Quantar false keying on weak signals

[radioactive69]

I have two UHF 25w Quantars running in Astro CAI Capable mode (mixed) on the same frequencies within the coverage area of each other. What I call Site #2 and thus programed in the software, keeps false keying on weak digital signals when the NAC codes don't match. If the incoming signal, with incorrect NAC, is strong (quiteing beyond flutter) enough, it won't false key. I sit at Site #2 and see this occur.

This occurs only when site #2 is enabled with the multi-NAC table. If site #2 is programed with a single NAC and PL this falsing will not occur.

Both programed with same R14.06.00 RSS.

Both Site #1 Site #2 Control firmware R02.14.038 and Codeplug V.14

Doug.

[Will]

Set the Squelch parameters for AND, both correct NAC AND correct quieting level to key. Set the loss of keying, receiver Squelch to OR... loss of carrier OR loss of proper NAC.

Check the receiver carrier detect/Squelch for proper quieting.

[radioactive69]

I don’t see those screens under the" configuration" folder. If they appear under the" service" folder, I be back at the site in two days and check those items.

[radioactive69]

Since I only have an IFR1500 on hand, I was only able to try the increased squelch setting from 19% to 34% and this only caused more falsing. It went from quieting at -124dbm to quieting at -121dbm in analog PL. I have not conducted and RSSI or BER as I have never done those alignments. I can get access to a P25 service monitor but, would need instruction on those tests.

I looked through the configuration settings under the both Configuration and Service in the RSS and found nothing for AND NAC nor OR NAC. I found a drop down menu for carrier AND PL/DPL under channel information.

I do have a Chip-Angle gas-fet preamp on the RX, just incase this might be an issue. Granted this only improved the RX by .15uv.

Any additional ideas? Please be specific to the folders in the RSS.

[Bill_G]

Since both stations are configured the same, I'd be tempted to first blow the programming of the good station into the bad station just in case there is something set wrong that I did not recognize at first. Failing that, I'd be tempted to swap both stations from one location to another to see if the problem stays or moves. If it moves with the station, then you know you probably have a failure in the control section. If the problem stays, then you have an external problem to identify.

[radioactive69]

Does the RPF file I save in RSS save the Service/ Alignment settings? When I open the RPF, the Service folder is greyed out.

I will first try loading the RPF of site# 1 into site# 2.

Also, could I just swap the control module?

I can swap stations. They are only an hours drive apart. I will try that later this October, during my vacation.

Thanks, Doug.

[Bill_G]

The radio programming does not contain the alignment. So, you can clone it into a station safely. I know swapping a whole station sounds like overkill. But, considering the problem, and how easy Quantars are to move, if cloning the programming doesn't fix it, bringing station #1 over to station #2 for a test would be worth the effort. It will at least tell you if the behavior is normal, and if you are fighting an external problem.

I'm fascinated about why tightening the squelch makes the falsing worse, and makes me wonder about your preamp being saturated. But, you could be being led astray and arriving at the wrong conclusions. Having the two stations side by side for a while will tell you which way to go.

[The Pager Geek]

This occurs only when site #2 is enabled with the multi-NAC table. If site #2 is programed with a single NAC and PL this falsing will not occur.

Doug.

This statement bothers me. It leads me to believe it is a codeplug or firmware issue failing to be selective enough under certain conditions. There were known squelch issues with versions just prior to .038 so I wonder if this is a problem.

What are the rest of your FW revisions?
Was it upgraded from another version, or original build 20.14.038?

Also, do you have a digital service monitor to test it? (Try to replicate the issue with a service monitor so the station is off of the antenna system? Rule out interference, pre-amp, antenna, etc.)

tpg

[radioactive69]

Firmware History: I don not know the original FW. I gave both stations to a friend that works on 100's of these for public safety and he upgraded the FW. I didn't use them until after he aligned them both a couple of years ago now.

I will contact another engineer I know to help me run some P25 tests and alignments on site #2 and report back those tests. Hopefully we can meet this week to do this.

Both sites are identical in there configuration, preamp, duplexer, multi-NAC enabled and mixed mode. Analog PL, NAC codes and antennas are different.

[The Pager Geek]

. Analog PL, NAC codes and antennas are different.

... and locations.

I would rule out antenna system first before you try to chase firmware bugs that may not exist.

A p25 service monitor generating a carrier at threshold with the invalid NAC will prove it's the station if it still demonstrates the problem, or (if not replicated) show that it is an antenna system or interference issue.

tpg

[AEC]

Passive Intermod(PIM), may be a culprit.
How old is the antenna/feedline?
When was the duplexer last touched?
Are you getting spurious emissions from the preamp oscillating?

Are you using isolators(single or dual)..are they adjusted properly?
If the loads are hot, you are getting garbage back into the transmitter and reradiating...harmonic mixing, third order products Etc...And all feeding into your station, which can also feed back through the preamp and cause even more troubles.

I would also check nearby stations...sniff the area for strong in and out of band signals...

Anything relatively new placed at the site since your problems began to show up?
RF door alarms, wireless cameras, SCADA system?

Start at the simplest problem and work outward, write down your findings as they may come in use for future reference.

Remove the preamp and see if the problem persists, if not, you may want to pad the output with a 3dB or slightly higher attenuator.

just things I like to look at to find gremlins...

[radioactive69]

I have loaded in the site# 1 RPF file and have just changed the NAC Table and PL. This I will test first and report back.

Here is some more answers :
The site is my house and is clean of any major RF.
The antenna is 10yr. old 4 pole dipole with enclosed cabling, good shape.
Duplexer tuned 3 months ago. Moto T1500 model.
No isolators.
Reflected power checked with watt meter and is flat, 25 out, 1 back.
Cable is 1/2" and is less then 6 years old.
I can remove the preamp next but, will run the P25 service monitor tests first.
Thanks for the suggestions. Doug.
I'll get back to you guys.

[radioactive69]

Still waiting to get access to the COM120 service monitor.

Once I do the weak signal testing, I'll report back.

Doug.

[radioactive69]

That will be a IFR 2975, not a COM 120.

[radioactive69]

Sorry to all following this issue....however, it is not easy for me to get the P25 service monitor from
my friends radio shop. He keeps this thing with him for the after hours/on call needs. So, once I can
corner him to sacrifice some time for his help, we will report back with our findings. Stay tuned.

[radioactive69]

Time to report on the efforts over the last few months to find out what fixed this problem:

Utlimately I changed the frequency by less then a megahertz and that resolved the issue.

Before that I changed the run of heliax, two other antennas, jumper cables, two other duplexers, NAC codes
and feature Multi-NAC enabled and disabled.

I did not find any signals on or near the input or output frequencies from a 5db base antenna and the service monitor's stubby antenna that would of mixed with the repeater.