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Author Topic: Cold temperature desense  (Read 758 times)
K6ECS
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« on: December 04, 2017, 06:07:07 PM »

Our repeater site vendor does not provide heat for the vault. Ironically, he does provide air conditioning. Now that it has turned cold again our duplexer seems to be desensing even though during warmer times of the year it does not. What are some ideas available to keep the duplexer at close to a nominal temperature? I am blessed with LIDS that know of this anomaly and come by and key the machine up just to trigger the desense. Over and over and over and... The duplexer is a Sinclair Res-Loc Q2330e.

Thank you in advance.
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K5LXP
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« Reply #1 on: December 04, 2017, 09:23:59 PM »

Just to be accurate, duplexers don't desense, receivers do.  I'm not sure what you mean by "trigger desense", you don't hear desense with just a kerchunk.  You're implying that the repeater cycles on its own, which is a different problem.

In my experience duplexers don't shift that far over their spec'd temperature.  The data sheet for the model you listed shows -40 to +60C, so if yours are drifting out of spec at temperatures within this range either there's a defect, or it's something else in the system that's inducing your symptom.

It would seem then you need to isolate the problem to the root cause, than to just assume it's the duplexer.  May mean a trip to the site when it's cold, and verify the subsystems independently.

Mark K5LXP
Albuquerque, NM
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NA4IT
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« Reply #2 on: December 05, 2017, 04:03:34 AM »

100 watt light bulb next to the problem child.

I actually installed a small 12V DC bulb inside a RX compartment with a small thermostat mounted to the case of the box. The bulb would turn on at 40F. Drilled a 1/16" hold in the lid to see if the bulb was on. Worked well.
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K4JJL
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« Reply #3 on: December 05, 2017, 07:11:43 AM »

It could be a buildup on condensation, too, especially if it's large temperature swings a short period of time.

Try the technical tap.  Sometimes you can narrow down the issue by banging on it with a screwdriver handle.
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NA4IT
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« Reply #4 on: December 05, 2017, 11:30:35 AM »

Come to think of it, I think the 12VDC bulb was put in the TX compartment. Probably wouldn't hurt to have one in each. Thermostat was a simple HVAC wall thermostat.
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N8EKT
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« Reply #5 on: December 08, 2017, 03:58:14 PM »

Sinclair specs are from -40 to 140 degrees so i would call Sinclair and ask them why this is happening.
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K3GAU
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« Reply #6 on: December 12, 2017, 08:21:52 AM »

It could also be that the squelch sensitivity changes with temperature. 
In any case, you might try using a battery warmer for heat.  It is kind of like a heating pad for batteries. They are usually wrapped around the outside of the battery to help keep it warm in very cold temperatures.  It may even have a thermostat built into it.
GL
Dave K3GAU
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WB0DZX
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« Reply #7 on: December 13, 2017, 06:33:19 AM »

If any part of this post is truncated and I can't abort, I'll have to send it to you in an email.

Sorry for the length of this regarding your repeater's "desense." By way of intro, I have been a long time two-way radio troubleshooter, system designer, and an active member on two-way radio interference and spectrum committees. Many times I have assisted the FCC with identifying and tracking interference and spectrum management.
   
As off the wall as some of this may seem, all of the following is from my real world experience in troubleshooting and solving the problem (and similar ones) you're having, often after others have failed. The problem may not be what you think it is (desense). Unfortunately there could be one or more causes; therefore, no one solution. Forgive me if I included too much basic info.
   
I have seen that you have asked for help with the problem in the past. I do see (at one point) what some of system consisted of. Forgive me for making my comments generic, rather than specific to your system. (I will often use a phrase similar to "If you have..." knowing full well that you do or don't.) Others with a non-identical system may have a problem related to your problem. If your system is open user access, thank you! If your system supports emergency communications, thank you! If your system is closed, that is your prerogative and you are still entitled to not having problems.
   
Desense is not caused by the guts of a properly adjusted duplexer, appropriate to the site, when it simply receives a normal signal, even if that signal was one or more kerchunks by somebody. Desense occurs internally in the receiver. Its definition is a reduction in receiver sensitivity, usually due to one or more additionals (including the repeater's transmitter) and constant, to desired RF signals. Usually this is a result of certain misadjustments (or rarely, malfunctioning) of the duplexer, allowing the repeater transmitter to get into the receiver.
   
"Capture effect" appears to be a reduction in sensitivity (desense) *** only when two or more received signals are received *** because the weakest signals are "attenuated." If your repeater transmitter and an improperly adjusted duplexer are involved AND a user is not at the site, those are the two signals. The repeater transmitter will usually win - BUT it may not be instantaneous, mimicking a kerchunk. The repeater transmitter may generate a momentary problem until it "settles down." Likewise, the receiver may be itself (or due to a misadjusted or now incorrect duplexer) susceptible to the transmitter. See "circulator" below.
   
