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RF Link Antenna Questions

smcclellan06

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May 20, 2020
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Hey all,



I’m working on a project and I need some guidance on how to approach this antenna setup for my project.



I have repeater A on top of a mountain and I am working on an RF link/range extender by using 2 mobiles to link repeater A to portable repeater B on site.

Here are the details of my setup:



Repeater A- Motorola SLR5700(Analog):

RX- 469.500 TX- 464.500



Repeater B- Motorola SLR1000 (Analog):

RX- 456.500 TX- 451.500



RF Link (Analog):

2- Motorola XPR5550e



Here is the setup for the XPR5550e radios:



Mobile A-

RX- 464.500 TX- 469.500



Mobile B-

RX- 451.500 TX- 456.500



Both mobiles are currently setup with an RA-2T cable, which, I have been told, is normally used in a crossband repeater setup.



I am able to get both mobiles repeating bi-directionally so far.

What I don’t know/understand is the best method for an antenna setup. If each mobile were only using simplex, then I would imagine I would need a duplexer. Because each mobile is setup for duplex, what is the best method for an antenna setup? I would prefer to use 1 base station antenna for both mobile radios. Do I need to use 2 duplexers and an antenna combiner? I’m not well versed in antenna setup for this situation, so please forgive my ignorance.



Any help would be greatly appreciated.



Thank you.
 

mmckenna

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Are the mobile radios working as subscriber radios on the other repeater?

If so, you don't need a duplexer.

But you need to figure out how to keep the system from getting into some sort of locked up loop (I assume you've already figured that out).

I'd run the mobiles at the lowest power possible and use a directional antenna pointed at the other site.
 

smcclellan06

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Thanks for taking the time to reply!

Yes, mobile radio A is a subscriber to Repeater A and mobile radio B is a subscriber radio on Repeater B
Both mobiles are linked via cross patch cable.

I haven’t run into any loop issues yet (fingers crossed). If I do, what would be some ways to mitigate that?

I currently have both mobiles set to the lowest TX power.

Would it be possible to use 1 base antenna for both of these mobiles? If so, what hardware would be required to hook that up properly?
 

ramal121

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Getting two mobiles with duplex channels combined to one antenna is going to be downright impossible. I would say go with two separate antennas and call it a day.

I think what you're thinking is to put a duplexer backwards on each mobile to break out separate transmit and receive. Fine but then what? A normal antenna scheme would have a transmit combiner and receive multicoupler and require an antenna for each. Now you are back to two antennas again. I guess there might be a way to frankenstein something to combine it all into one using pass and notch cans and isolators for good measure but would have to be engineered and expensive.

Just use two yagis with as good of separation as you can get. Transmit to receive freqs are at 8 and 18 MHz and are doable. I have done it this way and works fine.
 

mmckenna

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^^^ what he said ^^^

You'll spend a lot of money trying to combine this into one antenna. Much cheaper to use a dedicated Yagi antenna on the subscriber radio pointed at the other radio site. That'll allow you to lower link radio transmit power which can really help you in a number of ways. It also doesn't send a bunch of RF out in directions it's not needed, and prevents annoying as many other users on those very commonly used channels.
 

smcclellan06

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Thank you both for your responses, they are very much appreciated.

That makes total sense on sticking with 2 separate antennas. I will stick with that.

mmckenna- You mentioned a possible loop that can cause things to lock up. I’m currently not having this issue, but could you please elaborate In case I run into any issues down the road?
 

ramal121

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Having two radios screwed together for cross band or cross channel repeat is usually pretty safe. Never had a problem with that. Too bad EasyLink dropped their analog linking gizmo, but with IP running everywhere nowadays there's more than one way to skin the cat.

This "loop" thing crops up in other configurations. Consider not hooking two mobiles together but instead each directly to its respective repeater. Fine, if a repeater becomes active the link radio will talk to the other repeater. But what if there is a squelch tail on the opposing repeater? When the link radio goes back to receive it will hear the other squelch tail and bring up the original repeater. Now the cycle repeats ping-ponging back and forth. Even if the squelch tails are set for zero, if there is the slightest latent hiccup on the COR line as the link radio reverts from transmit to receive it will start the ball rolling and you are off to the races until the link is broken. As a greenhorn I won a tee shirt.

As I said you're pretty safe for what you want to do. The nice thing is that you are not constricted where you put the linking radios. As long as they can access both repeater with a reasonable signal things will work out just fine.
 

mmckenna

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Yeah, careful programming and you should be OK.
Those frequencies are very commonly used, so it would be super easy for some random dude to interfere with the link radios and come up on your repeater. Make sure you pick your CTCSS/DCS code carefully. I have some of those frequencies programmed into my scanner at work. I hear a LOT of traffic on those. If this is a permanent setup, you really should get a set of coordinated frequency pairs that are not itinerants.

