A 100w N7DDC auto-ATU for £33? It must be useless!! (No, it isn’t)

I was browsing AliExpress, as you do, and spotted a fully built N7DDC open-source design Chinese auto-atu for only £32.53 including shipping! There had been an article in PW that suggested they were OK, although I knew Chris G5CTH said he’d built one from a kit which didn’t work well. The spec of this one looked ok though. It worked from 1.8 – 55 MHz, was in a metal case, had a 4-row OLED display and a (probably optimistic) 100w power rating. It was also the 7 x 7 Extended version, which meant it had 7 inductors and 7 capacitors, rather than 5 of each in the ‘cheaper’ 5 x 5 version. This means it can tune a wider range of antenna impedances. Even better it had a built-in 1500mAh lithium battery! It didn’t have as many buttons as some versions but at the price it seemed worth a punt.

N7DDC auto-atu on top of an LDG AT-100 Pro II

As you can see, this version, branded SZ-PLZ Co, only has two controls. A red power switch and a big blue Tune button. It turned out you didn’t need to press the tune button to initiate an auto-tune. Just hit transmit and if the SWR is worse than about 2:1 it will go into an auto-tuning cycle. You can press the tune button if you want to force a retune, for example to see if a better match can be found.

The OLED display shows RF power in, SWR and the capacitance and inductance values used for the match. When in TX the bottom two rows show the power being delivered to the antenna and percentage efficiency.

The case seems to be aluminium with a PCB front panel and metal rear panel carrying the SO-239 connectors.

Internally the build quality of the atu itself looked fine. Only 6 inductors are visible but there are 7 – the smallest is under the coax connector & shiny black coax cable at bottom left. There is a multiway connector to the PCB front panel, which carries the display, switches, charge LED and USB-C charger socket.

The battery arrangement looked a bit more Heath-Robinson as the battery pack is held onto the lid with what looks and feels like silicone sealant! It is connected to a charger PCB, also held by the same material. The battery pack itself seems to have some (protection?) circuitry built-in. Unfortunately something seems to take a little power from the battery, so it runs itself flat if the atu isn’t used for a while. The power switch seems to stop power going from the charger PCB to the main board, so the charge circuitry is always connected. Whatever the reason, it is far from ideal and the biggest drawback of the atu. Presumably why it only cost £33 inc shipping.

N7DDC 7×7 atu PCB
N7DDC atu battery & charger PCB held by sealant

Colin, G8RLZ, had been kind enough to loan me his LDG AT-100 Pro II auto-atu and I ran some comparison tests with the N7DDC after it arrived. The LDG is undoubtedly a better atu – it has a far better range of control and also has latching relays, so takes little to no power except while tuning. It also tunes from a very low power level. This N7DDC atu isn’t really configurable and needed the rig set to 15W output power to be certain of getting it to tune properly.

Here is how the two atu’s compared when connected to my 15m long doublet, which is fed by twin feeder connected to a 4:1 balun. There is a coax run of about 20m to the shack, which is where the ATUs were connected up. The loss on the coax will have attenuated the reflected signal, making the antenna easier to match.

SZ-PLZ N7DDC AT-100 Extended 7×7 SWR
12m1:13:1 then 1.5:1
160mNo matchNo match*

*At a later date it achieved a match of 1.74:1 on 160m

As you can see the performance was similar. The Chinese atu was a bit less reliable in finding a match and needed far more power to do so. In theory it only needed 5W to tune but I found my TS-590 had to be set to 15W to find a reliable tune on all bands. This wasn’t true of the LDG, which would happily tune with less than 5W from my Hermes-Lite 2 QRP SDR rig.

Both atu’s would tune 6m well but it was a bit pointless as there was so much cable and balun loss that little RF seemed to escape the antenna! The N7DDC atu produced a match with an SWR of 1.37:1.

All in all I think I got a good buy, despite the battery issue. I suspect I could fix that with a switch to disconnect the battery. The ATU works and I bought a rainproof box so it can be used in the right place, next to the antenna where there is little to no additional feeder loss when the antenna SWR is poor. Have I run 100W through it? No, but I have put 30W of FT8 through it and it has survived, with no appreciable heat generated even on bands with an iffy match.

Mike, G4RAA

Update 12 Mar 2024, correcting location of smallest coil, added detail on PCB front panel & the fact that the rear panel is metal, not PCB as originally thought.

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