Icom FR4000 "noisy hollow pipe" sound, seems duplexer related?

[KevinC]

The dummy load on the transmitter that @prcguy suggested is a good idea.

 

[prcguy]

That's where I'd start.

I would still start with terminating the transmitter with a load and listen to the repeat audio. If its normal then connect the duplexer but terminate the antenna output with a load and listen, should be very easy to isolate the problem to the repeater or duplexer or cables/antenna. You should have enough info to pinpoint the problem in less than 5min. No need to complicate things or go off on tangents, just concentrate on the basics.

 

[ElroyJetson]

It's a larger duplexer than that, about 7x9 inches overall size, with N connectors.

I know, LMR isn't optimal for this application but it's not expensive and is easy to work with and is the de facto standard
for lower cost radio installations, at least in central florida. It's almost rare to find anything else in use.

I don't spec what goes in, that's not my job. I maintain and install what I'm told to maintain and install.

If it were my job to spec out systems, I'd be trying my best to sell better products and systems to my customers.
But this is lowest competent bidder work. Keep that in mind. The safety of your kids at school may depend on the
performance of repeaters and portable radios that were selected because of LOW COST. Feel better now?

And I've got to make it run right when it misbehaves...and its normal state of operation is never all THAT far from misbehaving.

 

[prcguy]

It's a larger duplexer than that, about 7x9 inches overall size, with N connectors.

I know, LMR isn't optimal for this application but it's not expensive and is easy to work with and is the de facto standard
for lower cost radio installations, at least in central florida. It's almost rare to find anything else in use.

I don't spec what goes in, that's not my job. I maintain and install what I'm told to maintain and install.

If it were my job to spec out systems, I'd be trying my best to sell better products and systems to my customers.
But this is lowest competent bidder work. Keep that in mind. The safety of your kids at school may depend on the
performance of repeaters and portable radios that were selected because of LOW COST. Feel better now?

And I've got to make it run right when it misbehaves...and its normal state of operation is never all THAT far from misbehaving.

So are ya gonna get a dummy load and start testing?

 

[WRMD298]

LMR400 isn't optimal for duplexed systems.

Exactly. That's why I asked.

 

[WRMD298]

Testing with a dummy load as has been mentioned already would give you an idea whether you have "in house" problems or not. I completely understand about the LMR400 being less expensive than other options, but as others have stated, certainly is not good for duplex operation. A slight increase in cost will get you LMR400DB, which is the "direct burial" version, and the braid is flooded with sticky mess, which actually helps alleviate semiconductor-like properties of oxidized/loose metal-to-metal contact between the braid and foil of the cable. I doubt it's your sole problem at only 40 watts of power. I am still thinking you have local mixing. Any secondary source can be a suspect. Nearby broadcast transmitters, WX service transmitters, cell sites, etc. If the agency isn't willing to spend some money on infrastructure to correct the problem (post-filters on the TX, a good BP/BR duplexer, Heliax-type cable, etc) then I guess they should be willing to accept sub-par performance.

 

[ElroyJetson]

The dummy load test was done. As was dropping the power output of the repeater. The duplexer is suspect, still, and will be replaced with a slightly different type as soon as it is shipped.

 

[Project25_MASTR]

I might have missed it but what actual tests were performed? All I've seen thus far checking the tuning with a R8000, an insertion loss test and some sort of dummy load test. 0.5 dB of insertion loss is very suspect as having dealt with some 2000 RFS/Celwave flatpacks over a three year span...you should be seeing something more akin of 1.5-2 dB of insertion loss. Wouldn't be the first time I've seen it and it doesn't always mean bad duplexer though.

I'd be more interested to see what the receive sensitivity is through the duplexer is, how much desense is present as RFS/Celwave flat packs rated at 50W will always begin to show signs of desense around 32 W of input power unless you are using an additional preselector. A FR4000 specifically will typically hit 12 dB SINAD around -116 dBm to -117 dBm. How much desense was present? What was the effective receive sensitivity? Did your numbers change any after eliminating the desense and testing the receive sensitivity while the repeater was enabled?

Personally, I think RFS/Celwave makes a well known product using low cost materials at a premium price post RFS acquisition due the the name and the fact it's made in the US (or at least used to be). That being said, they tend to make a fairly consistent product (which CommScope was never able to say after moving the DB line to Mexico production facilities). I remember one specific RFS failure where the duplexer looked fine on the R8000 but as soon as you applied power to it it would show issues to the point where anything more than 2W was a problem.

 

[Ubbe]

I looked at the FR4000 schematics and it has a tiny SMD two diode voltage clipping circuit at its RX input. I've never seen that in a professional repeater. They will work as a mixer when the input signal goes above 1,2V peak to peak, that's 30mW. So it will probably be very sensitive to any RF coming into the RX. Those diodes recovery time are 4nS at max that equals to 250MHz. At 400MHz they will never recover and be saturated. Diodes will work as mixers and create multiples of frequencies.

This calls for a very high quality duplexer not letting anything from its own TX pass thru and not having anything from other RF sources going thru the duplexer. I bet the repeater have excellent sensitivity at the test bench but will be extremely prone to RF issues at a site, if not installed far from any other RF sources.

/Ubbe

 

[ElroyJetson]

I can't do a thing about the cost-conscious/cheap-ass/budget-oriented approach of my customers. And yet at the same time, these modern schools built to hurricane-proof construction standards are quite often a challenge from an RF propagation perspective. You'd THINK that 40 watts would be enough power to cover a school campus that's not even a quarter mile in size, but even with the antenna located centrally in the main building, quite often there are portable radio coverage issues at the ends of the building. Less than a hundred yards away.

