2 radio-linked remote sites via 1 antenna

[Rob_Brookes]

For those of you who have more than one remote radio site where they are fed from your base by a radio link, here's a suggestion confirmed by the engineers at Simoco. If the link-channel frequencies are kept apart by a suitable amount (about 4 to 5 MHz) then a diplexer can be used taking it's feed from two of the link radios and using only a single antenna on your base roof to handle both sites. Although this would work for semi-duplex systems such as the current wide-area repeaters use, it's potentially more valuable to those who use fixed rebroadcast. There's currently only one UKSAR channel available for this purpose, 147.350MHz. However, as these are closed links where no other station but your own base needs access to them, there's nothing lost in buying additional channels from Ofcom, the remote site output is still on your team working channel so everyone can still talk to you. How it gets back to your base is irrelevant to users of the system. As MR is a charity recognised by Ofcom, you get 50% discount so at the time of writing such a channel would cost you about £38 per year.

A number of teams currently use fixed rebroadcast with low band link channels so that changing the operating channel on the remote site is possible without any messing around with DTMF. Again, a suitable diplexer on the antenna leads to two such link radios would require only a single antenna on your roof. There's no major electrical or operational advantage to doing it this way, apart from the increased isolation achieved using a diplexer which will almost certainly be greater than if you are using horizontal antenna spacing, but those teams who are short of places to put any more antenna on their base roof, might see this as a way of at least reducing the number of antenna needed.

Using an SRM9030 radio at your base also means both sites can be independantly controlled so you don't have to transmit on both sites simultaineously. You do need a base radio operator who's on the ball though even though the radio display will change to tell you which site is currently being received. Methods of doing this vary from simple routing of the CAT5 control head leads via a switch box (where theoretically any number of radios can be controlled), to a bit of innovative programming of the 9030 control head and a Dmap accessory card which brings the number of controlled sets down to two. If the 'Repeater Mode' is not programmed but 'Radio Select' is, then both radios, whilst being completely independant of each other, can be selected for transmit via a single control head. Both will always receive so you never miss any transmissions from the field but only the control-head selected radio (and hence, the selected remote site) will transmit. Press the button and you're on the other site instantly.

This system is in the process of being developed by Langdale Ambleside MRT and anyone who needs any more information on this can contact me via the website and I'll do what I can, mostly at the moment it's food for thought. Anyone who wants to know how a low band remote link can be used to change the channel on your remote site high band TWC radio can get the information the same way.