Recomedation frequency separation of two close by repeaters

[tropiradio]

I wanted to ask for opinions and advise on a tricky matter of frequency separation between two UHF repeaters that will have to operate in very close proximity. Repeaters are Motorola SLR operating and Hytera RD982i (unfortunately could not source another SLR) and both operating in TRBO/DMR mode and at 50W TX power. Both will also be equipped with a Sinclair Q3220E duplexer. One repeater (the existing one) is already operating at R464.100 and T469.100. My question would be what would be the recommended frequency spacing/separation for the second pair of frequencies of the repeater to be added in order to minimize the chances of IM or mutual interference due to having to operate in close proximity? I am hoping to minimize the chances of that happening by at least carefully and wisely choosing the new frequencies for the new repeater. I have been in the LMR business for over 40 years, but in scenarios like this I always like to hear opinions from others. And also wanted to mention I really cannot consider combiners as a solution as the number will not allow it.

I fortunately have some freedom of choosing the frequencies before licensing them so that would allow me to possibly optimize them to deal with any IM or interference if it happens and perhaps minimize or solve it by moving around a bit one of both frequencies of the new repeater. Or if it would be better I could potentially even also distance them 1,-2... 3 MHZ away while always retaining a 5MHz separation between R/T. Again very flexible in that sense. Will also preemptively try to put as much horizontal distance as the top of the building allows (about 30ft) and some vertical distance as well (about 5-6ft) between the two antennas, and keep both feedline routing (approx 40ft of LMR400) as separated as possible. I could also try (if necessary) to lower TX power down to 25-30W as the prime coverage area of both repeaters is really limited to about 1 mile. BTW both repeaters will be using the same model of vertical antenna, Telewave ANT450 due to its ruggedness and long life for this environment.

I was looking at the Sinclair Q3220E response curves and it seems that it might make sense to stay close to the existing RX and TX frequencies, say 100Khz away (?), so as to take advantage of the band reject characteristics of both duplexer being able to attenuate each others signals as well. For example use say R464.200 and 469.100. Would this make sense? Or what would be a better choice given the physical installation constrains in this case?

And I always also like to kind of get my memory refreshed in regards to what else would be the recommended best practices in terms of RF handling for this case besides the one mentioned earlier and aside from optimized frequency allocations?

Thanks in advance for any pointers and suggestions.

 

[K4EET]

To calculate IM products from a pure mathematical standpoint, you will need a complete list of frequencies at the site to select a pair that is compatible with all site residents. If you want to do the actual engineering of what antennas need to go where to avoid interference to other site residents, then you will need all of the system specifications (power, antenna models, antenna placement, coax type, coax length, etc.) to avoid other more involved system design issues. And don't forget about structural integrity like tower loading, etc. Be sure you cover all of the bases for whatever work you all plan to do. This isn't a trivial task.

 

[AM909]

Are you outside of the US, or did you reverse the transmit and receive freqs above?

 

[mrsvensven]

What you really want to avoid is having a receiver close in frequency to a transmitter. Since the receivers at 469 are all 5MHz away from the transmitters at 464, you should have no problem even if your frequencies are very close to each other.

This assumes that your incoming portable radio signals aren't right near the radio site (within 500 feet) and overloading the adjacent channel receivers. If that's the case, then 25kHz+ of frequency separation would be a good idea.

I would avoid braided LMR400 cable for TDMA use. It isn't rated for low PIM and could cause RF noise issues. Any repeater installation (especially TDMA) is better off with solid hardline.

 

[tropiradio]

What you really want to avoid is having a receiver close in frequency to a transmitter. Since the receivers at 469 are all 5MHz away from the transmitters at 464, you should have no problem even if your frequencies are very close to each other.

This assumes that your incoming portable radio signals aren't right near the radio site (within 500 feet) and overloading the adjacent channel receivers. If that's the case, then 25kHz+ of frequency separation would be a good idea.

I would avoid braided LMR400 cable for TDMA use. It isn't rated for low PIM and could cause RF noise issues. Any repeater installation (especially TDMA) is better off with solid hardline.

I always use the higher frequency of the repeater pair as transmitter, being the reasoning so that the portables can use a lower frequency Tx at which they would be more efficient. Probably not very relevant nowadays with modern equipment, but that is what I always remember was done.

