At our PD, we have a low band radio that is attached to an antenna on our roof. It is used to activate our tornado sirens. . We also have our 800mhz Harris radio for dispatch, that needs an antenna for reception. Can we run a splitter and run both off the same antenna or will that create and issue?
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Antenna splitter question
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prcguy
Member
A splitter is not the right device for that but a diplexer is. A diplexer has a high pass filter and a low pass filter in one package that will allow both radios to transmit and receive without one transmitter blowing up the other receiver from high power being fed to it. There are some amateur grade diplexers that will work but I'm hesitant to recommend one due to your public service status. I would recommend having your radio supplier research models and I suspect they will actually recommend a triplexer as I don't know of any commercial quality diplexers that will work. A triplexer is fine and they would just have to put a 50 ohm termination on the unused UHF port. Some commercial triplexers made by Motorola, Panorama Antenna or Sti-Co may work at VHF lo band and some may not so it should be thoroughly tested.
Awesome. Thank you!
mmckenna
I ♥ Ø
Which bands are you using on the Harris radio, and what band(s) is the base antenna designed for?
BinaryMode
Blondie Once Said To Call Her But Never Answerd
Is the antenna designed for 800 MHz AND the low band you mention?
Antennas are designed for bands. Some are so-called "all band." Also, receiving and transmitting are two different animals as well...
In a nutshell I wouldn't do it. Continue to use the low band antenna for the radio used for the sirens and you'd use the 800 MHz band antenna for Comms. Again, transmission and reception are two different things. Especially when it comes to using some kind of splinting hardware for both radios.
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CcSkyEye
Member
I'm guessing these two radios run on different bands and would suggest a dedicated antenna for each. Especially with one being for civil defense siren activation.
I'm not sure about your position or level of experience, but any antenna/radio system used for mission critical systems, should be designed and implemented by a professional system installer.
The fact that the one system is used for tornado warnings (read life or death situations), makes me nervous that anyone other than a professional would touch it. I can't even imagine what liability you could open yourself up for should something happen and the warnings not be transmitted or dispatches not heard.
Please don't take this the wrong way; I'm sure that you've got the best intentions in mind and are probably trying to save your department money, but this is probably not something that you should touch. Radio professionals on here should probably chime in for their opinions.
Be safe.
freddaniel
Member
PRCGUY is right. The diplexer, or also known as a Crossband Coupler is the correct product ON EACH END of the coax. These are typically used on very tall towers where very long expensive coax runs are used. Commercial grade diplexers, made to Public Safety standards are in the $1,000 each price range, plus installation labor. Therefore, in your case, it would likely be cheaper to run another coax to the roof.
There is also the issue of creating a single point of failure for both systems. YES, there are Amateur grade diplexers out there, but they are far from Public Safety grade. I have used some of these and the most common problem is moisture causing failures over time. In Public Safety and commercial applications, moisture protection at the coax/antenna connection is of paramount concern, as many failures are traced to poor weatherproofing. So, adding one more point of failure is risky.
There is also the issue of creating a single point of failure for both systems. YES, there are Amateur grade diplexers out there, but they are far from Public Safety grade. I have used some of these and the most common problem is moisture causing failures over time. In Public Safety and commercial applications, moisture protection at the coax/antenna connection is of paramount concern, as many failures are traced to poor weatherproofing. So, adding one more point of failure is risky.
Ubbe
Member
Using a roof antenna, of any sort, to listen to a 800MHz repeater that probably will be line of sight will work just fine.
Then you only need a diplexer down at the radios. Most diplexers are a low pass filter and a high pass filter, so no frequency limitations.
If you get one for HAM frequencies it will have a crossover at around 300MHz. As the diplexer will be indoors it will not be critical and you can use the $30 100W kind below here. Just order some pigtails, connection coaxes, with the correct connectors at each end, try to avoid adapters and secure connectors with some amalgamating rubber tape. You can put the diplexer in a plastic box for protection.
Ebay Diplexer
Coax pigtail
Amalgamating Tape
/Ubbe
speedway_navigator
Member
That sounds odd, typically an 800 MHz portable will receive indoors but not be able to transmit back to the repeater or receive site.
There are portable antennas that are resonant at different frequency bands but I don't think I've seen a low band and 800 in the same antenna, especially a base station antenna.
If your 800 receive is weak but consistent you might get away with a mag mount antenna on top of a metal file cabinet or above the ceiling tiles, but this is a life safety system, why take chances? These are the kind of things a lawyer will bring out in a line of duty death if he/she wants to put blame on the agency.
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