A split in half Capacity Plus site drops talkgroup calls after 3-5 seconds 😕

Our Customer uses a small LCP system consisting of three sites [...]. The biggest site has an issue of intermittently dropping local calls after 3 to 5 sec. [...] This site has four repeaters, two of which are connected to one antenna on a tower via a combiner and the other two repeaters are in another location a few hundred meters away with a seperate antenna system.  Our theory is that maybe the ATEX radios (1W) working on that site, sometimes are unable to initiate or maintain the call when rest slot rotates through repeaters on different towers (Switching from one tower to another). In other words we have a split in half Capacity plus site working on two antennas on different towers. [...] Would you consider this to be a possible cause?

Without seeing the installation and based on your description, the non-standard way in which the biggest site was installed seems problematic to me. 

All repeaters on a specific site should all be connected to the same antenna system. This not only reduces the likelihood of interference but also provides consistent coverage no matter which slot the rest channel is on at that site. 

Despite this obvious potential cause, my suggestion would be to try to eliminate all other possible causes, as redoing the RF filtering system will cost money and result in some downtime. The customer may not be pleased if you do all of this work and the reported symptom is still there.

Based on my (anecdotal) experience, the common causes for dropped calls in Multisite Capacity Plus systems are:

• Misconfiguration

• Interference

• Everything else

Misconfiguration includes radio; repeater; router configuration as well as the design and installation. Interference includes desensitization; intermodulation and external (accidental and deliberate) interference. In some cases, the root cause might be both misconfiguration and interference. There might be things I’ve forgotten to mention (or haven’t thought about) hence everything else BUT the first two are the leading causes.

Another thing to be aware of, is that the ATEX radios have a transmit power of 1W (with an ERP of around 250mW). The transmit power from the repeaters could be anything up to 50W (though I suspect RRT will probably limit you to 5W) with an ERP of anything up to 100W. The net result is an imbalance between the outbound (repeater-to-radio) and inbound (radio-to-repeater) coverage.

There could be circumstances where a radio gets a good signal from the most appropriate rest channel but when requesting a call, the inbound signal is marginal resulting in dropouts.

If the symptom is being seen on both mobile (fixed) radios and portables, then the above is not a likely cause.

Without digging into equipment configurations etcetera, there are some questions you can ask your customer (or yourselves), which will give you some pointers:

When (date) was the system first installed and when (date) was this symptom first seen? If the symptom appeared sometime after the system was installed, what changed on the day or shortly before the symptom appeared? Even unrelated things can have an effect?

When does this symptom occur? For example, at certain times or on certain days. If so, what is unique about those times? For example, an intermod product might only appear if signals from transmitters TXA TXC and TXD are present.

How often does this symptom occur? Is there any pattern? For example, four calls go ahead okay then four call attempts exhibit this symptom then the problem goes away. This might indicate a problem on one or two repeaters such as the two installed at the location 300m away.

Is this symptom only seen by certain users? For example radio A makes talkgroup call but it is dropped after 3-5 seconds. Then Radio B makes the same call and it goes ahead normally. In this case, the symptom is only seem by the user of Radio A. If this is the case, what is the difference between radio A and B? Where is the user of radio A located?

If you (the customer) have RDAC, try pulling the repeater logs as there might be some clues there.

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** The question has been edited to remove potential identifiable information and for brevity. Image source pxhere.com.