Yes the potables are within 1 mile of the repeaters, sometimes even less. As I mentioned, the prime area is within 1 mile of the repeater.
The 25KHz separation that you suggest would be so the Br of both duplexers will have effect on both frequency pairs?

So RPT1: T469.100 R464.100 (existing)
and RPT2: T469.125 R464.125 (new to be added)

I assume this is the frequency spacing that you would recommend?

So 1/4" hardline would be a better choice? Due to the short distances from equipment to antenna, and only 50W, I assume this would be adequate.

Thank you for the useful input.

 

[tropiradio]

Are you outside of the US, or did you reverse the transmit and receive freqs above?

Yes, outside US.

 

[tropiradio]

To calculate IM products from a pure mathematical standpoint, you will need a complete list of frequencies at the site to select a pair that is compatible with all site residents. If you want to do the actual engineering of what antennas need to go where to avoid interference to other site residents, then you will need all of the system specifications (power, antenna models, antenna placement, coax type, coax length, etc.) to avoid other more involved system design issues. And don't forget about structural integrity like tower loading, etc. Be sure you cover all of the bases for whatever work you all plan to do. This isn't a trivial task.

There are other 2 repeaters in the immediate vecinity, but not at the same building. These other repeaters are each about 1/2 mile away, and one operates in the R455-T450 split at 40W (XPR), and the other uses a T459-R454 split at 50W (SLR). I know both use small mobile duplexers but so far there are no reports of interference from my repeater at R464/T469 probably due in part because of frequency separation of 10MHz or more and that their own portables operate within 500 yards of the respective home repeater 90% of the time. All antennas are on the roof of their own 1-2 story buildings, so no common towers involved anywhere.

Thank you.

 

[tropiradio]

To calculate IM products from a pure mathematical standpoint, you will need a complete list of frequencies at the site to select a pair that is compatible with all site residents. If you want to do the actual engineering of what antennas need to go where to avoid interference to other site residents, then you will need all of the system specifications (power, antenna models, antenna placement, coax type, coax length, etc.) to avoid other more involved system design issues. And don't forget about structural integrity like tower loading, etc. Be sure you cover all of the bases for whatever work you all plan to do. This isn't a trivial task.

I found this online IM calculator but it only accepts up to 3 frequencies:

Calculate intermodulation products from 2 and 3 frequencies.

www.arcticpeak.com

But realistically in my scenario I would need to also consider two other repeaters in the immediate vicinity about 1/2 mile away, so it would be total 4 TX frequencies to analyze for IM. Can anyone please point me to an IM calculator that can handle at least 4 frequencies?

 

[jeepsandradios]

Personally I would use at minimum 1/2"hardline.

 

[popnokick]

50 watts RF output for a one mile or less coverage radius is excessive (unless you’re putting signals through 3 foot thick concrete or underground tunnels). Have you modeled the predictive coverage? Reducing power (if possible) also reduces chances of IMD and other interference.

 

[tropiradio]

Personally I would use at minimum 1/2"hardline.

Yes that is what I meant to say, NOT 1/4" like I had written earlier. Thank you.

 

[tropiradio]

50 watts RF output for a one mile or less coverage radius is excessive (unless you’re putting signals through 3 foot thick concrete or underground tunnels). Have you modeled the predictive coverage? Reducing power (if possible) also reduces chances of IMD and other interference.

Well you know what they say ...more power more joy.
But seriously, in TRBO I always try for the strongest possible signal at the portables to avoid people sometimes sounding like they have half a banana in their mouth at difficult locations. But in this case I should be be able to lower the repeater transmit power down to half, and those -3dB should not have much impact on the quality of signal at at the portable radios. They are all within 1 mile of the repeater at the most.

 

[mrsvensven]

Yes the potables are within 1 mile of the repeaters, sometimes even less. As I mentioned, the prime area is within 1 mile of the repeater.
The 25KHz separation that you suggest would be so the Br of both duplexers will have effect on both frequency pairs?

It has nothing to do with the Br of the duplexers. I don't think that really matters, since the other transmitter is on it's own antenna and is therefore going to me much much weaker than the transmitter actually sharing the same antenna.

The 25Khz (minimum) separation is so that if a really strong portable is talking in on the 464.1 input, your new receiver isn't overloaded by that strong portable while trying to receive a weaker portable further away on your new receive frequency. The more separation the better, 25kHz is just the bare minimum I would want. (in practice, I actually specified minimum spacing of 75kHz for a large new system I just designed)

 

[mrsvensven]

Yes, outside US.

