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Re: Running a Repeater on a FM Broadcast Tower

 

Circulator, which is a passive element allowing the passing of RF signals only in one direction, is a key element in almost any RF circuit technology.?
An isolator is based on a circulator, having a dummy load on the reverse way port, thus isolating the RF signal passage truly to one direction.

Usually circulators or isolators come in one- or two-pack sets so that there are one or two circulators in a chain. Double-pack-isolators have better isolation in general as there are two sets of dummy loads in row in the reverse path preventing signal entering back to the port where RF is not wanted to enter.

Such cases where RF is not welcome into the transmitter output port are exactly situations where there are multiple strong signals present, and an unwanted signal entering the transmitter amplifier could generate IMD and unwanted mixing signal results. Isolators also improve?RF circuit matching, requiring of course that the isolators are tuned to the frequency used.

In repeater systems isolators should be always used in the transmitter chain, they are a very good method of getting rid of many difficult-to-solve sporadic issues.

73,
Erik OH2LAK N5LAK TF3EY

On Fri, 14 Mar 2025 at 23:24, Chris Boone WB5ITT via <setxtelecom=[email protected]> wrote:
Circulators are used all the time in LMR...Combiners have them, I've used them on conventional systems where IMD was a problem...never called them isolators in my 50+ years of LMR..

Chris WB5ITT?

On Fri, Mar 14, 2025, 4:09 PM Matt via <al0r=[email protected]> wrote:

Isolators are only used on the tx side.

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An isolator is generally a 3 port circulator where one port is terminated. In some systems, such as radar, a circulator can be used to allow a transmitter and receiver to share the same antenna. It is possible to have more than 3 ports on a circulator

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You typically don’t see a circulator in “LMR.”

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You don’t need to tune into any frequency in particular. As mentioned, simply insert a power meter in the line and put a load on the other port. If you want to measure the level at a given frequency, then you’ll need additional equipment.

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As far as what to do… you can trap/stub it, add band pass filtering, or ignore it (if you have equipment that tolerates it.) Why would we do something about it? Because hams are cheap. BpBr duplexers are used, with the owners not understanding that they really aren’t “band pass” as they think they are. Or, notch only duplexers are used, or crappy rf decks. Insert a lot of reasons.

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An actual band pass filter, given adequate separation (i.e., FM broadcast/2m) can kill “all” the signal/voltage from the broadcast transmitter. Only passing the desired band, i.e., 2m, or even a narrower portion. Alternatively, you can trap the broadcast station, cleaning the line for all the downstream users, then add a band pass too, shielding your receiver from all kinds of unwanted signals.

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Matt

AL0R

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From: [email protected] <[email protected]> On Behalf Of steven harvey via
Sent: Friday, March 14, 2025 13:14
To: [email protected]
Subject: Re: [repeater-builder] Running a Repeater on a FM Broadcast Tower

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People have mentioned isolator and circulator any brand better than the other?? ?

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If I remember correctly.? ?The isolator are used on the TX and RX side of the repeater ports to the duplexer?

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The circulator is used after it combines on the duplexer output to go up the antenna?

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Also it was stated to bring a watt meter to look for potential RF energy coming down the cable.? ? I'm assuming you'll want too look for the RF on the repeater frequency or the radio station frequency?? ?What do you do if you find such a issue??

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Appreciate everyone help on.? Lot's of good information.

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Thanks

Steven H

N8RLW?

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