If your repeater has tone squelch (CTCSS, etc), desense can occur from a strong signal *** without the required tone *** on or near the receiver frequency. (See intermod below) It's not supposed to happen, but I have rarely seen some bursts of such a strong signal getting through (after the tone squelch) to activate the repeater. If any burst sound is heard, it will sound either like an unsquelched receiver, data, distorted paging tones, a syllable or two, or an unintelligible voice. Often, in this rare case, there is nothing heard other than the activated repeater, kinda sounding like it had been kerchunked. When that happens, the burst was *** very *** short.
   
In the off chance that there is a for-real kerchunker or someone gets the idea to start, EVERYONE involved in the investigation MUST NOT give a hint of what is being done or schedules or location(s). This means no discussion on the air (repeater or otherwise) or at club/repeater meetings. At least one secret meeting must be coordinated by phone.
 
DO NOT ACKNOWLEDGE A PERCEIVED KERCHUNKER! Now is not the time to make that known, except to the investigation team. The only easy ways to know if the perceived "desensing" is caused by a "kerchunker" is to:
1. Listen on reverse offset by multiple people, preferably at the site or by those on higher elevations with a good antenna.
2. Have what should be a stronger station identify over the top of the kerchunking (if it lasts long enough as one or multiple transmissions) and have a third station listen. If the kerchunking is still heard simultaneously, even at a lower level, two conditions apply:
1. The kerchunker is stronger than the one trying to override it (capture effect - NOT desense.) Try a station on high power with a beam. OR
2. The "kerchunking" is generated at the site (still capture effect or another cause below, maybe causing desense.)
   
The hard way is to go to the repeater site to listen to the input with a different radio and/or spectrum analyzer. Don't forget the antenna, coax connector adapters, or pigtails! Bummer if a human, alien, etc kerchunker has caught wind of the schedule and effort, refusing to "cooperate."
  
Almost all kerchunkers want feedback to complete their "reward." If the interval between kerchunks is very short OR they continue before any squelch tail ends OR the kerchunks go on for long periods, it is doubtful a human, alien, or Murphy is the cause. Seldom do two or more kerchunkers coordinate among themselves. If so, it involves slower and inconsistent kerchunk intervals, usually with differing quieting levels and background noise. Confirmed kerchunking can be minimized by:
1. Direction finding: watch/listen on the input for a null instead of a peak. If a link is involved, it won't work! or
2. Education, especially of new members and at training classes
Don't go so far as to say, "We will hunt you down like you're a rabid dog!"
  
I have also seen someone's transceiver left on, usually in an unattended vehicle, start kerchunking without human/alien/spirit involvement. This can be caused by:
1. A bad microphone switch or one affected by dirt that becomes intermittently conductive in high humidity, one partially wedged on a button, or the switch intermittently malfunctioning. If the later, a passing truck or train, or vehicle with loud thumping bass could set it off, if the "kerchunking" disappears shortly after it starts. OR
2. The vehicle's battery is drained enough to cause the powered on radio to not like it. This is related to the problem of cordless phones dialling 9-1-1 or memory numbers on their own. The radio "kerchunks" because, beyond a certain point, the additional small drain on the battery is enough to increase its internal temperature. The radio goes temporarily spastic due to the fluctuating voltage/current.
3. A kid playing with a microphone at home or in an unlocked vehicle.
4. An intermittently malfunctioning transceiver.
5. The transceiver is set to cross-band repeat, especially without CTCSS.
6. A handheld transceiver is wedged where the transmit switch is momentarily depressed, usually by body movement or other vibration. Sometimes you will hear someone talking or a radio in the background.
   
I assume the repeater is on a mountaintop. Does the problem occur with significant temperature swings, especially enough to have air conditioning on only during the day? Is the duplexer and/or repeater flush against an uninsulated wall? That would be a potential factor if the temperature dropped considerably after dark during seasonal change.
  
1. Does the "kerchunking" occur only at certain times of the day or night? If it coincides with starting or quitting time or is only during the day, one or more co-site business-related transmitters may be generating out of tolerance spurs or noise, perhaps yielding intermod.
2. When it started, were there any new transmitters added or antenna changes made to the site?
3. Are there nearby ski areas? They and ski patrol start up when the weather gets cold.
4. Is there a co-sited or nearby transmitter devoted only to support snow, avalanche, and rock slide removal or wildfire prevention/suppression?
5. How about other nearby or co-sited ham transmitters in the same band?
If any of the above apply, suspect intermod or a problem with the other system(s). Intermod may sometimes involve transmitters in bands other than your repeater's. Intermod product(s) showing up on a receiver does not have to involve any malfunctioning transmitter and/or may involve a poorly protected (duplexer now wrong for the site due to one or more changes) or malfunctioning receiver.
   