 

[Ubbe]

I'm sursprised by the bad specification for that Icom FR4000 repeater. It says intermod for the receiver is typical 70dB. Then I read this:
The IEC requirement for intermodulation rejection ratio is 68 dB, calculated as the ratio of the unwanted signal to the level of the wanted signal which degrades the receiver SINAD from 20 to 14 dB. It's just 2dB above the minimum value permitted.

If the receiver sensitivity are something like -120dBm 0,25uV that 70dB will be at -50dBm that equals microwatts that the receiver can handle until it's sensitivity degrades. I would say that this is not a receiver suitable to be used in a repeater configuration or on a site with other transmitters.

I would split the RX out from the duplexer to a spectrum analyzer and when that hollow sound appears to look at the spectrum where another transmitters peak appear and then use a notch filter for that. Or a narrow bandpass for RX. As it doesn't happen every time the repeater transmits it has to be another transmitter that has to be involved. It could even be a portable in the same system that are too close to the repeater when transmitting. I would remove that D1 protection diodes at the receiver input as that will only create problems.

/Ubbe

 

[Project25_MASTR]

I can't do a thing about the cost-conscious/cheap-ass/budget-oriented approach of my customers. And yet at the same time, these modern schools built to hurricane-proof construction standards are quite often a challenge from an RF propagation perspective. You'd THINK that 40 watts would be enough power to cover a school campus that's not even a quarter mile in size, but even with the antenna located centrally in the main building, quite often there are portable radio coverage issues at the ends of the building. Less than a hundred yards away.

I've found that issue to be an antenna selection issue 90% of the time. It was rare I would ever run more than 20-30W (10-15W was more typical) on school repeaters mainly due to the fact you are just uplinking with portables. Your schools are built to withstand hurricanes…mine are built to withstand tornados.

 

[fontanei]

ElroyJetson,

You might want to try a different power supply.

 

[paulears]

I've just once, had this sort of issue that in the end was not RF induced as in the usual intermod mixing but was actually RF getting into the transmit audio stage - and the FM signal being superimposed on the analogue audio producing a sort of phasey-swirly sound, a bit like the ringing in a PA system when feedback is approaching. In my example the audio link screen between the RX and TX unit had become detached and while the circuit was completed by the chassis ground, it let in the RF. re-soldering the cable fixed it.

 

[ElroyJetson]

After considerable bench testing the problem was determined to be in the repeater's receiver circuit itself, triggered under certain conditions all related to, basically, poor or non-existent filtering.
All these schools use low cost repeaters with no preselectors and cheap mobile duplexers with only so-so RF performance. The kind of equipment you'd NEVER find in a public safety grade communications system.

And so they have RF problems that could be avoided with better RF filtering.

I used to think of this kind of equipment as being normal and OK. Now that I've gained some useful experience with GTR8000s and Quantars and other more modern public safety grade equipment and their higher spec duplexers, and seen the performance and reliability differences for myself, I just can't look at a repeater without a preselector or equipped with a mobile duplexer with ANY degree of optimism regarding its peformance and resistance to interference.

But the customers are alway right? So they will spend 3000 dollars on a school repeater and call constantly about their radios problems, hoping to get it fixed for free, and after paying for a whole lot of service calls they've spent as much money as if they'd have bought a GTR8000 with a good full spec duplexer and gotten NO RF issues.

 

[Project25_MASTR]

I mean…I wouldn't buy a GTR8000 for conventional analog use as they can do some really weird things with analog. A Zetron MT-4E…I'd install all day long though.

Most of the dealers have been switching the schools over to digital in some way or another, thus the issues with hollow audio, PIM using LMR400, etc are masked so they do it because they can get away with it. You can have excellent mobile duplexer based systems but you really need go in with the understanding of how everything works together in addition to the limitations of it all. It was rare for me to put up a school system and set the power output to more than 15W (what good is it when your portables are only 5W) so you minimize the a lot of the issues.

Again, would help if dealers were hiring people who actually had the knowledge of equipment limitations…or if the guys getting ready to retire actually cared any longer.

 

[12dbsinad]

FR4000 one of the best 50w repeaters ever made for simple analog use. Have dozens that are 15+ years old and never touched, still on the air.

Of course cheap components would kill any repeater, especially at high RF sites. Sounds like nobody knew really how to troubleshoot and repair so the next best thing is to throw money at it. You'll solve it eventually.

 

[ElroyJetson]

Part of the problems I've been dealing with have been due to the fact that a single contractor installed more or less identical repeater solutions in every school when the schools are just too many and too close together, and yes, they run them at the highest power level they can get. And when I say it's too much I get backlash. Back down the power and that often solves problems instead of causing them.

I no longer work in that segment of the radio business. And I don't want to go back to it again.

 

[mmckenna]

And when I say it's too much I get backlash. Back down the power and that often solves problems instead of causing them.

I inherited a site that had 3 Bridgecom repeaters with internal mobile duplexers.
3 separate radio sites, all on their own pair, all linked over IP.
Extremely remote location. No RF interference.

I replaced the Bridgecoms and the cobbled together ham radio linking boxes with Kenwood NXR-1800's.
I tuned, tested and reused the flat pack duplexers that I pulled out of the Bridgecom's.
Works just fine at 10 watts out and users on 5 watt hand held radios.
i've talked into this system from 30 miles out.

There does seem to be that common urge to crank the knob to 11. Seems to be the CB radio mindset.

 

[KevinC]

So basically...

96 times out of 100 that sound you describe, along with the repeater hanging, is TX getting back into the RX.