Saturday, May 6, 2017

Galway 70cm Repeater - EI7AKR

The Galway VHF Group 70 cms Repeater is now located at its permanent home on Knockroe, Abbeknockmoy, Co. Galway. This is a prime site with a superb and unimpeded view in all directions. Initial predictions using Radio-Mobile show a pretty uniform coverage of the County of Galway in almost all directions. 

As can be seen below, the coverage is impressive. This will facilitate many operators who just can not experiment with communication over distance using UHF Radio. The primary objective is to give saturation coverage of the County and, more importantly, the main routes into and out of Galway City. Regrettably, this is not possible in the direction of Clifden due to the proximity of the 12 Pins and Maamturks, however, coverage into Oughterard, and just beyond, is possible. Whilst predicted coverage is impressive, the actual mobile coverage may bring in even more surprises.

The Individual components of the Repeater are as follows

 The Repeater is fabricated from a modified Commercial Kyodo UHF Repeater unit running 40 Watts fed into a 4 stacked dipole array with an actual gain of 6dB. Naturally, ultra low loss coax feeder will be used. The Repeater was supplied by WESCOM and modified by Enda O'Brien EI5GMB.

To achieve the isolation between transmit and receive it is necessary to insert a band pass/band reject filter in line. This is one area where compromise is not an option. A set of six Sinclair cavity filters were purchased. Expensive but perfect for the job. Careful tuning using a spectrum analyser and/or a VNA was required to achieve the best possible rejection. The specification suggested that there would be 120dB of isolation between the receive and transmit frequency and we were using a 7.6MHz split.

Ideally the cavity filter needs to be tuned into the feeder and antenna in use on site. Although the cavity filter can be tuned into a 50 Ohm load, it will be necessary to tweak it up on site. If one changes the antenna or feeder the whole process of tuning the cavity filters has to be repeated as minor changes to the load will alter the characteristic curves.Shown below are the characteristic curves on the receive tuned to 432.825MHz. 

We were fortunate that we were able to tune the cavity filters into the correct heliax feeder and antenna system prior to installation on the permanent site. This will reduce the amount of time spent tuning on site. It will probably be necessary to make minor adjustements before leaving the repeater once installation has been completed.

It is now possible to run 30 watts of power without any worry about desense or drift. With the present power setting it has been possible to get into the repeater with an S 1 signal received from it whilst it is tranmitting 30 watts. This may have to be balanced on site but at this stage it may not be necessary. 

There is no noticeable temperature effect causing the cavity pass or reject bands to drift. We had tried the small Procom type filters which actually heated up when our transmit signal was passed through them. This resulted in the characteristic curves to drift and resulted in desense after a couple of minutes of use. A search on the internet revealed the Sinclair cavity filters.These were by no means cheap but it is far better not to compromise the system for the sake of saving a few Euro. 

An RC-100 Repeater controller deals with the identification and timings of the repeater. The repeater is accessed using a CTCSS tone of 77 Hz as it is considered irresponsible to allow spurious triggering from a carrier operated squelch. Through this controller, the Repeater can be switched off at short notice if required to do so. 

The Antenna is the most important part of the set up.

The antenna system was obtained from Radio Structures Ltd in the UK. It was designed specifically for the task taking into account the Transmit and Receive frequencies. the Gain of the antenna is 5.5dB gain over a 1/2 wave dipole.The antenna is true omni-directional and the characteristics are shown below. 

There is little point in producing a professional system without a professional antenna fed with low loss coax. In our case we used Andrews heliax with negligable losses at UHF.

Status - Active

Call sign EI7AKR

Repeater Access by 77 Hz CTCSS Tone

Modulation Narrow FM (+/- 2.5 KHz)

The IRLP Node Number is 5705
The Echolink Node Number  5705

Echolink Node EI7AKR-R

Locator Reference for site IO53PK
Output Frequency 430.825 MHz  

Input Frequency 438.425MHz

 Frequency Shift + 7.6 MHz 

Please note Time-out 2.45minutes

Repeater Keeper - Steve Wright EI5DD
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