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Can a UHF repeater antenna be mounted on an AM radio station tower ?


JCase
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Greetings all,

 

There are two AM Radio broadcast (talk radio) stations in my area. One uses three commercial towers and the other uses one, not that the number of towers matters.........or does it ?

In my attempt to place a GMRS repeater on the air it has become painfully obvious that I am not able to achieve the antenna height needed to render this repeater to be of much use to most people, including myself, since the maximum range in all directions is just over 7 miles using a 25-30 watt mobile approximately 3.5 miles for handheld use.

I noticed that none of the towers that the AM radio stations have, have any (that I can see) antennas mounted to them and remembered learning somewhere, that AM Radio transmitters use the tower structure as the antenna so with that in mind, it’s understandable not seeing other antennas on the tower. But, that got me thinking....... I remember years ago on the opposite side of the county the local AM (country music) radio station and the State Highway Patrol (using VHF) shared a tower and it worked quite well for both parties except for the last 5-10 years. During the end of this setup, every time the Highway Patrol dispatcher would transmit, the music from the radio station would also come across but was at very low levels sort of like “background” (no pun intended) music so you could still hear the dispatcher with no issues. I’m guessing this could have been mitigated with either filters or some form of adjustment but wasn’t since the State was in the process of switching to an 800 trunked system.

 

So, now that I’ve written a short story...........

 

This answer might be obvious to some but as I’m still learning, I could use some help.

Can a GMRS/UHF repeater antenna be mounted on an AM radio tower without causing problems for both users? Is there additional equipment needed for either party, other than what’s already in place for the radio station and for the repeater side, the repeater with duplexer, feed line and proper grounding ?

 

I look forward to the always helpful and friendly advice and guidance I continue to receive from the members.

 

J

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That may not be possible. Frequently towers on the AM broadcast band “are the antenna” because the frequency is so low. The towers can be a 1/4 wave tall.

 

For example an AM station on 1000Kz a 1/4 wave antenna is about 246 feet tall. Attaching anything to the tower would disturb the functioning to some degree or another. And that’s neglecting the high power RF from the tower coupling into the UHF antenna.

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Yes you can do it but it requires an Isocoupler, most of the time.  https://www.radioworld.com/industry/how-to-use-am-towers-for-other-stuff

The only time you don't need an isocoupler is where the AM broadcast tower is shunt fed, meaning the tower is grounded and fed at a near mid-point on the tower for impedance matching.  All other AM towers that I have ever seen have a base insulator and the entire tower is the antenna.  The isocoupler is basically an air-core capacitor that provides DC isolation while passing the desired RF up to your antenna.  However, ANYTHING added to an AM tower will de-tune it and the station engineer will have to schedule downtime during off hours and completely re-tune it.  I would think that the AM operator that would allow you to do this is extremely rare unless they are getting rent or free use of the repeater or something like that.    

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