Transmit amplifier for digital repeater

[tropiradio]

I am curious if there are any reputable linear amplifier manufacturers that have models compatible with the fast switching that a digital system would require. Case in point (not originally installed by me) is a two repeater SLR UHF conventional (TRBO) system running both at rated 55W transmit which ends up being about 40W after passing through 3-stage isolator, low pass filter and duplexer. Side by side omni (looks like 6dB) antennas about 25ft apart facing each other and at identical height.

When I first approached this system for upgrades about two years ago, it was just two SLR repeaters thrown on a table and stacked on top of each other, and each with a flat-pack mobile (notch only) mini duplexer. It is certainly a testament to the SLR's performance that this system was even running that way without any mutual interference between the two repeaters. I did since upgrade some time ago the duplexers to Sinclair QT3220E, and also added the (Decibel) DB4326 isolators and DB4332-B low pass filters, plus a Polyphaser IS-50NX-C2 antenna surge protection.

Coverage area is roughly a rectangle of about 1.5 x 0.5 mile, but repeater is located at one far edge of the area, and antennas (omni/fiberglass) are mounted very low (due to very strict aesthetic reasons), so the whole radome is barely clearing a two story building ledge. I would have gone first trying to raise the existing antennas and possibly even change them to yagi type pointing inwards the coverage area, but management says that modifying antennas, or moving the repeaters to a more central location within the intended area are not an option at this point.

Complaint is that portable to portable communications through the repeater sometimes does not work, specially in cases where the portables are close to each other, and mostly towards the far opposite end of the coverage area where the repeater signal is somewhat weaker, and the problem seems to have recently escalated after they got a bunch of additional portables (HP782's in addition to the existing smaller fleet of XPR3500e). To me what seems to be happening is that the transmitting portable is overwhelming the receiver of the nearby portable, and making it difficult or impossible to receive the relatively weaker repeater fringe signal. This of course gets worse if both portable radios that are trying to communicate are both indoors as additional repeater signal attenuation comes into play.

In the old (analog) days, if I needed to raise the power of a repeater I would use a TPL linear amplifier with mostly good results, although in some cases they where a bit temperamental. But TPL seems to be no longer in business, so was wondering what other alternative brands could be considered, as it seems I won't have much other options than trying to raise the transmit power of the repeaters in order to try and also raise signal strength in the more difficult areas.

I'm of course also open to any other suggestions or ideas. Was even thinking that placing a couple of small power auxiliary transmitters might help, but then this will obviously create problems if not simulcasted, which would be a whole new level and cost to even consider.

 

[mmckenna]

What is the spacing on the repeater pair?

Two hand held radios shouldn't be desensing each other on a normal 5MHz split.
No reason a 40 watt repeater can't cover a campus that small even with antennas down that low.

I think you have something else going on. Throwing more RF power likely isn't going to fix it.

 

[KevinC]

A DMR repeater transmits on both timeslots all the time, the "fast switching" is on the subscriber side.

 

[ramal121]

Didn't realize TPL went belly up! Our other go to is Cresend Technonogies or we have used Henry amps a few times. As KevinC mentioned a fast switch amp is not required on a repeater.

I agree, 40W on a two story building should cover a way larger footprint (unless you're trying from inside a bank vault). I'm concerned that your receiver is not up to snuff and is partially deaf to the portables. What you describe can surely be from this.

Do this first and chime back. Run RDAC and take a RSSI reading from both repeaters at idle. Then transmit on one repeater, the other repeater, and lastly both together and note the RSSI on these. This will help tremendously to troubleshoot what's going on.

 

[tropiradio]

What is the spacing on the repeater pair?

Two hand held radios shouldn't be desensing each other on a normal 5MHz split.
No reason a 40 watt repeater can't cover a campus that small even with antennas down that low.

I think you have something else going on. Throwing more RF power likely isn't going to fix it.

Not exactly 5MHz, one repeater offset is 6.6MHz, and the other 5.16MHz.
What else could it be, interference on the receiving frequency of the portables?

 

[prcguy]

The limited range sounds fishy. I might temporarily disconnect the repeaters and connect a base radio to the repeater antenna then map out simplex coverage as a baseline. Maybe test in analog mode to simplify things. If you get the same poor range then find a way to camouflage the antennas and get them higher.

 

[ramal121]

interference on the receiving frequency of the portables?

Yes, that is a possibility also.

 

[ramal121]

How confident are you of the integrity of the feedlines and antennas?

