable amount of money into 4 digits, something the CRG, as a voluntary non-profit group could ill afford.

Following discussions with Arqiva, they have graciously agreed to absorb those costs and will not be charging us “our share” of local authority business rates. The CRG is grateful for the kind benevolent offer made by Arqiva, which will enable us to remain on their sites, providing coverage for Cambridgeshire and surrounding areas, for both “pleasure” and “resilience” use.

Arqiva are showing continued support for amateur radio these days, as it is seen as a breeding ground for the next generation of engineers.

The CRG & Camb-Hams, together with the other societies in Cambridgeshire, are keen to promote the hobby among the younger generations. The Camb-Hams have already helped Cambridge105 run a number of “build a radio” sessions with local schools, and will be looking to do more in the future. The Cambridge University Wireless Society continues to promote amateur radio within the world of higher education, and also offers courses for those wishing to obtain all grades of licence.  Everybody on our repeaters no doubt now knows someone newly licensed due to such efforts.

As a group we are keen to support these initiatives as can be seen in the CRG Student Radio Loan Scheme. If you have any suggestions on how you think the CRG, or the Camb-Hams, could do more to encourage new people to the hobby, or you would like to get involved in any aspect mentioned above, then please contact us.

GB3PE Update – 27th October 2012

Last week I visited site at Thorney Toll to change out the repeater controller for one that improved the audio quality through the repeater, and to give an inter-over indication.

It is still planned to change to a G1YFF logic in the future, though Jake has other projects to complete first, and this one will be his first FX5000 integration – not just plug in the back of the box, a proper connection into the repeater control module.

For now, I have swapped out the recalcitrant Zetron 37 for a far far better Zetron 38Max that I have purchased and loaned to the CRG for the job.

The first thing you noticed is the quality of the through audio – it’s much much better (couldn’t be any worse!), being fuller and far louder. If anything, it’s a little more “punchy” than it is when the FX5000 is placed in it’s own talk-through mode without the controller plugged in. It’s certainly easy on the ear when mobile.

And then there’s the “K”. I had considered using trailing pips, but that may cause issues with the Echolink that Andy G6OHM kindly provides for GB3PE.

When you drop carrier, you’ll notice a little squech tail. I have set the Zetron to do this so weak signals chopping in/out don’t send K K K K K K as they’re falling out (one of the reasons why the old box didn’t have a pip was it sending the pip the moment that CTCSS decode was lost!). People who have radios that provide a “reverse tone burst” when they de-key will have no squelch tail. (A “reverse tone burst” is a period of around 300mS where the transmitter remains keyed after the PTT is released, with NO CTCSS on the signal – it allows the repeater to close it’s input before the signal is lost, so there’s no squelch crash. It’s a feature on just about every PMR radio, yet only a few ham rigs do it).

Echolink is working correctly via GB3PE (when Andy has it switched on – see the CRG homepage for the status indicator). There were a few issues initially, but Andy has resolved those now. We still hope that at some point in the future we’ll be able to have Echolink, and other facilities like IRLP or AllStarLink on GB3PE 24/7.

MB7IPE – our new APRS iGate, built up by Gavin M1BXF, went on site too. At first there was a problem where PE’s TX was getting back into the RX and keeping the repeater open, but appearing like a very weak signal. Turning MB7IPE off made it go away. After some searching and testing, it was found that the power cable for the APRS unit was pretty well a 1/2wave at 2m, and causing some kind of parasitic re-radiation into the RX. Chopping it to “just long enough” cured the issue, as well as winding the aerial feeder into a choke, just in case.

The other issue I found was MB7IPE causing de-sense to PE’s input, even with a filter inline. Something that didn’t appear on any bench tests. So it was removed back to my workshop for tests & mods where necessary. With PE’s input only 275KHz away, it was going to be a problem! However, following a little head scratching, and a good suggestion by Rob M0VFC, I have added a relay into the PTT line, which is energised by the “COR” signal from the repeater (Carrier Operated Relay). This means that MB7IPE’s TX will be disabled when there is a valid signal on the input of GB3PE. I had hoped to use COR to control the DCD in the TNC, which would allow it to buffer packets until the coast was clear to transmit. Sadly the TNC-X that is used for the APRS digi does its DCD in software, and there are no hardware means of providing this.

I hope to get MB7IPE back on site early next week (w/c 29th Oct).

Again, at some stage in the future, we hope to have internet connectivity for our systems on site, which will enable the iGate for MB7IPE.

What we did find, during its short tenure on site last week, is that it can hear MB7PI, and MB7PI can hear it, which puts it in direct contact with our main iGate at Barkway.

