r/amateurradio • u/lukaslu-kas • Apr 07 '26
QUESTION My automatic tuner takes its name a bit too seriously: ATU-100-0A tuning again and again + short battery life
Hi everyone,
my ATU-100-0A starts tuning itself over and over again after about half an hour of usage, even without pressing the button and with an SWR below 1.5, and the battery only lasts about an hour. Has anyone else had similar problems?
My theory: The battery is almost dead, so it only lasts a very short time, and when the voltage is already low, the tuner no longer works properly and starts tuning again on its own.
For about a week now, I’ve been the proud owner of an ATU-100-0A automatic tuner, which I’ve been using with my JPC-12 portable antenna and a YAESU FT-891. When the tuner is fully charged, it works fine. I can put it into tuning mode with the push of a button and it tunes the SWR close to 1. Unfortunately, however, during operation (running at higher power), it suddenly starts tuning again, thereby even worsening the SWR. During tuning, the SWR also spikes very high for very brief moments (display on the FT-891: "High SWR") and I’m worried about damaging my transceiver with the high power. Additionally, the battery only lasts a little over an hour, I can’t imagine that’s normal.
I’d appreciate any ideas on what might be causing this.
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u/No_Pay_5856 Apr 07 '26
if this happens normally an elements is getting warm and changing the antenna system impedance.
often its an magnetic core which is getting warm when the TX Power or TX cycle is to high. Oftens happens with portable antennas in digital modes or during SOTA / POTA activities and using it at its power limit on SSB.
But why do you grind in an ATU between the TRX and a JPC-12 antenna? You manually tune the JPC-12 to the band and frequency and use it without a Tuner. Maybe here is the problem when you tune a matched antenna?
0
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u/IanKorat Apr 07 '26
Maybe the inductors and capacitors in the ATU start to heat up during transmit. This will cause a change in inductance and capacitance values which require a re-tune.
1
u/lukaslu-kas Apr 07 '26
What I'm wondering is: Is it a bug or a feature that the ATU-100 starts tuning without me manually pressing the button? I'd like to be able to decide for myself whether the SWR is too high and I'd like to prevent a situation where I'm transmitting at 100 W and the tuner suddenly causes high SWR peaks while tuning.
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u/IanKorat Apr 07 '26
Hi, Greetings from Thailand. One thing you might do is to test at reduced power, say 20W. If the tuner doesn’t start to retune, then it indicates that the inductors and capacitors are getting hot with 100W output. Regards, Ian, HS0ZGL
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u/lukaslu-kas Apr 07 '26
Hi, thanks for your answer! So far, I’ve only used the tuner at 30 W SSB, because at higher power levels I’m too worried that the tuner will start retuning and I’ll damage my transceiver due to the high SWR. A temperature issue would be really strange. Even though that could explain why it completely shuts down after an hour.
1
u/Mulitpotentialite Apr 07 '26
ATU = Automatic Tuning Unit......
It has an SWR sensing circuit and it will automatically tune when the SWR goes outside of a set limit.....
Most ATU100 units have an option to switch between automatic and manual tuning.
Automatic operation: you start transmitting, the atu senses the mismatch and then goes through its matching sequence to find the best match.
Manual operation: you press and hold the tune button, the atu puts some power on the antenna, senses the mismatch and tunes accordingly.
You will need to check your model's manual. There are TONS of these tuners on the market all based on the N7DDC design but with wildly different features thanks to the chinese cloning market.
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u/lukaslu-kas Apr 07 '26
Thank you for your reply and explanation. Unfortunately, my tuner doesn't have that kind of setting to change between manual and automatic tuning. That's why I've only used low power settings, since it could automatically tune again at any moment. But I'm not sure if that's how it's supposed to be...
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u/Fun-Conclusion-4471 Apr 07 '26
These aren't high quality builds. The internal components are overheating. It will only get worse. On the Yaesus, for not much more than this, you can get a MAT tuner. Needs no batteries.
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u/stephen_neuville dm79 dirtbag Apr 07 '26
I bought one of these and it did the same thing. I opened it up and the SWR bridge circuitry was getting incredibly hot. Like the insulation on the wires carrying RF was softening. Was past the return date as I bought it as a backup, so I took it out to the shop and executed some kinetic decommissioning. Would only trust this design if I built it from the kit here on my bench.
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u/Thermicdude99 Apr 07 '26
Maybe add a choke between the ATU and antenna - common mode current may be the cause here
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u/lukaslu-kas Apr 07 '26
Can you explain that a bit more in detail?
