The story so far…

While working for ARM in Cambridge, I spent a lot of time commuting to my university and family at weekends and late at night. I had just bought an Icom ID-5100 which supported D-STAR and lived in an area where the D-STAR repeater coverage was not so great.

I looked into the feasibility of setting up my own repeater, and thus a project was born. We had good connections with the Kelvedon Hatch Secret Nuclear Bunker through our activities at GB0SNB and so it seemed like the ideal place. Located between two well regarded D-STAR repeaters, GB7OK and GB7PI, a repeater at the bunker could fill in a large area of missing coverage, encompassing parts of Essex and the M25 motorway around London.

I applied for the Notice of Variation (repeater licence) on 3rd November 2014. I had originally wanted GB7NB for Nuclear Bunker, but it was all ready issued to the Norfolk Broads repeater!  GB7KH was selected for Kelvedon Hatch, and the NoV was issued by Ofcom on 3rd February 2015, with output frequency 439.6125 MHz and input frequency 430.6125 MHz. An ERP of 12.2dBW (~16W) was licensed.

On the 14 February 2015, the Skymasts 414.04-405-T5 antenna was acquired with all the required mounting brackets, along with a high IIP3 low-noise BFG541 preamp. Below, the VSWR and return loss plots for the antenna.


The following weekend we roped the cable run to gauge the length of feed required (93 metres from antenna base to repeater backplate). The heliax in this picture is some of the existing feeder used at GB0SNB by the SNBCG radio club.


Further work at the tower end of the feeder tunnels to remove the ground water was successful – it took around 40 minutes using a small DC submersible pump, which frequently silted up with the last few millimetres of water.

wpid-img_20150308_114241.jpg wpid-img_20150308_115858.jpg

On the 24 February 2015, the original Icom ID-RP4000V UHF repeater with ID-RP2C controller arrived.

wpid-wp-1424828666774.jpeg wpid-wp-1424828705236.jpeg

I spent a few weeks trying to understand the very bizarre networking requirements that the Icom G2 D-STAR software imposes. Because of the way the internet is installed at the bunker, we don’t have many options, and so using a VPN to tunnel traffic out of the bunker was the only workable option.

I enlisted the help of Rob M0VFC, shown below battling it out against OpenVPN at the 4th March 2015 Camb-Hams Pye and Pint; an attempt to meet the specific networking requirements. Rob nearly had it working in the pub, but it needed a little more tweaking. A few days later, with a fresh head and steeped with optimism, I read lots of information on OpenVPN and port forwarding, and was able to get the rest going myself, pinging the odd email back and forth to Rob when I got stuck!


On the 7th March 2015, the feeder was collected from our friends in the fens who had ordered it in for us, ensuring we got new and genuine stuff. In the back of my car, among the other junk, 120 metres of Andrew LDF4-50.


On the 12 March 2015, a major breakthrough with the Icom G2 software, when I finally managed to get GB7KH configured and registered with Icom trust servers. Thanks to Darren G7LWT for his invaluable help. I had the repeater sitting in the indoor shack for a few weeks on low power just soak testing, using an old Dell laptop until I have time to fully commission the new server. Oh, and I also at some point in time borrowed work’s vinyl cutter to cut the callsign and frequencies out for a  bit of smartness!


The lower box with the stickers on contains a Procom duplexer which was purchased through Icom UK and tuned to the frequencies for GB7KH as well as an additional transmit and receive band-pass filters to provide an extra isolation between TX and RX, reduce the TX phase noise, and keep other signals present on the tower out of the receiver.

The brand new (built to order, just for GB7KH) gateway server was donated on the 20th March 2015 by Ben M0LGN of Onega Ltd. In true radio-amateur fashion, I took the lid off:


The weekend of the 21-22nd March 2015, I was at the Rosmalen Hamfest in The Netherlands, where I picked up a few bits and pieces for the repeater. A nice commercial switching power-supply and a 1U tray to house a few ancillaries. While away, the remote GSM switch arrived from BangGood.

The repeater antenna went up over the Easter weekend (4-6th April) 2015, with a massive effort from many peopleAaron VK3ABX managed to source 2 aluminium scaffold poles for the project, which stand the antenna 3 metres out from the mast.  These were collected in the morning from Aaron’s parents – no they didn’t come from Australia! They came on G8OCV’s car (in heavy rain)!


It would not have been possible without Graham M0PAX (who supplied all of the mast hardware, brackets, clamps and cable cleats for the 90 metre run) and Rob M0VFC (who climbed the tower alone, and done all of the work above ground).

PHA_4692 PHA_4643
PHA_4641 IMG_20150405_170633

John M0UKD took this picture some weeks after the antenna had been fitted, and it was still there!


Rob M0VFC took a panoramic with his cell-phone while up the tower.


There are many, many more pictures in The Photo Gallery, which you are encouraged to check out.

On the evening of 6th April 2015, long after all the others had headed home, myself and G8OCV were in the shack setting up GB7KH for its first use!


In an ongoing program of improvement, on the 9th May 2015, just over a month after it was installed, the RF deck was opened and the interconnects changed for higher quality double-screened cables, to help reduce desensitisation of the receiver from the leaked TX signals inside the cabinet. This is a well known and crucial modification to the Icom hardware.


When I left the site, the repeater was purring away on the bench.


On Sunday 14th June, we added a preamp and notch filter to the repeater. The notch was to remove some strong signals also present on the tower. The preamp helps to overcome some of the feeder loss. Initial reports seem to indicate there is an improvement.


Sunday 12 July 2014 saw the replacement of the old Dell laptop for a much newer, lower energy SuperMicro server (donated new by Ben M0LGN/Onega Ltd).  With the repeater case open slightly, you can see the front of the notch filter.  Once the rack case arrives from Graham M0PAX, this will all be tied with a new PSU and UPS.


On Sunday 23rd August 2015, Graham M0PAX, Chris G8OCV and myself (M1GEO) went back into the feeder tunnel to better secure the feeder.  With more time than we had during the initial install, we were better able to fit the LDF4-50A feeder inside the bunker, and make a better estimate of length. The feeder was trimmed to length and the repeater moved to a more convenient (permanent) location.


On Sunday, 30 August 2015, following the above relocation work, I noticed the network router for the repeater was reporting an increased number of dropped packets on the Ethernet uplink. This was due to damaged cable termination. Some higher quality parts have been ordered to prevent future failures. Thanks to Rob M0VFC for the advice on how to reduce the potential of repeated failures in future.

The story continues with the upgrade to the MMDVM Repeater, which is ongoing…

Future Work


We are looking to home the repeater on the surface in a cabinet alongside the cell phone equipment, since this would greatly reduce the length of feeder required, and thus improve the TX and RX performance of the repeater. To this end, Rob M0ZPU has sourced us a surface equipment cabinet, with cooling and trace heater, to house the repeater. We’re waiting for some warmer weather to lay foundations for the cabinet and run cables.

Any suggestions, advice or donations are gratefully received. Contact. Donate.