Hi all, has anyone got any experience of building a passive repeater for VHF frequencies (around the 2M band)?
I’ve been reading up and it appears potentially feasible to point a highly directional Yagi (several elements) at an active repeater site (in this case with a 7MHz split - it’s not amateur radio), then point another somewhat less directional yagi antenna into a valley that is shielded from the active repeater site.
Then just connect the driven elements of the two Yagis with a short length of co-ax.
Effectively it creates a fixed version of something similar to aircraft scatter, only somewhat more tuned and permanent!
It might allow a very unobtrusive solution to get into a “dark” valley without resorting to some sort of off grid powered “proper” repeater. All you need is a short pole and the two antennas.
I know it’s used commercially at microwave bands, but seems to be less common at VHF.
I designed a UHF one for a commercial application. You need to do the path loss calculations carefully. They are generally not very effective at lower frequencies.
I need to buy several Yagi antennas for the project anyway. So they’ll get reused if it didn’t work.
Just as a rule of thumb, based on no real science whatsoever, I was thinking of a 10dBi gain antenna pointing directly at a 10W repeater 4Km away. The other would be a 5dBi gain antenna to point into a side valley which extends for about 6Km. The reason for the lower gain is to maintain enough spread into the valley.
The handheld radios will be 5W into a reasonably tuned helical that I am told is better than -3dBi.
It’s all running on DMR.
(This isn’t a commercial operation. It is for volunteer Mountain Rescue).
The alternative is a high site 35Km away (!) that will illuminate the upper parts of the valley with a highly directional beam connected to a proper 10W repeater. It’s the upper parts that matter because that’s where 99% of the rescue activity takes place. We’ve actually tested that with a temp set up and it works.
It’s not possible to put a self powered (solar/wind) site where I propose the passive repeater might go for reasons of visual intrusion and the chances of getting planning permission are zero.
Suggest that you design it properly with a pen and paper to avoid wasting time on something that won’t work. Active repeater is far more likely to work.
That was what I thought (and as I say we’ve actually tested that from the 35Km site).
But I’m just wishing we could cover the lower parts of the valley.
One option is a vehicle mounted repeater with 4G backhaul. But even that is tricky due to very marginal 4G signal where the vehicles get parked before team members dismount to go up the hill.
I’m just soliciting ideas from this community. We’re all interested in hills and radio and there’s some good experience out there.
My view Gerald is if this was just an amateur repeater you could do all sorts of things that would probably work and do piecemeal improvements over time. But it’s for something much more serious… you may already have someone in difficulty and so having a comms system the mountain rescue need that is iffy etc. puts them into more danger when they are already doing something challenging.
The relentless application of cash will always solve these problems but you will have limited funds no doubt.
Vehicle mounted gear is a way to get around planning issues. If the backhaul is difficult from the normal vehicle parking place, consider dropping a repeater in a box somewhere between the vehicle and the site you are trying to contact. Drop it off on the way out and pick it up on the way home.
If you went for the 4G backhaul you might manage to get a 4G antenna with some gain that might give a somewhat improved coverage, but it might take a bit of time and skill to set it up. In Teesdale we knew the spots where there was poor or no coverage and had a map in the base which indicated where the vehicle had to be parked to solve the problem. (This was the old VHF analogue stuff but the same probably applies to finding the bits with an acceptable 4G signal) ) At least part of the issue is that the system needs to be both reliable and foolproof as when it is needed it won’t be used by someone who enjoys solving a technical problem at 3AM in the rain. If you went for the high gain antenna on the real repeater with a 4G backhaul plan B system, and then spend a day driving round the valley identifing the best parking spots that give coverage you might manage the best of both worlds. Good Luck Paul
Hi Gerald,
The path loss calcs are not that complex. Without the gain from the rx/tx in an active repeater it is always going to be difficult. An active repeater adds about 10 to the14th power gain, 140dB, that is the magic. Hence the high gain aerial passive microwave approach vers vhf. I suggest some meaningful calcs before expenditure.
Regards
David
G0EVV
A comms company I worked for in the 80’s tried a passive repeater on an estate in Perthshire, didnt work very well from memory and was dismantled. That was a 25w erp base to 5w handportables on VHF high band. I think it was a pair of 5 or 6 element yagis, one with LOS to the base station and other pointing up the glen. That probably only yields about 15dB of gain which is not much, perhaps ok for point to point but not point to multipoint.
I know that two Mountain rescue teams up here (Lochaber & Cairngorm) are making use of latest DMR single frequency repeater feature on vhf high band. Some fixed repeater, some on mobiles in vehicles and on some of the latest handhelds which also do SFR. Transforms how you can achieve wide area coverage, everything on one simplex frequency and some units acting as SFR. Professionally I helped Lochaber get a rent free site share for a duplex repeater and supported them to get it running, I suspect that will get changed out to a SFR also.
Sites so far not a problem for fixed sites, except for planning restrictions in national parks.
We actually need the antennas anyway elsewhere in the project at a later stage, so the cost would be time and effort.
But appreciate you calculations that put the whole thing into perspective!
Yeah… that’s still a problem as the one bit of hill I need to cover has no spot for a vehicle to park to provide proper coverage. It would help, but not fix completely.
Yeah, that’s been considered. A couple of problems though. No roadside/trackside site actually gives coverage. It’s a very high footfall area, theft or damage is a significant risk, based on other examples.
Thanks all for suggestions - they are all very useful and add into the mix of thinking the project team have.
I built a passive repeater for 460 MHz. many years ago. It worked very well and gave coverage of a 2-way system into an area that we wanted. Several years later, we were done doing what we did and stopped needing it, so I scheduled it for removal. It took a long time to get to it (years). When I finally removed the antennas, I heard the county radio tech got a lot of complaints about no coverage in the valleys where they used to have coverage. I just got a chuckle out of it.