7075 P25 Digital Under Test!

Colonial Knob 7075 has become, at least temporarily, the first Amateur Radio P25 digital voice repeater in New Zealand! How has this happened? Read on…

The Colonial Knob repeater at 7075 has undergone a comprehensive upgrade and is now Project P25 capable as a result of the failure of the original repeater equipment. Unfortunately this has happened at the same time that on-going repairs to the power line that feeds the Belmont 710 site has meant that there is only sufficient power to maintain the UHF National System facilities at Belmont.

These circumstances made the immediate repair of 7075 a high priority so that good 2M coverage was available for VHF Convention this Easter. The 7075 repeater was originally built for 710 by Peter Williams ZL2ARW more than 35 years ago! While it has given sterling service it has begun to become rather unreliable. Two weeks ago a team visited 7075 to investigate the latest failure. While on site the issue of coverage for Convention was discussed since it was known that a spare mixed mode commercial VHF repeater was available capable of both analog FM voice and digital P25 operation the decision was made to recommend the temporary use of this equipment to VHF Group committee. Since 7075 was already designated a mixed voice and data repeater it made sense to take advantage of the capability to explore P25 at no cost to the Group.

The Committee quickly agreed to support this initiative so a FMTAG Form 10 has been drafted for discussion and a decision at the next VHF Group meeting by members before forwarding to FMTAG.

Why do a Form 10 for an existing repeater? Well the trustees would like to raise the power of the repeater from the present 14 dBW while the use of the P25 modulation (4 level C4FM) requires the addition of the emission designators 8K10F1DDN (P25 voice) and 8K10F1EJN (P25 data) to the exiting licensed 16K0F3EJN. These technical changes require an amendment to the licence in SMART hence the Form 10 process. And of course we want to publicise this advance in amateur radio!

OK so what needed for working 7075? For normal analog operation an existing 2M FM rig will be fine, analog FM in results in analog out. For P25 digital program the default NAC of 293 and you’re set to go, digital in results in digital out.

There will be a lot more information on Amateur Radio P25 at VHF Convention this year but to get started have a read of the following QST article from 2008: http://www.qsl.net/kb9mwr/projects/voip/apco25/Project%2025%20for%20Ama…

Just like any other mixed voice and data repeater you’re going to hear voice and bursts of data when listening in FM mode. If you don’t like the buzzing data sound of P25 the repeater has a handy CTCSS tone on its output that is present only with analog FM. If your rig has a CTCSS decoder, enable it to only open the squelch when receiving a 123 Hz sub audible tone. You DO need CTCSS to access this repeater. Note that with CTCSS on you will need to check to see if the channel is ‘busy’ before transmitting.

John ZL4JY
Trustee 7075

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