GB7IP / GB3IP

GB7IP/GB3IP

Leicester and Stafford quasi-synced repeaters

STATUS

This will be updated as things progress...

10/3/2017: Repeaters both finished, tested, working. Testing reveals that when one radio is warm (approx 50 degrees) after long periods of TX and the other is cold (approx 5 degrees) upsets the quasi-sync, causing faint hetrodyning to be heard. This is really just being very fussy in an extreme case, but since we cannot guarantee that both radios will stay the same temperatures on both sites then the only choice is to use external PAs. At 5W TX power the temperature only rises to 35 degrees, and at 1W the temp is about 23 degrees (in an 18 degree room). Therefore we will use 1W into the external PA and this keeps the radios temperatures far closer together, rather than using the internal PA.

11/3/2017: Antenna arrived for Leicester site (Stafford already has aerial installed). 2x Kuhne 2m EME preamps ordered, 2x PA's ordered. Enquiry sent to bandpass filter manufacturer that looks promising reference 2x Bandpass filters.

ETA on air approx 31st March 2017 (if I say the day after that then nobody will take it seriously!!!)

History

This project started from frequent enquiries from amateurs in the coverage area of GB7SK (Rutland and East Leicestershire) from people who consistently said they really loved the coverage, since it did an area that nothing else on ANY band reliably covers. However many users were daunted or put off by DMR, wanting plain old analog FM.

Shortly after GB7SK came on air, we enquired about the possibility of having a secondary repeater on the site for 70cm FM. The response was very positive, but unfortunately it would be impossible to accommodate it using the existing 9mhz Digital repeater channels, since they are digital only. It was not possible to combine any other split (7.6mhz or 1.6mhz) up the same antenna, and due to conflicts with other commercial equipment on the site, the proposal was shelved.

However after about a year of enquiries (at least one per month, all from different people) regarding either making GB7SK dual-mode or installing another repeater, we noticed that there was actually a spare channel in the 2m bandplan. We therefore applied for GB7KS.

Once the application had been processed, it was accepted that we had found one of the very last "holes" in the 2m repeater network coverage, which considering it's location in the Midlands was rather surprising it had not been filled before. Therefore there was strong support for this proposal.

However it became apparent that the proposed frequency may cause considerable difficulties with another local repeater just 12.5khz away, and around 15 miles away. Therefore this proposal would fail. The only alternative was to look at something different.

I knew that Jon G7PFT had lost enthusiasm for GB3IP, and that it was long-term off-air. Furthermore the frequency was clear, apart from overlap between the two repeaters. The idea then emerged of quasi-syncing the two repeaters to allow frequency sharing effectively without huge complaints for users of either device regarding co-channel interference. Furthermore being the keeper of GB3FR (12.5khz away), we could further mitigate interference issues if necessary. Effectively co-channel interference in this case would actually be desirable, and at least you would be having the same conversation!

I had previously developed a quasi-sync solution that had worked rather well for commercial radio, using both Markfield (GB7LR site) and Skeffington (GB7SK site). The concept is therefore proven, and the equipment already developed, it would simply need the 68mhz transceivers replacing with 145mhz transceivers, and the Markfield equipment moved to Stafford.

I am also a strong believer that a well engineered wide-area 2m repeater, particularly one with some "quirky" technical aspect, will attract like-minded technical operators, such that traffic will naturally increase. If you enter a new area and call out on a repeater and get no response, the next time you are in that area you will probably "downgrade" that repeater in your list, or not listen to it again when passing through. Conversely if you get a successful contact, then you will probably "upgrade" that repeater, possibly listening to it even when there is no traffic, in the hope of responding to another call. Thereby traffic generates traffic.

Technical characteristics

Both repeaters will be almost identical technically (it is necessary as part of quasi-sync to use equipment as similar as possible, particularly in terms of receiver/transmitter equipment). Both use single aerial collinear antennas, both run 25W ERP, both use modified Tait TM8000 Series radios (to give frequency lock), and both have identical GPS frequency reference.

To use the repeater(s), you do nothing clever or complicated. Simply receive on 145.7625/145.1625 (RV61) as you would for a "normal" repeater. Both repeaters use CTCSS on receive (mandatory, no toneburst or carrier access), but neither transmits CTCSS, to make the quasi-syncing easier. We may address this at a later stage.

It must be noted that this repeater will have a 40 second time out timer. The reason is that this encourages a "chatty" conversational style that is more suited towards a wide-area repeater where there will be multiple users at any one time, rather than longer "overs" which are more suited to local repeaters where the demand for their use is less. This may be revised in time in light of experience, but this will be the initial setting.

It is intended at a later date once the quasi-sync has been established and proven to work correctly then to assess the possibility of using DMR across this system. This is subject to equipment availability for the infrastructure, successful trials of DMR quasi-sync, and support from users.

Donations/support

I don't need or want any donations. I find it simple that way. If you want to give something back then please enjoy using it.

Quasi-sync is a rather interesting technology and this repeater has been installed in the spirit of allowing radio amateurs to carry out the first clause in their Ofcom license - "Self training in Radio Communication". You can therefore learn and experiment with DMR for yourselves to see what it does.

If you do have any questions then please telephone me on 07824 364 541. It really is the best way to get hold of me because I am rather busy and don't have time for emails.