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Old 05-30-2018, 10:15 PM
KG5LVU KG5LVU is offline
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Default Questions about repeater and antenna setup

Hi, I help run a small event (about 3000 people) but we have to cover a pretty wide area (about 3 square miles) so we added a repeater setup a few years ago and haven't looked back. However, as more and more teams are spread out over the whole property we decided to move to a trunked repeater setup this year (Motorola Capacity Plus). It mostly worked really well until I had teams tying up the first repeater's 2 time slots. As the second repeater's antenna was 10' farther down the mast it didn't always work. So I'm trying to figure out a solution to get the second repeater to work better.

Ideally I'd love to get one antenna for both repeaters, but from what I can tell that takes a ridiculous amount of equipment and introduces high amounts of signal loss.

So I was thinking about trying to consolidate the TX and RX antennas instead of having each repeater on it's own antenna like now. Put the RX antenna at the top of the mast (and maybe get a high gain antenna for the RX while I'm at it). Can I just get my existing duplexers retuned for this or do I need different equipment to consolidate the repeaters in this manner? Any other downsides I'm not thinking about?
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Old 05-31-2018, 01:15 PM
paulears paulears is offline
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Location: East Anglia, UK
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Default Re: Questions about repeater and antenna setup

Sorry - I'm a bit confused. You have two repeaters on the same mast, trunked? There are a number of problems here, how close are they in frequency? If you are using the usual duplexer, the bandwidth is quite narrow, so you don't have to be far off the tuning for the performance to suffer - it's quite possible that your receiver isolation from it's transmit frequency is good, but the other repeater might be far less reduced in level causing desense on the other repeater, and vice-versa. If the site is big, would you not be better to do what trunking is really designed for, and shift the repeaters apart, and then do an IP link between them - or just run the two repeaters from both ends of the site so they work perfectly and independently. No reason you can't simply run two slots on one frequency and two slots on the other - which would be far, far easier. Trunking wouldn't really help - here. you'd not get any benefit as if you use the trunking normally - you still only have two effective channels - because for much of the time, both will be transmitting the same information. To use two channels close in frequency on a single mast needs much better and tighter filtering - a simple 5 or 6 chamber cavity just doesn't have the narrowness to make it work.
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Old 05-31-2018, 11:35 PM
KG5LVU KG5LVU is offline
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Join Date: May 2018
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Default Re: Questions about repeater and antenna setup

I've got about 10' of vertical separation between the two antennas that should be roughly 55dB of isolation so I don't think that's the problem.

As for putting the repeaters at different parts of the site and IP linking them, that isn't really an option. This is unimproved camping ground that we don't own. So adding the line to do that would be difficult at best.

And you and I have very different opinions about what trunking is used for. We're trying to let multiple groups use the repeaters since we don't use all 4 timeslots at once for the teams that were on the repeaters previously. We're using trunking to make those 4 TDMA/FDMA channels available to about 7 different talkgroups. That part worked great except for when people on the edge of the property got directed to the bottom repeater because the top one was full.
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Old 06-01-2018, 06:42 PM
paulears paulears is offline
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Location: East Anglia, UK
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Default Re: Questions about repeater and antenna setup

So you're doing a hierarchy access then? some get priority and use the A repeater, and the other is an overflow - and the problem is the overflow is a poor performer?

Silly question, but have you swapped the antennas around and seen of the problem swaps - the lower antenna becomes the good performer and vice versa?

I suppose you could invest in an antenna combiner and splitter and run single antenna working. I'm just left wondering as if both repeaters work fine on their own, the only likely cause of the grief is desense - and while your isolation figure is typical for 10' can you get that with the separation you have in TX-RX frequency? Some of the smaller mobile type duplexers struggle to work with the usual splits we have here in the UK, and I guess the lower antenna height reduces the overall efficiency.

The only feature I don't have the maths for is how two TX emissions interact in the separation calculations. you have the desense caused by the shared antenna for TX and RX, AND the additional desense from the other channel 10' away. I've no experience of how they interact in this way?
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