Some intermod is strong enough to get by the duplexer, especially if the duplexer (now due to site changes) happens to be the wrong type which definitely for VHF being compact rack mounted and/or does not have discernable modulation when it triggers a carrier squelch receiver. In that case, the "kerchunking" is often due to the RF level being at the squelch threshold.
   
Intermod usually involves one or more transmitters transmitting *** continually, *** such as telemetry, links and order wire, weather radio, etc. Otherwise, it could involve one or more of the following *** contributing *** factors:
1. A malfunctioning transmitter (spurs and noise).
2. If one or more nearby or co-sited transmitters have the same CTCSS tone (see below) as your repeater, suspect interaction.
3. If there is (recent or not) metal, no matter how small, discarded or stored around the site, it could - only in conjunction with the appropriate transmitted signals - cause intermod, especially considering expansion  above a certain temperature and contraction below a certain temperature, wind, and moisture/humidity. The metal acts like an antenna.
4. A high VSWR on someone else's system could cause it to malfunction, especially during weather or humidity changes.
   
You will likely need to get info from the site manager and any nearby sites' managers. One key piece of info is the year the repeater problem started - to see if that coincides with any site changes. That might even point to something you did to the repeater.
   
1. How old is your repeater? The older it gets, the more problems can occur. Layers of dust, insect, and/or rodent activity can also be/contribute to the cause, especially if the A/C filter is not changed often enough and on circuit board traces.
2. If your repeater receiver is slightly out of spec, cold weather (possibly with a humidity change, remember the air conditioner?) may exacerbate the (intermod) situation.
3. Did the "kerchunking" start after any movement (earthquake?) of the repeater and/or duplexer and/or wiring was changed? If so, rarely your duplexer's adjustments could have changed. This could result in one "kerchunk" (the result of intermod, other transmitter occasionally acting up, other user duplexer being slightly misadjusted) appearing to be many "kerchunks." It should be years and years, if ever, of temperature swings that would cause an unmoved duplexer in a shelter to need readjustment.
4. Skip could be a factor, especially due to fronts, temperature inversions, and wildfires, sometimes coupled with terrain. Skip does not have to be booming in. Airplane and meteor refections could be "skip." Weather radio transmitter frequencies may be used to help determine skip, but mountaintop conditions can be much different than in the valley. Remember California weather this year has not been normal and may not be for some time.
5. A co-channel repeater close enough could be rogue or coordinated. One or more of its users may get strong enough to trip your repeater. The user(s) may head for an in-between mountain for recreation, possibly on a seasonal basis. Check with the coordinator(s) for applicable coordinations or complaints, especially for reverse offset repeaters.
6. Is a crossband repeat, link, remote receiver, and/or a receiver voting system attached to your repeater? If so, take it/all of them off-line. If the problem goes away, only put the most important one on-line for a long enough time. Keep doing that for each subsystem until the problem reappears. If the problem continues after all is off-line, reconnect them right away.
In other words, unless one of the situations above applies, your duplexer is probably not at fault, unless it happens to not be inappropriate for your repeater in relation to its and nearby sites.
  