 

[tropiradio]

Do this first and chime back. Run RDAC and take a RSSI reading from both repeaters at idle. Then transmit on one repeater, the other repeater, and lastly both together and note the RSSI on these. This will help tremendously to troubleshoot what's going on.

I actually went about 3 months ago to do a routine check, and receiver sensitivity on both repeaters was around -119dbm from the antenna port at the duplexer. I did check them with RDAC but only to look up any alarms and the log, and BTW both repeaters had fan errors.

Also been monitoring comms for several days and rarely hear of any issues during a conversation between any two portables. BTW there is also 6 base stations located throughout the property with 45W HM782 radios, incidentally one of them in the next room where the repeaters are located, and another in the area where the issues are being reported to be happening. Not sure that could be related to the issue.

 

[AM909]

I think the first thing you have to determine is if the problem is handheld receive or transmit or both, by positioning handheld "X" in the problem area and handheld "Y" somewhat near the repeater, where coverage should be assured, though not close enough to risk desense to either the handheld or repeater. Then, try to make it fail. If X always hears Y, but Y does not always hear X, you have a repeater receive problem. If Y always hears X, but X does not always hear Y, you have a repeater transmit problem. That tells you where you should be looking. Ordinarily, a 40W repeater TX is way enough for 5W handheld coverage.

[add] If the problem is in both directions or indeterminate, look at feedline and antenna, which is probably the most likely cause. Easiest thing is measure TX power/SWR into the feedline and TX power/SWR at the base of the antenna. What do you get?

 

[prcguy]

I actually went about 3 months ago to do a routine check, and receiver sensitivity on both repeaters was around -119dbm from the antenna port at the duplexer. I did check them with RDAC but only to look up any alarms and the log, and BTW both repeaters had fan errors.

Also been monitoring comms for several days and rarely hear of any issues during a conversation between any two portables. BTW there is also 6 base stations located throughout the property with 45W HM782 radios, incidentally one of them in the next room where the repeaters are located, and another in the area where the issues are being reported to be happening. Not sure that could be related to the issue.

Is that -119dBm sensitivity during transmit, full duplex?

 

[tropiradio]

Is that -119dBm sensitivity during transmit, full duplex?

I tested under both conditions, static and full duplex, and during transmit there was just a tiny bit of interaction noticeable but nothing that would degrade the reception of the repeater.

BTW since someone also asked about the condition of the antennas, for both repeaters tested 1.2 - 1.3 SWR.
Also when I inspected the antennas, noticed they where mounted onto a wall with plastic brackets, like those that are used on boats to hold the longer marine antennas or extensions. So those antennas seem to be technically ungrounded except for the coax cable shield (estimate cable run to each antenna be about 50-60ft of RG214).

 

[tropiradio]

I think the first thing you have to determine is if the problem is handheld receive or transmit or both, by positioning handheld "X" in the problem area and handheld "Y" somewhat near the repeater, where coverage should be assured, though not close enough to risk desense to either the handheld or repeater. Then, try to make it fail. If X always hears Y, but Y does not always hear X, you have a repeater receive problem. If Y always hears X, but X does not always hear Y, you have a repeater transmit problem. That tells you where you should be looking. Ordinarily, a 40W repeater TX is way enough for 5W handheld coverage.

[add] If the problem is in both directions or indeterminate, look at feedline and antenna, which is probably the most likely cause. Easiest thing is measure TX power/SWR into the feedline and TX power/SWR at the base of the antenna. What do you get?

Thank you for the test suggestions, perfectly understood.

But by listening in I know that for instance the issue does not happen in the same area where it does happen only to close by radios. If in that same affected area a portable transmits to talk to another portable radio that is -not- in the immediate vicinity, the communication goes through just fine. meaning that the issue being reported only happens when trying to talk to a radio in the next room or very close by, but not when its a radio that is further away. That is why I though that receiver desensitizing by the nearby transmitting radio was playing a role.

So as some have suggested that there might be a reception problem at the repeater, why do those other communications between non adjacent portable radios just work fine? And this is the case of probably 99% of all the communications, with rarely one having some some form of digital artifacts (echo, or other form of distortion) that may require a "repeat message" request from the receiving end. Asa matter of fact, after having listened for 2-3 days straight, I never hear anyone reporting the issue at hand, only those occasional "repeat message" situations.

