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using VHF repeater antenna with a UHF repeater


WRTZ750

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I have the possibility of using an old VHF antenna that's up high on a water tower but want to test it to see if it would work for GMRS.  Is this a definite no it won't work or do you guys with repeater/tower experience think it would be worth trying my small mobile radio at the base of the water tower to see if the SWR is crazy high?  I spoke with the 3rd party company that does comm work in this tower and he said that antenna was put up there before his time and he doesn't know what type of antenna it is.  He said I could hook my mobile radio and an SWR meter on the input side of the duplexer and test that way.  Does that make sense?  Is that possible?  Does that mean to disconnect the antenna coax on the duplexer where it says "antenna" and hook my mobile radio up there to test?

For obvious liability reasons they don't want me climbing to the top of the tower but this is the closest I've been to finding an actual good elevation site for a repeater.

Another related question.  If there are multiple antennas on the top of this water tower, and I know which hardline I'm able to tie into at the ground, how does a guy/gal test which antenna that coax is going to?  

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Would need to know which part of the VHF band the antenna is for.  If it was for an amateur radio 2m (144-148 MHz) repeater, it probably won't work very well for GMRS, since the GMRS frequencies are not a harmonic of 2m.  If it was used for MURS, you might be able to get away with it, but performance will still suffer.  Also, I wouldn't use a radio with an inline SWR meter to do your testing.  You really don't know what you're transmitting into.  I would find someone with a quality antenna analyzer (like a RigExpert) that covers a wide range of frequencies.

You also don't know the state of the hardline, it might have a break in it.  Probably don't want to be keying a radio into a broken coax.  An antenna analyzer can also tell you the length of the coax, so you'll have a good idea if the entire length is still good.

As for knowing which antenna the hardline is going to, aside from tracing it, I don't know of a good way to test that.  More seasoned forum members might have some ideas.

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