My memory is not as good now as in the past. Plus I'm too lazy to find and review manuals (sorry) for further ideas. But, by what memory (of solving similar problems) I have left, here are some circuits and/or programming *** within your repeater *** that may be involved in the problem. CAVEAT: Do not bite off more than you can chew! Get knowledgeable help. Show them this email so they can get my "drift."
1. Squelch threshold too loose. Temperature (swings) can also cause it to loosen. This is not much of a problem in programming unless it was (accidentally) misadjusted or its associated circuitry is involved.
2. Schmitt trigger and associated in/out components
3. Carrier operated "relay" AND repeater controller: Any adjustment close to the threshold could be affected by temperature and/or humidity. A repeater controller connected to a phone line, even if not used for phone patching, could be affected by intermittent phone line noise and/or crosstalk, sometimes caused by the aforemention fluctuations, insects, rodents, etc.
4. Subaudible (CTCSS) tone system (missing or malfunctioning). If you're still using the ancient burst tone repeater access, it's probably not a good idea with today's crowded spectrum. If you don't have CTCSS, consider adding it after getting the tone coordinated.
5. Receiver front end and filtering, especially if there were recent adjustments. One or more components could have been "adjusted" enough to make it susceptible to intermod, site noise, or off frequency signals (including from your repeater's transmitter) making it through the duplexer.
6. Receiver may have to be switched from low to high side injection, or vice versa; if it can be done by programming or swapping a crystal in a channel element, due to changes or additions to the site or nearby ones.
7. Transmitter's local oscillator and maybe a stage or two thereafter: Sometimes these stages are on constantly. Check it on a spectrum analyzer for spurs and noise, especially on or near an receive frequencies. If there is at least one spur, (it may not even close to the input), assume it goes up and down the VHF band, for whatever reason. Clean up your transmitter (and not with a cloth, car wash, or steam sprayer).
8. Using a scanner (and a separate transceiver for simultaneously monitoring your repeater output), monitor (without CTCSS) co-sited repeater inputs and outputs within the same band as your repeater. If you get lucky, you may uncover a(n intermittent) broadband noise that simultaneously occurs in their systems. It would be difficult to track spurs unless the scanner is programmed in frequency sequential order up and down from your input. If your repeater or links carrier squelch, the offending signal may not be a problem if you added CTCSS.
9. You may have a circulator functioning as a duplexer. Any VSWR will cause the transmitter to get into the receiver, desensing it.
10. If you don't have circulator, installing one with the appropriate duplexer prevents many problems.

   
It is often unsaid, but should be: DO *** NOT *** TOUCH ANYONE ELSE's EQUIPMENT, no matter if you suspect they touched yours! Doing so will only invite escalation! However, you can look at it on a spectrum analyzer connected to a telescoping antenna, but unfortunately you may have to wait for the transmitter to transmit. If you suspect yours has been touched, recheck its operation. If it only appears that your duplexer has been moved, recheck its connections. A loose connector can create problems, especially with vibration (earthquake or less) and/or temperature swings. In extremely rare cases, the visible part of one or more tuning rods have been bent.
 
For decades, sheltered and properly functioning repeaters with their duplexers have operated over a wide temperature range, even with mechanical relays and no shelter heating. Intermittent internal low temperature faults might be coaxed to reappear with canned freeze. Canned air may not be cold enough. Any extra heat required for operation does not fix the problem, which may get worse. A stop-gap measure could be to cabinet the repeater, if it isn't already. The heat buildup may be just enough to sidestep actually solving the problem. The duplexer shouldn't be in the same cabinet or require a separate one.
   
So there's no misunderstanding, don't try to heat up the duplexer. That would more quickly hasten out of spec changes. Besides the heating will be uneven. A light bulb will also do no good for an exposed (open) rack or an enclosed (cabineted) one with an open or missing door. For a cabineted repeater without an internal (inappropriate) duplexer, place a 40-60 W *** incandescent *** (NOT LED, fluorescent, or "equivalent to") light bulb inside at the bottom, where it will hopefully be less likely to cause shorts and more evenly heat the inside. Don't get it near any manuals or other paper or CD's. As another poster indicated, a thermostat, like for heat tapes and engine block heaters, should be used to avoid extra heat and overly taxing a generator or solar power system. Farm supply companies usually have the best price rather than automotive supply. In California, you may have to get it from a store in higher elevations or on-line.
   
Closed circuit: Now, what specifically do I think is going on with your system?
1. If real direct kerchunking is happening, attempt to isolate which site receiver(s) is/are receiving it and/or where someone hears it on the repeater input. DFing on nulls is then required.
2. An inappropriate duplexer should be replaced.
3. Ideally any system, being at a crowded site, also requires a circulator and isolator.
4. Consider an extra filter for any affected receiver.
4. Check your system, especially wireless links and connections!
4. I suspect on-site or nearby snow/ice and possibly (transient/itinerant?) wildfire related comm systems may be seasonally involved - contributing to intermod, having out of spec transmitters, and/or, in the case of a repeater, having a duplexer inappropriate to the site.
5. Rarely, but the Santa Ana winds may have caused it, a fiberglass "stick" antenna may have flexed (often) enough and/or suffered from (repeated) lightning to disable an internal element or cause an intermittent connection, contributing to intermod.
 
Hopefully, you have not had to evacuate due to wildfires. For a mountaintop repeater such as yours, here's wishing you luck using a sno-cat, snowmobile, FWD, ATV, or helicopter to get you and your equipment safely up there. Watch out for wildfires and smoke. Be safe and good luck!
     
Being long retired, I have tried to remember all of the potentially relevant situations I have encountered, possibly leaving out the obvious and the not so obvious. I was called in as the last resort on hundreds of two-way radio systems having problems. Again, sorry for the length, not getting real technical or specific (this post would be much longer!), and attempting thoroughness.
   
Mike WB0DZX
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