Make sure you set a time out timer on the system. On the off chance something does get into it, you don't want your system keyed up long term.
 

smcclellan06

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Having two radios screwed together for cross band or cross channel repeat is usually pretty safe. Never had a problem with that. Too bad EasyLink dropped their analog linking gizmo, but with IP running everywhere nowadays there's more than one way to skin the cat.

This "loop" thing crops up in other configurations. Consider not hooking two mobiles together but instead each directly to its respective repeater. Fine, if a repeater becomes active the link radio will talk to the other repeater. But what if there is a squelch tail on the opposing repeater? When the link radio goes back to receive it will hear the other squelch tail and bring up the original repeater. Now the cycle repeats ping-ponging back and forth. Even if the squelch tails are set for zero, if there is the slightest latent hiccup on the COR line as the link radio reverts from transmit to receive it will start the ball rolling and you are off to the races until the link is broken. As a greenhorn I won a tee shirt.

As I said you're pretty safe for what you want to do. The nice thing is that you are not constricted where you put the linking radios. As long as they can access both repeater with a reasonable signal things will work out just fine.
Thanks ramal121, that makes sense.
 

smcclellan06

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Yeah, careful programming and you should be OK.
Those frequencies are very commonly used, so it would be super easy for some random dude to interfere with the link radios and come up on your repeater. Make sure you pick your CTCSS/DCS code carefully. I have some of those frequencies programmed into my scanner at work. I hear a LOT of traffic on those. If this is a permanent setup, you really should get a set of coordinated frequency pairs that are not itinerants.

Make sure you set a time out timer on the system. On the off chance something does get into it, you don't want your system keyed up long term.
Thanks mmckenna. I agree, it would be super easy for someone to interfere. Yes, the best case scenario would be to have a coordinated frequency pair, but this portable repeater setup isn't a permanent install, but rather a temporary setup that moves locations quite frequently, hence the itinerant frequencies. The main focus for this setup is to extend the range of my main repeater when I'm at the edge of the zone in various locations. Ideally, I would run my SLR1000 as a satellite receiver, but unfortunately, I cannot rely on an IP connection for the locations that this will be in. Microwave links could be an option, but I'm trying to keep it more "simple" by means of an RF link.

Let me take it a step further- I've also thought about using this setup in the digital (DMR) mode. The main advantage of running digital for me is I can have 2 simultaneous talkpaths and privacy so nobody can interfere as easily. I've started to do some testing. Would there be any new things to consider/look out for if I try running this setup in digital mode?
 

mmckenna

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Thanks mmckenna. I agree, it would be super easy for someone to interfere. Yes, the best case scenario would be to have a coordinated frequency pair, but this portable repeater setup isn't a permanent install, but rather a temporary setup that moves locations quite frequently, hence the itinerant frequencies. The main focus for this setup is to extend the range of my main repeater when I'm at the edge of the zone in various locations. Ideally, I would run my SLR1000 as a satellite receiver, but unfortunately, I cannot rely on an IP connection for the locations that this will be in. Microwave links could be an option, but I'm trying to keep it more "simple" by means of an RF link.

Makes sense. I figured that is what your application was.


Let me take it a step further- I've also thought about using this setup in the digital (DMR) mode. The main advantage of running digital for me is I can have 2 simultaneous talkpaths and privacy so nobody can interfere as easily. I've started to do some testing. Would there be any new things to consider/look out for if I try running this setup in digital mode?

If you are talking about linking analog repeaters with DMR linking:
Analog <> digital <> digital <> Analog usually ends up sounding poorly. The translation from analog to digital and back to analog usually brings out the worst in vocoders. Certainly, give it a try, but you may find that keeping it all analog sounds better.

Running digital on the links may help prevent the "ping pong'ing" effect that Ramal mentioned above.
Encryption would prevent someone from finding and messing with the link, but since it is a temporary setup, that's likely not going to be a big issue.

I inherited a system set up similar to this, but it uses 3 separate repeaters at 3 separate sites on 3 separate coordinated pairs. Linking between the sites is done with IP via some cheap boxes and some basic point to point IP links. It has taken a while to get all the bugs worked out, in fact, I have to go work on one of the sites on Tuesday, but it does work well. Higher end linking boxes would have solved a lot of the issues, but I had to work with what they had.
 

alcahuete

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Depending on how close the repeaters are, and if they are line of sight or not, just go with a microwave link (i.e. Ubiquity) and call it day. Far less cost than a single XPR5550e, you don't have to worry about additional antennas, interference with the repeater frequencies, etc. You then just use the built-in networking on the repeaters, and you're good to go. About the easiest setup you can think of.
 

WT4FEC

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Jun 1, 2020
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Location
La Belle Florida
Are the mobile radios working as subscriber radios on the other repeater?

If so, you don't need a duplexer.

But you need to figure out how to keep the system from getting into some sort of locked up loop (I assume you've already figured that out).

I'd run the mobiles at the lowest power possible and use a directional antenna pointed at the other site.

This is the correct answer, use two yagi's seperated vertically as far as possible and keep the power to just enough to make the link work.
 
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