GB3IM
Project to Enhance Coverage
Of The Island Using
VOIP and Internet Technologies

Background to the project

The Isle of Man has been served by two co-located repeaters for many years (at least twenty to my recollection, and certainly more than that). During that time technologies have changed and appeared which make it possible to greatly enhance the effectiveness of our repeaters, particularly with regard to coverage of the Island itself.

One often cited shortcoming of the Snaefell repeaters is the fact that, whilst the location offers superb coverage of the Irish Sea and surrounding lands, it often fails to provide adequate coverage of its immediate surroundings.

This leads to the situation where off-island stations enjoy excellent service, whilst on-island stations often struggle to be heard.

An additional drawback of Snaefell is ease of access. The only motorised access to the summit is either by means of the Manx Electric Railway, which is operated only during the summer season, or by "hitching a ride" with the Civil Aviation Authority's on-site engineers in their Diesel-powered rail car. Again, the CAA staff only visit the site on a few days in the week.

At all other times, the only other access to the site is by walking - not an enticing prospect when you're carrying tools and test equipment.

The weather is also an issue with Snaefell. It can be a very hostile environment for both people and antenna hardware. Opportunities to effect repairs can be few and far between, especially in winter, which could lead to extended periods of "Downtime" in the event of an aerial fault.

The Conclusion

Whilst keeping Snaefell on the air is highly desirable, the group feels that additional, supplementary, coverage is needed in order to maintain communications if the site should be lost for whatever reason (Weather, power failure, antenna fault etc). Furthermore, it is felt that Snaefell alone does not provide adequate coverage of the island.

These requirements have led to the birth of this project.


The Proposal

The project was first floated at a meeting of interested parties in December 2007. This discussion re-inforced the already well-known views of many local amateurs that we needed to become less dependent upon Snaefell as our primary repeater site, and that infill coverage of major communities was required.

During the following year, various technical discussions were held with the Emerging Technology Co-Ordination Committee (ETCC) of the RSGB to determine whether we might be permitted to achieve our aim of providing greater coverage of the island by using multiple internet-linked repeaters.

As it turned-out, once the ball had started rolling it quickly gathered momentum. The ETCC were keen to encourage this project, especially as the design deliberately avoided extending the coverage of the repeater beyond the island's boundaries.

The question was asked, and was quickly rejected, as to whether we could do anything on 2 metres. One of the reasons for rejecting 2M was the absurdly high price of duplexers but, additionally, recent studies by the ETCC have determined that new proposals for this band will not be readily accepted.

Once it was agreed that 70cm was the way to go, the ETCC were exceptionally helpful in moving things along at breathtaking speed.


The initial plan is as follows:

  • Re-Work the configuration of GB3IM.
  • Retain Snaefell on its present frequencies.
  • Install two additional repeaters at strategically located sites.
  • Link the three units via the internet so that they operate as one repeater.

The primary node for the project is located at the Carnane Radio Site (GB3IM-C), located south of Douglas. This node will house the main computer hardware and the primary internet link.

Snaefell will be reconfigured to be a secondary node, triggered by an uplink from Carnane.

A third node will be added in the North of the Island to provide infill coverage in areas tradionally poorly served by Snaefell because of terrain screening. The site is about 3 miles north of Ramsey and is designated GB3IM-R. This node will be internet linked to Carnane.

Carnane and Ramsey (GB3IM-C and GB3IM-R) will operate on wide-spaced channel RU66 (Output: 430.825MHz, Input: 438.425MHz). Snaefell will remain on RB5 (Output: 433.125MHz, Input: 434.725MHz)

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