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Thread: UHF mobile duplexers

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    Default UHF mobile duplexers

    The end of the school year is approaching, and several new school radio systems will be going in this summer. But, the design for one of them troubles me. Using the low cost UHF mobile notch duplexers is pretty common when there is only a single repeater involved, but this project has six schools linked by IPSC with most schools having two or more repeaters so they can have wide area and local coverage on hard time slot assignments. ie: Poorman's trunking to avoid the cost of LCP. I predict these notch duplexers are going to cause self interference.

    I have over twenty repeater pairs spread across 451/456 and 452/457. So, spreading the freqs out hoping for some natural rejection isn't there. I'm considering putting the multiple repeater receivers on a single antenna through a window filter and splitter rather than their individual duplexers. There is no budget for combiners, or receiver multicoupler shelves.

    Any other ideas to beat this project into submission while keeping the costs down?
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    The only practical way would be to wither run separate antennas if cost is a huge factor. If you have a bit of budget then you would be better to use a TX combiner combination and a single band pass filter to isolate the RX freq.

    Near freq pair de-sense should not be an issue as the 5meg offset should fall well within the radios internal filtering. If in doubt a single notch filter tuned to the average center tx freq should drop any unwanted signal to a ignorable level.

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    Quote Originally Posted by Notarola View Post
    The only practical way would be to wither run separate antennas if cost is a huge factor. If you have a bit of budget then you would be better to use a TX combiner combination and a single band pass filter to isolate the RX freq.

    Near freq pair de-sense should not be an issue as the 5meg offset should fall well within the radios internal filtering. If in doubt a single notch filter tuned to the average center tx freq should drop any unwanted signal to a ignorable level.
    In fact, the design calls for a separate antenna on each repeater. No combining, and no filtering besides the mobile reject notch duplexers. That means I have to try to separate the repeater freqs by 500khz so the portables can operate in the environment, and then get as much horizontal distance between each antenna plus turn the power down to keep desense and intermod minimized.
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    Default UHF mobile duplexers

    Quote Originally Posted by Bill_G View Post
    In fact, the design calls for a separate antenna on each repeater. No combining, and no filtering besides the mobile reject notch duplexers. That means I have to try to separate the repeater freqs by 500khz so the portables can operate in the environment, and then get as much horizontal distance between each antenna plus turn the power down to keep desense and intermod minimized.
    Sounds like a stupid design thought up by an idiot salesman trying to save a clueless customer a couple pennies.

    Almost would have been better to get the arm chair RR engineers to design it.


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    Quote Originally Posted by Viper1-6 View Post
    Sounds like a stupid design thought up by an idiot salesman trying to save a clueless customer a couple pennies.

    Almost would have been better to get the arm chair RR engineers to design it.


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    I think you nailed it.
    Bow wow wow yippie yo yippie yay

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    Quote Originally Posted by Bill_G View Post
    In fact, the design calls for a separate antenna on each repeater. No combining, and no filtering besides the mobile reject notch duplexers. That means I have to try to separate the repeater freqs by 500khz so the portables can operate in the environment, and then get as much horizontal distance between each antenna plus turn the power down to keep desense and intermod minimized.
    that really does limit your options. The Tx side wont be a problem since your running off seperate antennas. If there is any way to take the rx side from all the duplexers pass it through a single pass filter then route it to the seperate recievers you will be able to highly reduce any chance of TX de-sense. It wont be pretty but it will work.

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    Quote Originally Posted by Notarola View Post
    that really does limit your options. The Tx side wont be a problem since your running off seperate antennas. If there is any way to take the rx side from all the duplexers pass it through a single pass filter then route it to the seperate recievers you will be able to highly reduce any chance of TX de-sense. It wont be pretty but it will work.
    I thought about that, but that would require I combine three down to one, into a window filter, than spread it back out to three. Insertion losses would approach 8db. AND while the duplexers would notch their individual xmitters, they would pass the other two negating the notch advantage. I think a window filter off one of the antennas is the way to go.
    Bow wow wow yippie yo yippie yay

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    Wait! Do i get a free arm chair if i sign up at RR? I could use a new one.
    armchair.jpg

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    The big question is: are we talking municipal public schools, or college campuses? Acreage and building size changes the whole paradigm.

    If it were my baby, and they were going into medium or large municipal PS campuses I'd turn power down to 5W ERP and run the damn things into unity-gain fiberglass sticks. For a medium or large college campus with multiple multi-story buildings I'd probably go for DB404s and 10W to the antenna, and try to spread them around between buildings to maximize separation.

    And while it's not exactly kosher I have known shops to install yagis on the roofs of tall buildings and point them straight down. Supposedly the in-building coverage on UHF works very well using this method, even down into basements.
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    Quote Originally Posted by motorola_otaku View Post
    The big question is: are we talking municipal public schools, or college campuses? Acreage and building size changes the whole paradigm.

    If it were my baby, and they were going into medium or large municipal PS campuses I'd turn power down to 5W ERP and run the damn things into unity-gain fiberglass sticks. For a medium or large college campus with multiple multi-story buildings I'd probably go for DB404s and 10W to the antenna, and try to spread them around between buildings to maximize separation.