I am not even sure why one would want to call a radio that is almost in front of you, as the transmitting users report not being able to hear their own voice on the other radio, so that to me means that they are basically standing in front of each other, or the next room. I assume they are doing it purely as a test, which I know sometimes will just not work. Sometimes when I program digital radios for a customer I quickly usually test them through their repeater to make sure IDs are correct and voice is working etc. But if the transmitting portable is too close to the other the voice and other wise will just not get though. I either have to switch to low power or separate the radios a bit from each other for it to work.

 

[tropiradio]

Furthermore, and something I had not mentioned, the customer has about 12 groups spread across the 4 available slots, and only one of the groups is reporting the issue. Most groups are mostly symbolic for "when needed" and get very little if any usage at all. But there are a couple of higher usage ones as well, some of them sharing the same time slot. And being the case that the group that is reporting the issue is one of the busy groups, if not the busiest one overall in terms of repeater usage, and sharing the same slot with another group that has also quite a bit of traffic, although nowhere near as much, I though that there might be some call collisions happening that where creating the issue (this was before someone actually explained to me the nearby radio not working situation).

But after monitoring both of the groups sharing the same slot with separate radios at my end, I never saw that while this other group was accessing the repeater, that seemed to create an issue like leaving the first group out in the middle of a conversation, or something similar. Simultaneous call requests from both of these groups that share the same slot seem to be unfrequent, and in any case would have generated a very distinctive BUSY channel tone on the radio, something which has so far not been reported at all as being related in terms of the ongoing issue.

 

[mmckenna]

Not exactly 5MHz, one repeater offset is 6.6MHz, and the other 5.16MHz.
What else could it be, interference on the receiving frequency of the portables?

how close are the 2 transmit frequencies and how close are the two receive frequences?

 

[ramal121]

There's a couple of conditions you mentioned from your OP. First is the repeaters footprint seems to be awfully shortened for what your system is. This can be a number of things including both transmit or receive problems hence all the troubleshooting suggestions to look at all the different areas.

Now second your concern of desense problems to close by receiving radios is more specific. All radios can suffer from this to varying degrees.
The main determining factor of how close radios can be together is repeater transmitter signal strength. The better the signal to the receiver the better it will fight desense from any off channel energy making its way into the receiver. So your premise of increasing transmit power is not out of line however to go from 40 watts to 100 watts is only a 4 dB increase and this would be just barely noticeable.

So to combine this with the poor coverage area I would lean to some issue with the antenna or feedline. My axiom is a bad VSWR indicates a problem somewhere but a good VSWR reading doesn't mean squat. Using prcguys' suggestion of using a simplex frequency to map out tyhe receive coverage is a good starting point. It should be way bigger than what you state. Once you have that making a repeater match it or exceed it on the transmit side is no big deal as long as everything is running correctly.

Is there any way you can duplicate the no receive condition at a standard distance and then measure the strength of the repeater transmit at that location?

 

[prcguy]

Right now there is a long list of potential things that may be limiting coverage from local noise to questionable antenna/feedline, compromised antenna location, etc. You can spend a lot of time testing and tinkering with each item on the list and never get anywhere or do a baseline simplex test. And I would suggest using one or both repeater antennas fed with a handheld then a second test with units in poor coverage areas with just a handheld on a roof with stock antenna or somewhere high and in the clear replacing the repeater. This will give you an immediate answer on potential coverage and also discover if the current repeater antenna locations are compromised and limiting range.

If a handheld on a roof covers all areas perfectly and the same handheld into an existing repeater antenna has poor coverage, the antenna and feedline can be tested and if ok then the location is crap and you need to move the antennas higher. The building could have metal lath or shielding under the surface as with stucco or some concrete finishes. If there are people holding back moving the repeaters or antennas then have the radio users and whoever has the radio $$ budget petition management telling them this is a go/no go situation and if they want radios they have to compromise and allow the antennas to be in a more favorable location. In many cases you can't have both and I suspect there is a lot of $$ invested in the radio system and you may never get the intended use out of the radio system if its antenna location is holding it back.

 

[tropiradio]

how close are the 2 transmit frequencies and how close are the two receive frequences?

RX1 to RX2 are 600Khz apart. (both on 462 segment)
TX1 to TX2 are 1.5MHz apart. (456/457)

 

[jeepsandradios]

I assume your not in the US ? I ask as the splits your talking about aren't normal.

 

[mmckenna]

RX1 to RX2 are 600Khz apart. (both on 462 segment)
TX1 to TX2 are 1.5MHz apart. (456/457)

Is the RX/TX from the hand held radio point of view, or the repeater point of view?