Recomedation frequency separation of two close by repeaters

[12dbsinad]

Thanks for the info, yes absolutely that is ideally how it should be done, and I've done this before only when its a single repeater for longer runs or higher power even 7/8. One time years ago had to use 1-5/8 I think its the size, and even fill in the gas into the cable, that was quite a job to deal with that salami!

Usually use Andrew brand but its now it seems to be twice the price compared to the RFS Cellflex which I have never used before altough it looks good on paper. So just in case wanted to hear from others that have used it. Was not planning having to use hardline this time because of such a relatively short antenna feedline run, so it was not in the budget and trying to make ends meet. But given that now I was made aware of the higher chance of IM with braided cables like 9913 or LMR400, which is my GP cable what I used most times, guess I have to bite the bullet on this.

I understand, and to answer your question RFS cable is perfectly fine. Have used it dozens upon dozens of times with good results. It's actually what we use most of the time over Andrew.

 

[tropiradio]

Would it still be OK to use the short high quality NM-NM RG213 24" jumper that is included with the ANT450F2 it might be tricky to connect directly the 1/2" cable as the female connector on the antenna is a bit recessed but doable. Just wondering if it would make any difference in terms of potential IM to use that included coax jumper.

 

[mmckenna]

Would it still be OK to use the short high quality NM-NM RG213 24" jumper that is included with the ANT450F2 it might be tricky to connect directly the 1/2" cable as the female connector on the antenna is a bit recessed but doable. Just wondering if it would make any difference in terms of potential IM to use that included coax jumper.

That's what it's intended for. You wouldn't want to connect hardline directly to the antenna, puts strain on the connectors.

 

[tropiradio]

That's what it's intended for. You wouldn't want to connect hardline directly to the antenna, puts strain on the connectors.

Yes I agree, but since everyone here said to stay away from braided cable in my present scenario because it can generate IM specially when using TDMA/TRBO, just asking -and learning a bit more. Thank you.

 

[mmckenna]

Yes I agree, but since everyone here said to stay away from braided cable in my present scenario because it can generate IM specially when using TDMA/TRBO, just asking -and learning a bit more. Thank you.

Braided cable with dissimilar metals is the problem. And then only if it gets wet. LMR-400 can work in repeater installations, as long as it stays dry.
The cable that comes attached to these antennas is designed exactly for this type of installation. The manufacturers know it's going to be used in repeater installations and designs accordingly.

 

[12dbsinad]

For full duplex it's best to use double shielded cable like RG214 for jumpers

 

[tropiradio]

For full duplex it's best to use double shielded cable like RG214 for jumpers

LMR400 also has double shield.

 

[mmckenna]

LMR400 also has double shield.

Right, and it'll work fine if it stays perfectly dry. The problem with LMR400 is that it uses different metals for the foil shield and the braid.

Idea is to not introduce possible failure points to the system that might bite you in the rear later on.

 

[tropiradio]

Braided cable with dissimilar metals is the problem. And then only if it gets wet. LMR-400 can work in repeater installations, as long as it stays dry.
The cable that comes attached to these antennas is designed exactly for this type of installation. The manufacturers know it's going to be used in repeater installations and designs accordingly.

The jumper that comes with the Telewave antenna is actually made of RG213. So what you are saying it would still be permisible to use LMR400 for the antenna feedline if done carefully and kept dry? I assume a typical humidity entry point to the cable would be at the antenna connector, which I normally make sure its as protected as possible (Vapor lock, self sealing tape, I even put several tie wraps onto the sealing tape to keep it from slowly becoming undone and loosing a tight seal.

 

[tropiradio]

Right, and it'll work fine if it stays perfectly dry. The problem with LMR400 is that it uses different metals for the foil shield and the braid.

Idea is to not introduce possible failure points to the system that might bite you in the rear later on.

Right, aluminum foil, and thinned copper I think it is. Just stripped a piece.
OK thank you for that explanation, I was trying to figure out why the LMR400 cable would produce IMD outright as it sounded when people where warning me to stay away from it, and this is finally the first clear explanation I got. So it turns out its not the cable from the devil after all, just potentially it could become if not handled correctly or like you say humidity gets into it. Thanks again.

 

[mmckenna]

So it turns out its not the cable from the devil after all, just potentially it could become if not handled correctly or like you say humidity gets into it. Thanks again.

Yeah, I've got a site with a piece of it. Works fine, but it's indoors 95% of the way.
Proper connector sealing would be necessary, and you'd need to be 100% sure the jacket was good all the way. Coaxial cable won't last forever, and usually the jacket is where it fails. It's a site with just one repeater, so little risk of issues from other strong RF fields.

Still, if this was anything even remotely critical, it's best to avoid something like LMR-400 that will probably eventually give you issues. And, it's not the greatest coax ever made. You can do better pretty easily.

 

[tropiradio]

Yeah, I've got a site with a piece of it. Works fine, but it's indoors 95% of the way.
Proper connector sealing would be necessary, and you'd need to be 100% sure the jacket was good all the way. Coaxial cable won't last forever, and usually the jacket is where it fails. It's a site with just one repeater, so little risk of issues from other strong RF fields.

Current repeater with LMR400 also about 95% indoors, just goes through the side wall outside and then up about 4-5ft to the antenna.
Would be exact same scenario for the second soon to be added repeater.

Still, if this was anything even remotely critical, it's best to avoid something like LMR-400 that will probably eventually give you issues. And, it's not the greatest coax ever made. You can do better pretty easily.

