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Retevis RT97 Heat Dissipation


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RT97 “Duty Cycle”

 

I had a replacement RT97 come in today as a replacement for one that failed. The prior failed as it would not longer talk to a computer for programming. The TX/RX worked fine. I decided to test this new units “duty cycle” and heat displacement abilities.

 

The first thing I did was open it up and see what it has internally to bridge the transmitter to the aluminum outer housing. The bottom of the RX/TX unit has large fins cast into its aluminum body. It appears to be bedded in thermal paste to make a conductive path the aluminum outer housing.

 

I took a food thermometer and put it into the thermal paste. The unit was at 70 degrees.

 

The “Test”:

 

  • Stage One: I had it transmitting on high power. The transmission was broken up into three 1 minute sections with 10 seconds between each followed by a whole 2 minutes of TX. The transmitter rose from 70 degrees to 82 degrees. Hardly warm to the touch. (12 degree rise for 5 min Total TX w/ 40 seconds rest)

 

  • Stage Two: I gave it about 2 minutes of rest and hit it with two more sessions of 2 minutes transmissions, separated by 20 seconds. It had fallen to just below 80 prior to and rose to 89 after.(9 degree rise for 4 min Total TX, w/ 2 min 20 seconds rest)

 

  • Stage Three: I finally let it sit for 1 minute and did five more sessions of 2 minutes transmissions, separated by 20 seconds. The temp started at 88 and rose to 102 (14 degree rise for 10 Min Total TX w/ 2 min 20 seconds rest)

 

End total of 19 min of TX w/ 3 min 20 seconds of rest. The temp rose from 70 degrees to 102 degrees.

 

What I find interesting is that after the radio “warmed up” it took significantly more time to heat up further. I expected the 10 min spent TX'ing in Stage 3 to raise it more than it did. At the end after only about 60 seconds of rest it already had dropped to 94 degrees from a high of 102. I did not repeat this test on low power but I can only assume it would take longer to heat up.

 

Being as my use with these repeaters are outdoors here in Alaska it doesn’t appear that heat will be an issue for me. During the winter our avg daily temp is around 20 degrees and we only get to an avg daily temp of 60-65 in the summer. This leaves a lot of head room. The cooler ambient temperatures should further increase the rate of heat dissipation as my home was 70 degrees to start with. Anyone see any flaws with my logic?

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Thanks for this. I couldn't find a duty cycle rating anywhere for this unit. This gives an idea. 

  I was considering one of these, for family and friends, to cover my small in Northern Vermont as gmrs repeater coverage is almost non-existent here. The nearest repeater to me is 14 miles away and can chat on it from my mobile in my car, at home, I have a j-pole I'm installing in my attic for a base antenna which should help. Other than this repeater, nothing anywhere nearby. 

  The 5 watt output of the rt97 should handle all of downtown and then some if I decide to pick one up. 

  

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I applaud your effort in making the measurements. Isn’t experimentation fun?

The duty cycle rating of a piece of equipment is going to vary based on some given set of ambient conditions and target performance and longevity criteria.

Looking at the RT97 specs, the manufacture rates ambient operating temperature range of -30 to 60C (-22 to 140F). Theoretically the product should perform within spec when operated within this temperature range. However, the manufacturer does not specify how long the product can sustain operation at the higher temperatures. It could be 30 seconds, minutes, hours, days, months or years. And they do not spec longevity at any other temperature either.

What we all know is that one of electronics worst enemy is heat, but we also know that all electronics produce heat and operate at temperatures greater than ambient. It is the rating of the individual components that ultimate determine the capability of the product. Most all common electronics work incredibly well and live incredibly long lives when operating temperatures are held near room temperature (20C, 68F). But is not uncommon for component operating temperatures to reach 70C (158F) and selectively well beyond in some cases.

In the case of a repeater, the duty cycle of the product is ultimately the percentage of time it can transmit continuously at some given power level without exceeding some target operating temperature and performance variance. In a more practical sense, the duty cycle is the percentage of time it can transmit continuously at some given power level and do so reliably for years and years without heat-induced failure.

Absent detailed knowledge of all the components used in the design all we can do is speculate and/or measure longevity ourselves.

If I were doing an at-home duty cycle experiment myself, I believe I would start by determining what percentage of time I could transmit and limit operating temperature of the output components at the heat-sink connection to a rise of no more 20F. 20F being selected arbitrarily because it is a modest amount, and 20F above 68F is a temperature that electronics can run at reliably for decades. Beyond that, further knowledge of the specific components would be required.

I hope these additional insights are helpful.

Regards,


Michael
WRHS965
KE8PLM

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  • 2 weeks later...

You can buy "tell tale" temperature stickers that you install inside electronic equipment to determine the temperature extremes. While they don't tell you how long or whether internally or externally generated, they might be useful to determine if a failure was temperature related.

Commercial repeaters are rated at 100% transmit duty cycle. In Public Safety use that is a norm.. for trunking systems, it is also a norm. Ham radio repeaters are frequently subjected to 100% duty cycle for a half hour or more.