    And while it's not exactly kosher I have known shops to install yagis on the roofs of tall buildings and point them straight down. Supposedly the in-building coverage on UHF works very well using this method, even down into basements.
    They are K-12 public schools spread out over a county district. They purchased some monster Telewave 6db fiberglass colinears (about 100 inches tall) that I have to figure out how to mount. So, I am expecting to be playing the turn-the-power-down game, as well as spread them as far apart as possible on the given rooftops.

    I was given the do-not-question-the-engineering-department speech yesterday, and told I live in a bubble having spent too much time working on public safety systems with deep budgets. This is value engineering, and it will work. I don't think I paraphrased very much.

    On the plus side, at 64, I don't have much longer to deal with this.
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    Quote Originally Posted by Bill_G View Post
    I was given the do-not-question-the-engineering-department speech yesterday, and told I live in a bubble having spent too much time working on public safety systems with deep budgets. This is value engineering, and it will work. I don't think I paraphrased very much.

    On the plus side, at 64, I don't have much longer to deal with this.
    Your ‘engineering’ and ‘sales’ departments needs to be fired. Period.

    ‘Value engineering’ is double speak for ‘we are too stupid and cheap to do this right’

    I feel bad for you. I feel bad you have to work with such incompetent, stupid and useless people.

    Hopefully you do not need to stay to your next birthday.

    I would nope the fuck out of there.


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    Quote Originally Posted by Bill_G View Post
    I was given the do-not-question-the-engineering-department speech yesterday, and told I live in a bubble having spent too much time working on public safety systems with deep budgets. This is value engineering, and it will work. I don't think I paraphrased very much.

    On the plus side, at 64, I don't have much longer to deal with this.
    The problem is when it does not work, it's never the sales guy's fault and Engineering should know better to begin with....

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    Your options are certainly limited. I guess all you can do is go for maximum antenna separation both horizontal and vertical. One possible option is a ice shield type of barrier between the vertical antennas. This would have the effect of 'hiding' each antenna from the other. It may only give 20-35db isolation but thats still better than nothing and it would be cheap to construct and install.

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    Quote Originally Posted by Bill_G View Post
    They are K-12 public schools spread out over a county district. They purchased some monster Telewave 6db fiberglass colinears (about 100 inches tall) that I have to figure out how to mount.
    Soooooooo kilobuck antennas didn't break the budget (I think those antennas run north of $4 grand MSRP) but a real duplexer would have? I'm loving this more and more...
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    Quote Originally Posted by motorola_otaku View Post
    Soooooooo kilobuck antennas didn't break the budget (I think those antennas run north of $4 grand MSRP) but a real duplexer would have? I'm loving this more and more...
    You noticed that too. Yeah.

    I worked up the difference between doing this LCP with one rptr at each school as opposed to poorman's trunking in IPSC with two and three rptrs at each location. ie: five rptrs instead of ten, and five lower cost antennas instead of ten expensive ones. Less cable. Less labor. Easier codeplug.

    But, from inside a bubble, it's hard to tell ...
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    sinclair makes some pass/reject mobile duplexers that are about $400 apiece

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    Quote Originally Posted by escomm View Post
    sinclair makes some pass/reject mobile duplexers that are about $400 apiece
    Thanks. I wandered through their online catalog, and couldn't find a low cost pass/reject mobile duplexer. They have a $800 four section rack mount P/R, but all their mobile style are standard six section reject.
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    Those telewave 6db are going to have a very narrow vbw. you'd be better off with unity fiberglass. I think antenna vbw and spacing more of issue than duplexers.

    I wouldn't worry about the mobile duplexers for only 2-3 transmitters. A pass reject duplexer is not a replacement for rx preselector + circulator anyway. The biggest difference is power handling and price.

    I wouldn't even set the Trbo repeaters to Narrow IF filter, leave it on Wide.

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    since you have notch filters and multiple antennas you may be able to "optimize" the notches.

    Assumes these are the typical three notch setup per leg .

    So on the duplexer set two of the notches for the attached TX/RX pair and one for the other non connected pair.
    If the pairs are actually close to each other may get enough reject just doing a traditional alignment.

    I would of course verify the actual desense or duplex performance and by adjusting the power if needed for single repeater into a load.

    Separate antennas will provide some isolation to start and with one of the notches may be enough to optimize the environment along with some power adjustments.


    I have had to take this approach when having multiple VHF public safety repeaters ( four ) sharing the same site. Stuff was tacked on by multiple agencies over the years so getting everyone to spring for a shared antenna components was a non-starter.

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    Quote Originally Posted by motorola_otaku View Post
    And while it's not exactly kosher I have known shops to install yagis on the roofs of tall buildings and point them straight down. Supposedly the in-building coverage on UHF works very well using this method, even down into basements.
    During protests at our State Capitol such a configuration was used to get signal into the lowest levels of the building with good results. If a remember correctly VHF, UHF, and 800 were all used through this setup using a Daniels multi-band portable repeater.

    Due to the heavy construction of the building they had difficulty reaching these areas uniformly with other systems already in place and that had been setup for the incident.