There is a reason every repeater in the US transmits on 464 and receives on 469 and they don't ever flip. You don't want to mix transmitters and receivers on the same frequency, even many miles apart. If you're trying to receive a weak portable on 464.1 and there is some other repeater on a mountaintop 50 miles away transmitting on 464.1, it's probably going to overpower that portable and cause you problems.

 

[tropiradio]

There is a reason every repeater in the US transmits on 464 and receives on 469 and they don't ever flip. You don't want to mix transmitters and receivers on the same frequency, even many miles apart. If you're trying to receive a weak portable on 464.1 and there is some other repeater on a mountaintop 50 miles away transmitting on 464.1, it's probably going to overpower that portable and cause you problems.

Understood.

 

[tropiradio]

It has nothing to do with the Br of the duplexers. I don't think that really matters, since the other transmitter is on it's own antenna and is therefore going to me much much weaker than the transmitter actually sharing the same antenna.

The 25Khz (minimum) separation is so that if a really strong portable is talking in on the 464.1 input, your new receiver isn't overloaded by that strong portable while trying to receive a weaker portable further away on your new receive frequency. The more separation the better, 25kHz is just the bare minimum I would want. (in practice, I actually specified minimum spacing of 75kHz for a large new system I just designed)

I was talking about the way the Tx signal from one repeater would still fall into the Br region of the receiver of the other repeater in terms of potential irradiated RF noise. At least that is what it would look like on paper when I look at the duplexer response curves with both repeater's Rx and Tx respectively spaced away 25K from each other. The only drawback is that the Tx signal from one repeater would have easy access to the PA of the other repeater, so that is where circulators might come in handy.

I do get what you mean about weak portables might be overwhelmed by stronger ones if separation is say 25K. But keep in mind all portables will be within 1/2 mile of the repeater, so I don't think there will be much in the sense of weak signals getting to each repeater receiver from its respective portables. In any event nothing is set in stone yet, and perhaps your 75KHz separation makes more sense. I have the advantage of being able to try out the new frequency pair, and make changes if deemed necessary before signing off to license them. One other advantage is that the 460-470 spectrum at this location is essentially unused.

I will think about all these factors and consider any additional comments and suggestions over the weekend and make a decision on Monday. My main priority now is to try and define a potential frequency pair for the new repeater so the folks at Sinclair can send their nice Q3220E duplexer fully tuned and ready to go. That will save me some bench time if the new frequency pair works well out of the box.

In any event I very much value your input and observations. Thank you.

 

[12dbsinad]

There is a reason every repeater in the US transmits on 464 and receives on 469 and they don't ever flip. You don't want to mix transmitters and receivers on the same frequency, even many miles apart. If you're trying to receive a weak portable on 464.1 and there is some other repeater on a mountaintop 50 miles away transmitting on 464.1, it's probably going to overpower that portable and cause you problems.

We got it right with UHF bands but the cluster you describe is the exact hodge podge of the VHF band. If anyone deals in that band can for sure relate.

 

[tropiradio]

Question for the people that suggested using 1/2" hardline instead of LMR400, would this cable be adequate?

LCF12-50J-P7 1/2" Foam CELLFLEX Cable - Attenuation/100 ft @ 450 MHz 1.44 dB

https://www.tessco.com/product/1-2-foam-cellflex-cable-70393

I did not budget for hardline, so trying to find the least expensive solution which seems to be this cable and at least on paper it looks good. Anyone have experience with it? Or perhaps any alternate recommendations?

 

[12dbsinad]

Either RFS or Andrews hardline will work fine. Other brands like Times make hardline as well. The bad part is there is nothing cheap about it. Don't forget to include the appropriate connectors and lighting protection. Most common practice is to terminate the hardline at a ground buss inside the shelter with a Polyphaser then use a short piece of RG213, SuperFlex or equivalent to the equipment. You'll want that jumper as short as possible. You're also supposed to install grounding kits on each end of the cable run, search 1/2 CELLFLEX grounding kit on Tessco and you'll see what I mean.

Now, with all that said, some just buy the cable and throw it in without anything and it will work. I'm just telling you how it would be installed if done professionally.

 

[tropiradio]

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.