I would repeat the test over 100% key down and monitor the temperature rise vs time. Also monitor transmit power and current draw.

Sent from my SM-T350 using Tapatalk

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Is it bad that I'm looking at those internal pictures and thinking heyyy, it looks kind of doable to bypass that duplexer?

Add one of the higher rated (50w or so) mobile units, and a small linear amp in the transmit path...Depending on the amp, low power on the repeater should be able to keep it under 50 watts.

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Yes, it is bad.  You'll probably create massive desense in the receiver, and do nothing to create robust repeater coverage. 

Want to improve repeater coverage? Improve receiver performance. 9 times out of 10, that's where the system is lacking. 

Horsepower makes all the headlines, but there's more to the story when it comes to real world performance.

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Most common issue with repeaters is too much power and desense. In the LMR world we installed alot of 15-20 watt repeaters that covered alot of the county. LOS is king. If your entire fleet is 3-4 watt portables there is absolutely no reason to install a 50 watt repeater. We spent lots of time balancing the systems we installed to be close to the subscriber. It eliminates false expectations and user interactions. A decent antenna will make all the difference. Sorry but an Ed Fong and a DB404 is night and day. a DB404 with a RT97 on a good site with proper RF line would be a great combination. 

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1 hour ago, Radioguy7268 said:

Yes, it is bad.  You'll probably create massive desense in the receiver, and do nothing to create robust repeater coverage.

 

1 hour ago, kb2ztx said:

If your entire fleet is 3-4 watt portables there is absolutely no reason to install a 50 watt repeater. We spent lots of time balancing the systems we installed to be close to the subscriber. It eliminates false expectations and user interactions. 

Thanks for the reality check. My urge to tinker doesn't always pay off with good results in reality.

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4 hours ago, kb2ztx said:

Most common issue with repeaters is too much power and desense.  

It's worth repeating.

If you're running a compact "flat pack" duplexer - and you're pumping out over 20 watts, there's a real good chance that your system's measured desense is 1.5 to 2 dB.  Seems like a small amount, right?  However, if you can improve your receiver performance by 3dB -you have effectively doubled the area of coverage that your portables can talk back in from.

One of my first Ah-Ha! moments in 2-way radio was when an old timer took me out to a tower site where a customer on the repeater system was complaining about poor coverage with their hand-held portables. When we got on site, he spent a bunch of time setting up his test equipment and taking measurements. He already knew what he was going to do, but he took the time to educate me. He showed me the performance of the system as it was and showed me the measured desense. He let me listen in to "weak" portables that were operating on the edge of the system.

The customer had been sold a brand new high power 100 watt "high performance" repeater, but was running it all through an existing flat pack 6 cavity notch duplexer. They spent their money on more POWER, not on the duplexer - they already had one of those.  Long story short, after showing me the system performance as it was - he turned the power down to 10 watts. Suddenly, the units "on the edge" were coming in much more clearly, and users that had not been able to get in at all were now using the system, but still were scratchy.  He then added a tuned bandpass cavity between the duplexer and the repeater's receiver, and WOW, now they ALL sounded good. Then he went back and showed me what zero desense looked like. Couldn't even see a difference in the noise floor when the transmitter was keyed up locally. Now the repeater had good ears - an ability to listen that matched it's ability to talk out. Balance.

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It's worth repeating.
If you're running a compact "flat pack" duplexer - and you're pumping out over 20 watts, there's a real good chance that your system's measured desense is 1.5 to 2 dB.  Seems like a small amount, right?  However, if you can improve your receiver performance by 3dB -you have effectively doubled the area of coverage that your portables can talk back in from.
One of my first Ah-Ha! moments in 2-way radio was when an old timer took me out to a tower site where a customer on the repeater system was complaining about poor coverage with their hand-held portables. When we got on site, he spent a bunch of time setting up his test equipment and taking measurements. He already knew what he was going to do, but he took the time to educate me. He showed me the performance of the system as it was and showed me the measured desense. He let me listen in to "weak" portables that were operating on the edge of the system.
The customer had been sold a brand new high power 100 watt "high performance" repeater, but was running it all through an existing flat pack 6 cavity notch duplexer. They spent their money on more POWER, not on the duplexer - they already had one of those.  Long story short, after showing me the system performance as it was - he turned the power down to 10 watts. Suddenly, the units "on the edge" were coming in much more clearly, and users that had not been able to get in at all were now using the system, but still were scratchy.  He then added a tuned bandpass cavity between the duplexer and the repeater's receiver, and WOW, now they ALL sounded good. Then he went back and showed me what zero desense looked like. Couldn't even see a difference in the noise floor when the transmitter was keyed up locally. Now the repeater had good ears - an ability to listen that matched it's ability to talk out. Balance.

That really is a great story. Thanks for sharing that.

Around me there is one particular amateur repeater with great Tx coverage, but it has been shared many times that it’s ears are bigger than its mouth. There are times it seems it should not by all rights pick up some distant HTs, but it does so with seemly no effort. I understand the setup has been online for decades, its filters are first rate and it is meticulously maintained.

Regards,


Michael
WRHS965
KE8PLM
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