combining assistance

[FatBoy]

I have a new customer who is using a dozen or more mobiles (looking to expand to 20 or 30 total) for thier dispatch center. They are currently using 3-5 element uhf yagis that are pointed towards various LTR trunking systems around the area. They have figured out what towers each diapatch mobile can hit reliably. The current system if feed via 100-150ft of rg-58 for each antenna. I know the correct way to do this is 1 radio for each channel fed into a console (mip5k, c-soft, mcc, etc). I do not have the experience in combining to reduce the number of antennas on the roof. I was told to use a hybrid combiner, but the tessco rep is less than helpful. I know they use something for the fed/gov offices in my area as they have a bunch of mobiles in their office and dispatch area with few antennas on the roof. So an engineer friend of mine recommended a yagi pointed at each site fed from a power divider which is fed from the hybrid combiner, then all of the mobiles are connected to ports on the hybrid combiner. I know there will be 7+db of loss per port on tx for the combiner.......I know this is not rocket science , I am just new to this level of infrastructure. My initial plan was to reduce the dispatch mobiles to a few watts each in order to reduce the desense/IMD products, but was told that is an option of last resort. Thanks, FB.

[MSS-Dave]

Not rocket science but hold onto your wallet if you want a out of the box solution...

TX-RX, DB Spectra and Sinclair all make them for your band of interest. I use 2 8 channel units for my backup 800 MHz consolettes (I think they are Celwave, can't find the info right now...), use seperate TX/RX antennas. You can get duplexers for them too. And you are right, loss is about 8 db or so if I remember right. No problem, 30 watts in about 4-5 out to 3 element yagi. UHF losses may be higher per channel but it will depend on the number of ports on the unit, more ports, more loss.

Dave

[FatBoy]

Dave,
does it make sense to reduce actual base radio tx power to 1-5w if the customer is ok with the antennas and slight desense? Right now, it is $20k list for an 8 port unit and my customer would need at least 2 to start. I think the engineers who pointed me in this direction are used to fed/gov where budget is not that big of an issue. FB

[Bill_G]

This is a tough nut to crack when all you have to work with are single mobile radios. If they had two for each channel (one tx, one rx), then they could consider using more conventional, slightly cheaper, xmit combining and rcvr multicoupling equipment. It really depends on the freqs of interest. If everything is UHF LTR, putting all the rcvrs on a single antenna would be a piece of cake. And if all the xmit channels are 250k (or more) apart, the combining would be simplified. Multiple four port combiners can be combined themselves to a single antenna with tees and tuned stubs. But, if they have close channels less then 100khz apart, then it starts getting expensive, lossy, and requires more than one xmit antenna that should be in different horizontal planes (ie; separated vertically 25 or more feet). That would still look better than ten or fifteen antennas scattered around a rooftop like I've seen at some AAA call centers that dispatch numerous indy towing companies. When you have that many antennas in the same plane, things get ugly fast, and the crazy spaghetti wiring of RG58 all over the place actually benefits the site.

http://www.dbspectra.com/categories.php ... red&page=2

Looking at the dbSpectra site, who seem to be the only people attempting control station combining, the costs of passive cavity combining versus their control station combining is about the same. Their control station combiner allows 0khz channel spacing as opposed to passive cavities which can only get down to 100khz separation. Much more loss, but very small rack space.

[MSS-Dave]

FB..

Bill is right on with his take on this. Only thing worse than having to deal with multiple control stations is multiple repeaters on the same rooftop when the owners don't want to spring for combining (read: hotels in a tourist area that add repeaters and radios from whom-ever gives them the best $$$ deal, not the best technically.).

As far as lowering power, it should (legally) be licensed as a FX1 control station which limits power and height anyway. A good rule of thumb is your ERP from a directional antenna shouldn't exceed what the mobile ERP is using the same system. Minimizing your TX power **COULD** help your desense issues but there are lots of other factors to consider like antenna to antenna spacing, frequency seperation, quality of coax and antennas, etc. When you have that many transmitters on seperate antennas like this, it just screams Intermod.

I cases where I just HAD to make it work, I tried to get as much vertical seperation as I could between the antennas on the same support then tailored the TX power on the uplink to make sure I didn't overload the receivers on the repeater site.

Dave

[FatBoy]

MSS-Dave,
I think we are in agreement where there is the correct way, and then there are those times where it just has to be passable. I am not a fan of passable (no room in the link budget), but most of the shops in this area have declined this project at this point. I want to give the customers options, but with big caveats. Everyone else just says "no" and has moved on. The "cheapest" customers, where you make almost no money, are where you spend an inordinant amount of time.....FB

[Bill_G]

Well, if they bought two of the 16 port units at $40K ea plus labor, with 14db per chan insertion loss, and each radio putting out 26W/+44db, there would be 1W at the antenna port per channel. There is a similar insertion loss for the rx side. If the paths can work with that, then you would have a good sale in a tidy package. It could be made to look very pretty and professional for under $100K. Sounds gigantic at first, but would qualify as a capital expenditure that their accountant can help them with at tax time. This is the kind of expense that gets the guys in power ties excited in a good way. I say go for it.

[MSS-Dave]

Hmmmmm.... This is looking like a large cable provider I *ONCE* looked at in the dim and distant past in another area of Florida... lol

Bill's suggestion is the really right way to present to your customer. That is what we have here, 2 8 channel units on 4 antennas, 4 1/2" Heliax runs. Very neat, VERY expensive (read: Motorola engineered and installed...) but it was part of the bigger picture if you will and a necessity.

Is there anyway to reduce the number of mobiles needed by doing remotes within the center? I mean grouping all that need access to a talkgroup on a single radio? I can't imagine that they need 30+ talkgroups with separate radios for each on LTR. Maybe consolidate that way then the RF clutter might be more manageable. ANYTHING you can do to reduce the number of radios can only help if you are forced to proceed with that solution.

Dave

[d119]

I can vouch for these dBSpectra control station combiners in the UHF band. They work extremely well provided your path is there and they are very tidy and easy to interface. Definitely a product to be considered.

[escomm]

Popping in to offer Telewave and EMR as alternative resources. Much less expensive than TX/RX and dBSpectra, better lead times than Sinclair, oh and better pricing than Sinclair too.

[tvsjr]

Popping in to offer Telewave and EMR as alternative resources. Much less expensive than TX/RX and dBSpectra, better lead times than Sinclair, oh and better pricing than Sinclair too.

I've dealt with EMR for a few duplexers including one where it was a critical need (like yesterday) and they were excellent to work with. They turned my critical need around in 3 days order to delivery and didn't even charge a rush fee.