The noise is on the ATU power leads generated by something in the ATU. Connect the ATU power leads to the same battery as the radio and the noise is conducted into the radio through the radio’s power leads. Hence the term “conducted emissions”.
It may need nothing more than a bit of capacitance across the power rails. Try 0.1uF 50V disc ceramic across the “12V” input where the +ve attaches to the board and 0V. In fact try a 0.1 and a 100uF 25V electrolytic in parallel.
Doh! Thanks Andy - penny dropped.
I was thinking interference coming into the ATU - but this makes more sense - the ATU is putting EMI back down the leads to the supply, affecting anything else connected to the same supply.
OK, DC line filter will be added tomorrow.
Thanks & 73 Ed.
Well, the SAGA continues.
I have added an EMI suppressor board (inductors/capacitors) both in the feed to the ATU-100 and to the radio. The noise is still there.
To recheck my earlier findings, where I powered the ATU-100 off a separate battery, this time I powered the radio off a separate battery - expecting the noise to go away - it doesn’t - this tells me the ATU-100 is either radiating the interference or somehow putting it onto the coax, rather than the power lines. Both options are not an easy fix and as this unit did not previously cause this QRM, I am back to thinking it is some failing component or components inside it, that are the cause.
73 Ed.
I had a play with mine + KX2 and observed it doing the same. Then I didn’t make notes and have forgotten the exact setups. I’m out out dogging and will set it up again when I get back and say what mines does and when.
Hi Andy,
I’ve looked through some old threads and found this has all been discussed before (with me included in the thread) and I think why I now think this is a new fault, is because I am again testing into a dummy load and hence notice the interference. One of my last comments on the other thread is that when I have an antenna attached (rather than a dummy load), I can’t hear the birdies as the background band noise is higher. So it looks like this isn’t a new fault, rather one that has been with the ATU-100 from the start and is related to the LCD refresh when the unit periodically rechecks the SWR and reports it. Later firmware and the additional switch to turn off auto-tuning once an antenna is tuned, masks this problem somewhat. The problem is a pickit or similar is needed to load the later firmware and the cost of that set-up (for a single action) is more than buying a new ATU-100 with later firmware already installed and a auto/non-auto switch fitted.
73 Ed.
If anyone is interested, I built another ATU-100 for a friend.
It had to be used with an IC-705, and fit in a Windcamp carrying bag for that rig.
All connectors, controls and the display are mounted at the same side of the case, for easy access.
I made a report with many pictures on my blog:
73 - Luc, ON7DQ
Something stupid …and something good ! @HB9HRJ
Yesterday I went out for a POTA activation (no summits around in the flatlands where I live).
The goal was multi-multi : operate QRP and collect letters for the NAQCC Monthly Challenge (NAQCC December 2025 Challenge), do a valid POTA and BOTA activation, and chase some SOTA activators.
I decided to use my chinese 5.6m whip, extended with about 1.4m of wire to make it “non resonant” anywhere.
(see VERSION 2 in this blog post:
ON7DQ (KF0CR) HAM Radio Blog: Two versions of the cheap Chinese whip antenna from AliExpress )
At the base of the antenna I connected my ATU-100 (with it’s own battery, making it a remote tuner).
I checked the spots in Ham2k Polo, and saw a spot for EA7/HB9HRJ/P on 24.908 CW.
So I tuned up my antenna, simply sending a 5W carrier in FM, and SWR seemed OK.
I worked Alex and got a 559.
He was probably confused by my /P call and kept asking for my reference, so I gave him my POTA reference.
Anyway, after the QSO , I looked at my antenna … ooooh stupid me!
I had it all properly set up, but forgot one small detail : I had forgotten to extend the whip to its’ full length. So I worked Alex with an antenna only 53 cm long ! Sorry Alex.
The two 4m counterpoises on the ground may have worked along more than the short whip …
The good thing : that ATU-100 must be a very good tuner !
After that first QSO, I extended the whip to its’ full length, and I had 35 QSO’s in just over an hour, using 40/30/20/17/12/10m, all with the same antenna.
That included three USA stations on 10m CW, with only 5W.
73,
Luc ON7DQ
Tuning the proverbial wet noodle…
73 de ON4KJM / F4VXN
No I didn’t and no I wasn’t…oh wait HB9HRJ is also Alex. 
I admit I didn’t write your POTA reference down as it goes off your logs anyway.
I had a lot of QRM from various EU/RU fading in and out on the same frequency.
Thanks for the chase!
Well on 30m CW you were a huge signal with your 5 watts Luc. Excellent on 40m SSB too when I heard you work Alex GM5ALX/P.
Sometimes I wonder whether it is easier to chase stations using an ATU and a piece of wire than a resonant antenna, especially when multiple trips from the operating position are required to adjust links. Activating is different of course as it is one band followed by another and not jumping about between bands.