Academically speaking, what would be your -better- choice be? 9913, RG213, RG214.... or something else altogether?
I actually never used LMR400 before, it just so happened there was a the time no 9913 available, so my provider suggested LMR400.

 

[mmckenna]

Current repeater with LMR400 also about 95% indoors, just goes through the side wall outside and then up about 4-5ft to the antenna.
Would be exact same scenario for the second soon to be added repeater.

It's all good until it gets damp.

Academically speaking, what would be your -better- choice be? 9913, RG213, RG214.... or something else altogether?
I actually never used LMR400 before, it just so happened there was a the time no 9913 available, so my provider suggested LMR400.

I'd use 1/2" Heliax. Seriously. Not sure what kind of install this is, but if it's public safety or a paying customer, I'd want to deliver the best performance and reliability I could.

LMR-400 should be your last choice. Rg-214 would probably be better since it doesn't have the dissimilar metal/IM thing as a possibility. Don't get me wrong, in a perfect installation, LMR400 will technically work, until it gets damp, then all bets are off. Eventually you will have problems. I wouldn't treat a customer like that. I'd install the right cable from the beginning and not take unnecessary chances with someone else's stuff.

But I'd absolutely go Heliax. 40 feet isn't going to be a lot of money by the time you figure in installation, connectors, testing, sending a crew out there. The cost of the cable should be minimal in comparison to all the other costs.

 

[tropiradio]

I fully agree, I like to do things the right way. Was just curious in general context since you mentioned that it would be very easy to do better than LMR400, so just wanted to know what would be your cable choice in similar size in that sense.

 

[tropiradio]

I'd use 1/2" Heliax. Seriously. Not sure what kind of install this is, but if it's public safety or a paying customer, I'd want to deliver the best performance and reliability I could.

Quotes for the 1/2 cable and connectors just came in, my dealer said Andrew cable still the lowest price, and the numbers look quite manageable.
ANDREW LDF4-50A: $1.78/ft
L4TNM-PSA $24.00
L4TNF-PSA $24.00

I also found brand new 1/2 grounding kits on ebay for $15-$20 (~half price).

 

[mmckenna]

Quotes for the 1/2 cable and connectors just came in, my dealer said Andrew cable still the lowest price, and the numbers look quite manageable.
ANDREW LDF4-50A: $1.78/ft
L4TNM-PSA $24.00
L4TNF-PSA $24.00

I also found brand new 1/2 grounding kits on ebay for $15-$20 (~half price).

Those are good prices. Your dealer isn't doing you wrong.

 

[tropiradio]

Yes, been dealing with them for years.

Hey BTW I got another thread going on the repeater maker group, and there is one member that says that if I set both repeaters with the same Rx to Rx and Tx to Tx spacing, when both repeaters transmit at the same time then both receivers would be blocked. I really don't understand how and why that could happen. Any idea what he is talking about. The guy never answered my direct question and a few other members of the group are also disputing him, and asking that he be more specific but so far no more answers from him. His comment bellow, any idea why is he saying this?

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https://groups.io/g/repeater-builder/message/192503

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If both repeaters have the same TX/RX separation, say 5 mc,
they will interfere with each other when both TX key up at
the same time.

Run the frequencies using your pencil. They will mix past
the duplexer and antennas and generate a signal on the other
repeater.

This is akin to a broadcast station on 600 kc, will make a
ham 2-meter repeater jam up.

It does not actually matter what the actual frequency of
the repeater is.

Run the numbers. Be careful, this can be tricky.
--------------------

 

[mmckenna]

Run the numbers. Be careful, this can be tricky.

LMR UHF repeaters in the USA are 5MHz split. If this was a real issue, there would be no co-located UHF repeaters anywhere. Same on 700MHz, 800MHz, etc, anywhere there is a standard repeater offset. That would make trunked radio systems inoperable.

In other words, the term "full of poop" comes to mind.

You have to be -REALLY- careful when dealing with some of the ham radio crowd. The term "Just enough knowledge to be dangerous" is appropriate here. This is one of the reasons I don't participate on sites like QRZ, or other ham only oriented sites. My mind goes numb reading some of their posts.

In very general terms:
His example of a 600kHz AM broadcast station and a 2 meter repeater and all the mixing products can cause an issue, but how the hell this guy is extrapolating that out to impacting a commercial LMR repeater is beyond me. I can see "on paper" it could be an issue, but you'd have to be using some truly crappy components, coax, duplexers and a lot of other questionable stuff to cause this.

 

[12dbsinad]

It sounds like what this guy is trying to get at is a phenomenon called intermodulation. A study is typically done (intermod calculation) at commercial sites with all users to prevent problems. Usually it only becomes a problem at heavy RF sites. Depending on your UHF splits it is possible one repeater can desence the other receiver even with them being paired. This can happen if the transmit of 1 freq is close to the receive of the other. This however is very easy to figure out and it's possible to need extra cans on the equipment for filtering. At our sites all equipment is required to have things above and beyond just a Bp/Br duplexer with things like isollators (if it doesn't have combiners). Of course, these are public safety sites and equipment not some hammy repeater.

RF is very complex, I describe it as being married, you just never know... LOL.

 

[mmckenna]

Yeah, not an issue for a single repeater, and not really an issue at a commercial site when dealing with professionals.
It becomes an issue at sites that are not well managed, or have hams running a junky system that should have been tossed out when Nixon was in office.
Good components and proper design will address this.