Coming up next is the CRG AGM on November 28th at The Sun, Waterbeach, Cambs. All welcome (only paid up CRG members can vote) – I look forward to seeing you there!

GB3PE-R Echolink now active

Echolink on GB3PE – Node Number 741906
This is an attended link operated on behalf of the CRG by Andy, G6OHM. The link
is normally on between 19:30 and 22:00 local time, 3 to 5 days a week. It may also be
on occasionally during the daytime and at weekends.

The current link status is shown on the right sidebar of the CRG homepage.

More information on the GB3PE-R Echolink Page

Repeater Signal Strength Reports

We have added a form to allow Repeater Signal Strength Reports as it is always useful to receive reports of how well our repeaters are being heard and we really appreciate it when people take the time to tell us. If you have time please fill in the form to provide feedback on repeater strengths in various locations:  http://www.cambridgerepeaters.net/repeaters/

The Chairman’s Rant!

Repeater Timeouts – and why we have them.

Some people, as you know, seem to enjoy timing out on the repeaters in the CRG area. But why do we have a timeout in the first place? No, it’s not a hindrance, it’s there for a good reason.

While the repeater PA could handle 100% duty at its current setting, the idea of the timeout is to give other people a chance to make use of the repeater. The idea of the repeater in the first place is for mobile stations (in vehicle, or on foot etc…) to extend their operating range. It is not a platform for broadcasting, and it should be remembered that at peak times, with heavy traffic, there is a chance that a amateur mobile operator may come across a situation where they need to make an urgent call, and that means pretty well “now”, not in 3/6/9 minutes time.

What do I mean? Well, GB3PI has a 3 minute timeout. But some people think it’s OK to timeout, grab a reset, then have another bite, maybe another timeout, and some more…. That’s not good operating practice, and they should know better. Especially those who work in the commercial radio comms trade where we often set timeouts to 1min, AND have “hog” timers where users that monopolise community base stations suffer a further lockout if they exceed a pre-defined amount of use. It’s all to promote fair usage, and give other users a chance to utilise the equipment.

What does this mean for CRG repeaters? Quite simply, if the current situation continues, I am going to investigate a number of options which include an aggressive timeout policy on CRG repeaters, where the timeout is set at a generous 3min, but will reduce to 2min if the repeater times out again in a given period, and if it times out again, it will reduce to 1min, then 30seconds, and finaly the repeater will shut down for five minutes.

Another option includes utilising a transmitter signature system to identify offending stations and adjust the timeout according to their behaviour. That last option may be a little draconian, but it’s on the table!

Why has it come to this? – Comments have been made by some people that they don’t use our repeaters much these days because they can’t get a word in edgeways, and when they do, their journey is over before they get to have a proper go. So they just don’t bother turning it on. The next step is not having a radio in the car, followed by leaving the hobby. So why should the selfish actions of one of two people ruin the hobby for many more?

Look at it from that perspective. Your journey to work takes 15 mins. Enough time for a few overs in normal QSO. Then take it with our repeater hog on air – taking 2/3 x timeouts @ 3min each = 6/9mins per transmission. You *might* get one go, you probably will only get the chance to announce yourself on the repeater, and the next time it’s 73!! Not good at all.

Education has been tried, and it’s failed. It’s time for action!

Madingley Site Power Failure

Hi all,

Thanks to everyone for the updates regarding the loss of power at the Madingley site on 20/01/12, it sure gave the batteries a good test and I would say they worked well.

If you look at the telemetry MB7UM sends out here http://goo.gl/C6g7f you can see the 4th graph down showing the mains power state (010 for ON and 000 for OFF). Power went for somewhere between 16:50:41 and 17:02:20 and came back on between 00:00:17 and 00:11:53 (how cool is it we can tell this now!). That was a duration of about 7 hours.

Back on the telemetry, the 24v graph (2nd one down) shows a drop in voltage from ~27.5v to 25.0v, 27.5v is the charging voltage, but held steady at 25.0v for most of the time after, some slight fluctuations towards the end. The 12v (top graph) didn’t change at all (the fluctuations between 13.9v and 14.1v is probably ADC error). Note the graphs are the actual value x10 which allows greater resolution.

The only thing I need to work out is why the temperature jumped 8C when the power can back on (between 00:00 and 00:47). This *might* be an ADC reference voltage error, but I can’t see how as it is run off a 5v rail which shouldn’t have been affected. There is a heater in there so maybe it’s just a very good one!

Anyway great work by Phil G4BIK who not only updated GB3PY’s logic to send a ‘B’ between overs instead of a ‘K’ when on batteries, and for plumbing in the new rectifier and battery backup system which worked a treat over the 7 hours.


Gavin M1BXF

CRG Technical Coordinator

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