Perth to Geraldton

An idea on how to link Perth to Geraldton on 2m using existing equipment in one hop.

Home
The path between Perth and Geraldton on 2 metres is a good one with propagation possible much of the time.

For several years Noel VK6MF and myself Will VK6UU ran a Monday night sked on 146 MHz FM, and in the entire time never missed a QSO. We were using 25 to 50 watts into 10 element beams at each end. My QTH was a high point in Lesmurdie and Noel's, out near the light house in Geraldton.

My idea is to use the unused Vertex, (VK6RLM) and the original VK6RLM, an FM880 as a linking system between Perth and Geraldton..

The system requires some explanation as there are some options as to how it would work. First the overall diagram of the concept, followed by detailed explanation.

Perth to Geraldton

The basic concept diagram
The linking system to be described uses 2 repeaters and one transceiver
Step one original VK6RLM at Tic Hill
I'm calling it VK6RLM for ease of explanation.
Install original VK6RLM (FM880) at Tic Hill and connect to at least a single 10 element vertical beam. The beam or beams to be about half way up the tower as shown on the diagram above.

The antenna gain needs to be 10 dB more than the existing antenna gain of VK6RTH 2 metres, which should be 6 dB. Hence the gain of the vertical beam, or beams, needs to be 16 dB. This does require at least 2 correctly engineered 10 element beams. The reason for the design concept of at least 10 dB more is to make a considerable improvement in the signal North, as compared to the existing repeater VK6RTH.

The frequencies of VK6RLM (FM880) running into a duplexer will work with no desensing to the existing 2 metre repeater, VK6RTH, as each repeater's duplexer offers a good degree of isolation between repeaters, and visa versa.

VK6RLM is not connected to VK6RTH. VK6RLM operates as a normal repeater with a high gain antenna pointing North along the Brand Highway to Geraldton. The repeater not only acts as a link to Geraldton, but a normal repeater with considerable gain to the North. VK6RTH 2 metres is not changed in any way.
Geraldton
At Geraldton, the Vertex VK6RLM is installed, also into a high gain vertical beam, pointing South to Perth. Now this is where it gets interesting and a little complicated as there are options available, depending on what we choose to do.

Most important you can not just set up the Vertex repeater on the reverse frequencies to VK6RLM at Tic Hill. Sounds like a good simple way to go with the two repeaters hearing each other, and hence "linking". Trouble is this set up locks up with lots of audio howl around. It is like putting a microphone in front of a speaker. Each repeater keys up the other repeater and the systems remains keyed up until the time outs kicks in.

Perth Geraldton map

Perth To Geraldton and Brand Highway
Option one

If the Vertex can be frequency shifted on the fly this is what could be done.

 The vertex, in Geraldton, receives on 146.750 (beam antenna) and transmits on 147.300 to an omni directional antenna as shown on the diagram above. The Vertex transmits on 146.150 to the beam and receives on 147.900 from the omni.

This is complicated and requires the vertex to be able to shift frequency, both receive and transmit, and for the Vertex receiver to be able scan between 146.750 and 147.900. And added to this the Vertex has to have coax relays to switch between the required two antennas, the beam to Perth and the omni for the Geraldton users.

The only advantage of this option is it only requires the Vertex and no extra equipment, but I believe the complication out weighs the advantage, even if the Vertex can do all the frequency changes on the fly.

Option two

Option two requires a second 2 metre repeater and the Vertex to operate, not as a repeater, but as a transceiver, receiving and transmitting separately, not at the same time as a repeater does.

The new repeater operates on 147.900 RX and 147.300 TX into the omni antenna shown in the diagram, and is a stand alone repeater and can be used by the Geraldton amateurs as such. The reason for the frequency selection at the high end of the 147 MHz band is to minimise desensing to and from the Vertex link transceiver.

When an amateur in Geraldton transmits on 147.900, the new Geraldton repeater re-transmits onto 147.300 as a normal repeater, but also the Vertex transmits on 146.150 into the vertical beam pointing at Perth. Propagation allowing, this signal is received at Tic Hill, on the original VK6RLM FM880 and re-transmitted on 146.750 to Perth amateurs listening on this frequency.

The reverse happens when a Perth amateur transmits on 146.150, VK6RLM at Tic Hill re-transmits the signal on 146.750, which is beamed to Geraldton and received by the Vertex transceiver to be re-transmitted on 147.300.



In Summary
Using the original VK6RLM & the replacement VK6RLM, plus the addition of a new repeater, Perth to Geraldton could be linked in one hop. Perhaps not with 100% reliability, but with a high degree of link time. As an added bonus, a stronger signal North from Tic Hill would be achieved for amateurs using the Brand Highway.

Also amateurs in Geraldton, when traveling South, can access the Vertex link transceiver on 146.750 rx and 146.150 tx. These amateurs operate reverse repeater offset and have a much stronger signal South and are re-transmitted out on the Geraldton repeater on 147.900 - 147.300.

A further option, if the concept is considered a worthwhile amateur project, is to use all new commercial equipment, using higher power between 50 and 100 watts.

What are your thoughts....?

Will
VK6UU
will2@iinet.net.au