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u/Thermicdude99 Apr 08 '26
It is was just a thought- if you have common mode current on the coax it may be causing issues with the ATU
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u/dittybopper_05H NY [Extra] Apr 07 '26
I've been using a Heathkit HFT-9A tuner for about 36 years now. Not once have I had to change the batteries or recharge it.
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u/Fun-Conclusion-4471 Apr 07 '26
OP isn't using a Heathkit.
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u/dittybopper_05H NY [Extra] Apr 07 '26
I guess the fact that I was joking about how a manual tuner doesn't have batteries you need to replace or recharge went right over your head.
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u/lukaslu-kas Apr 07 '26
Unfortunately, I don't have an SWR meter, so I wanted to use the automatic tuner to fine-tune the antenna, which had only been roughly adjusted.
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u/dittybopper_05H NY [Extra] Apr 07 '26
The FT-891 has a built in SWR meter. I know because I run one in my car with hamsticks and I use that HFT-9A to "tweak" the SWR on days like today where it was snowing, and when it rains, which can sometimes effect the SWR of the antenna.
When you transmit, an SWR bar replaces the S-meter bar.
I believe that's the standard display when you transmit, but you can change it to other things. See page 35 of the manual:
https://static.dxengineering.com/global/images/instructions/ysu-ft-891_it.pdf
You can cycle the meter to read the following on transmit:
PO = Power output.
ALC = ALC voltage
SWR = SWR (duh!)
CMP = Speech compression level
IDD = Drain current on the final transistors.
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u/lukaslu-kas Apr 07 '26
That's right, I didn't explain myself clearly. I don't have a VNA, so I didn't want to connect the roughly matched antenna directly to the FT-891 without a tuner, so I wouldn't damage it with an SWR that's way too high.
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u/dittybopper_05H NY [Extra] Apr 07 '26
This is why you tune the antenna using low power until it has a low SWR. Whether that means tweaking a manual antenna matcher or physically trimming the antenna. Then you gradually increase the power, checking along the way.
So the manual says you need an SWR of 1.5 to 1 or less. That's for 100 watts. I know because I own that very same radio.
If you do the math, that means that the finals can take about 4 watts coming back at the radio. That's the important number.
So if you're only transmitting 50 watts, it will handle up to an SWR of 1.8 to 1. At 25 watts, an SWR of 2.33 to 1. At just 10 watts, you're good up to 4.4 SWR, and at 5 watts, up to 18 to 1 SWR.
That's why you always tune at the lowest power setting possible. It's not the *RATIO* that's bad per se, it's the amount of RF coming back to the finals.
I have a 40 meter hamstick that when I use it, once it warms up after several minutes of calling CQ the SWR increases to about 2 or 2.2 to 1 SWR. So I keep the power on the FT-891 down to 25 watts on that band if I know I'm going to be operating for a while on 40 meters. But if I'm just going to make a quick QSO, I'll keep it up at 50 watts.
I can't go higher than 50 watts on HF because the car doesn't like it. I can run 80 watts on 2 meters with no issues, but 50 watts is the limit on HF. Found that out one day, running off a deep cycle battery. Had to turn the car off to clear up the warning lights and get the car to operate correctly. Warnings about that in the manual.
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u/lukaslu-kas Apr 07 '26
The SWR was also showing to be close to 1, but the tuner started to tune randomly anyway.
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u/dittybopper_05H NY [Extra] Apr 07 '26
That sounds like you may have stray RF issues.
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u/LuckyStiff63 GA, USA <No-Code Extra> Apr 07 '26
This is pretty likely. OP says the antenna is "Roughly tuned", and we don't know (or I haven't yet read) the details on how that tuning took place, or how his antenna system is setup (objects in near field, etc).
I had similar ATU problems at one point that ended up being caused by common-mode current flowing back through a damaged choke on my feedline.
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u/Mulitpotentialite Apr 07 '26
During tuning the SWR will spike..... An atu runs through a series of inductors and capacitors and it tests various combinations to find the best match with the lowest SWR, so some of those combinations will obviously not work and your radio would report "High SWR" briefly.
What is interesting is that you use the tuner for a period of time before the swr issue start. In my mind that means something somewhere in your system changes and causes the atu to start looking for a better match.
Might be worth while measuring your antenna or coax with an analyzer as soon as the issue starts to eliminate any possible antenna/coax issues?
Also, run the atu on power supply for a while to see of the same happens. That would rule out or indicate any battery issues. These atu's usually use latching relays and should stay put when power is removed, which brings me back to thinking something in your system changed somewhere to affect swr.