Attached is a concept for the very important antenna installation on the Walliston tower. There may be another concept idea but I have not seen such as of yet. So to kick things off please have a read and comment.
Walliston RF concept
The antenna layout on the Walliston tower can be designed in several different ways, each with its own pros and cons.
The concept presented here relies on separate antennas for each band and split antennas, i.e. separate antennas for receive and transmit, except for the 70cm digital repeater and the simplex 23cm high data rate gateway.
This design is expensive in that it uses more antennas (8) and more coax (8) and hence takes up more mast space, but most important offers several important advantages.
Antenna failure, replacement and testing only takes out one repeater. If a triband or dualband antenna fails for example it affects 3 repeater systems. Antennas can be easily isolated tested and replaced.
Easier to achieve desense isolation with less reliance on duplexer performance. Good duplexer performance is difficult to achieve particularly on 2M. 70cm duplexers are cheap and don?t require as much isolation as 2M duplexers. 23cm duplexers are difficult to source. The antenna layout highlights this with only the 70cm repeater using a duplexer into a single antenna.
Below is a concept diagram of the Walliston site. It shows the antenna mast, which is 75? (23M) high along with the layout of the antennas. Yes there are a lot of antennas and yes there is more than one way to put together the antenna layout. The number of antennas could be reduced. This design uses split antennas on all systems except the 70CM digital repeater and the 23CM high-speed digital simplex system.
Split antennas means one for receive and one for transmit for each duplex repeater, rather than a duplexer feeding one antenna. This offers one major advantage, less filter isolation is required (cavity filters) and zero desensing is much easier to achieve.
What type of antenna to use?
Folded dipoles and in particular the ones you see on commercial installations are cheap, broadband, DC earthed for lightning protection, low SWR (1.2 or better), robust, come in a wide frequency range, can be custom made if required (e.g. 23cm) and can be stacked (more than one) to produce increased gain. And these antennas are designed to work side mounted on a tower, which is the type of installation we are looking at. Side mounting also produces 3dB gain, which is ideal, as the Walliston site should direct most of the signals to the West.
Due to the number of antennas and the number of coaxes required in this design there are some options to reduce this. For example the 2M receiving dipoles (2 of) could be phased together (3dB gain) connected to one coax and then split via a low gain RF amp at the bottom of the tower, giving two antenna feeds. Or only one dipole could be used (to save antenna space) and split as described.
The 4-dipole arrays on 23cm and 70cm could be reduced to 2 dipole arrays. It all depends on what the tower can support.
The diagram is only roughly to size but a to scale diagram can be produced to give us a better idea of the antenna space taken.
Walliston tower configuration
I hope this document encourages discussion and more ideas. The antenna installation is one important (if not the most important) aspect of this complex installation. If we wish to achieve the very best performance RF wise then considerable engineering excellence needs to be our number one goal.
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