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Thursday 22 May 2014

Comments on the Chinese ATU

Asked for some comments on the Chinese atu.

I get no benefit from the site or anyone else for this. The kit is about as cheap as it gets and I think you would be hard pressed to find all the bits for less than the ebay site charges.

The documents and instructions are a bit odd though and I have been asked to build one for someone and take photographs as I go. Will add the photos a bit later.

I started by checking everything is there, you can use the schematic

It used to be here:

I built the VSWR PCB first....

1. The small PCB... the switch provided does not quite fit into the PCB holes so you will have to use either small wire extensions, file the holes or file the switch contacts so that the switch fits. I used a different DPDT switch that fitted a little better.

2. The capacitor on the PCB maybe in the wrong place. It does not go across the transformer secondary.  I have two different PCB and on one it goes from the diode to ground, which is where it is supposed to. You have two ways of dealing with this, either solder the capacitor on the bottom or you could drill another hole next to the diode hole and folder one leg over and solder it in place.

3. The 1K resistor in series with the LED might not be bright enough for some, if not you can swap for say a 680 ohm resistor.

4. The T106-2 winding is probably the thing that most will have a problem with and it is worth spending time to get the taps to line up with the switch. You will need 36 turns with taps on the main inductor. This means if you start at one end you are going to be winding a 1.6 metre long length of wire initially. This works but I found it easier to start at about the mid point of the wire provided (you wind the transformer for the VSWR first) and put a twist in (use the switch as a former). This will become the 5th tap point on the switch. Then wind on turns and add taps as needed until you get to the end of the wire (I did this for the 10T tap end first). So counted 3T, tap, 2T tap, 3T tap, 2T tap, 10T and to position 1 on the switch. Then on the other side.... 3T tap, 2T tap, 4T tap, 2T tap, 2T tap, 1T and onto position 12 on the switch.
Do NOT cut off all the 10T end excess wire you will need about 2-3" left on.
Note you will want the taps to be regular spaced to line up with the switch so put them on one outside edge of the T106-2.

(see photo of mine).

5. Note the 12 way switch really is that... however you might find it doesn't seem to switch more that 5 or 6 positions. Do not panic you have not been sent a dud. Take the switch nut and grip washer off, you'll find there is a washer with a  tab that is slotted into a hole on the switch. As we want ALL 12 positions you do not want any to be used so I cut the tab off and put it back in place. You could just turn it over... and add a small slot in the switch hole to allow for it.

6. The VSWR transformer the primary is 2T and the secondary is 5T. Make sure BOTH sets of windings are passed through the same way. On the PCB the primary is the 'IN' the secondary is marked 'out'. You can mount the torroid vertical or horizontally.

7. The provided 51ohm 2W resistors are obviously bigger than the ones used originally to make the PCB so you need to bend them with one leg at 90 degrees to the body, the other you bend the leg in slightly under the body as well.

8. The LED will fit into a hole in the box so you should bend this over at 90 degrees. It will fit in a hole you cut, when the switch is put into the box (attached to the PCB) so it might need a bit of adjustment to the leads.

The instructions show fitting the capacitors and the switch first to make sure they fit.

Note drill the deeper half of the box! Drill BEFORE you add anything. Now you can use the templates provided and stick those to the box, or not choice is yours.

I drilled all the holes using a drawing pin to punch a hole in the centre of the templates held against the box (didn't use the templates).

The ebay trader provides BNC connectors. I was asked to use phono connectors.

The final wiring up is probably the most troublesome. The first kit had horrid wire so I used some 5amp blue and brown from mains flex. The second kit provided some red and yellow cable.

1. Fit one length from one of the BNC tabs all the way over to the other (pushing the cable to the side of the box.

2. Take a length of the same cable about 3" long and fit one end to the GND on the PCB.
Take two more lengths of cable (different colour is used in the pictures) one about 2" one 3" and strip and tin the ends.and fit the shorter one to the OUT on the PCB, fit the longer to the IN on the PCB.

This was how I did the next bits...

For now this is an image from the docs....

3. Take the two capacitors and fit them to the box, add knobs now or later doesn't matter. Take one long length of wire about 4" and strip about 3/4" insulation of both ends. solder the ends to the outer tabs of both capacitors with enough flexible in the middle to allow space to fit the switch and torroid (might need to offer it up to check. You want the wire to be free of the body of the switch. One way is to hot melt glue it to the box.

4. Take a 2-3" length of wire and strip and tin both ends. Solder to the central connector of the ANTENNA side and the other end to the centra lead from the left hand side capacitor.

5. Now fiddly bit.... take the nut and lock washer off the PCB mounted switch and push it in the hole, then fit it into place. Make sure the LED goes through the hole you made. (you could hot melt or glue it in place) You will then have the two BNC, two caps and the PCB in the right place .

6. Solder the central lead from the PCB you have in place (short one) to the central tab on the right hand side capacitor.

7. Solder the remaining PCB lead to the centre of the RIG side RF connector.

You might have to trim the wires to fit... the lengths I give are approximate.

8. Add a length of insulated cable to the central pin of the switch and feed it out between the switch pins and the torroid, or of through the hole. both seem to work. The other end (10T end) keep the enamelled wire lead long. Then fit the rotary switch into the central hole in the box WITH the torroid attached. Then make sure it is tight as this will be rotated.

9. The 10T end enamelled wire solder to the wire connecting both capacitors, the easiest place might be on left hand capacitor tab. Solder the lead you added to the switch central pin to the GND connection on the RIG side.

That's it the wiring is all done.

Fit the knobs and you are ready to use.

The trimmer caps on the capacitors you should set then so that are fully messed so you will have the capacitance available from them (few pF at most).

The white rubber string thing is a water seal, mine was a bit longer than needed. You fit it into the box lid, I used the end of a slot screwdriver.

Label and enjoy....

To use you switch IN the VSWR meter, adjust the C and L until the LED light dims the most or goes out. Use dit dit dit on your TX as the vswr could be way out!. Once tuned switch the VSWR out.

I think you 'could' replace the LED and 1K resistor with a tiny 50uA edge meter.

CARE: Watch out the capacitor mounting screws CAN be too long, do not put them in too far (use washers or a spacer (piece of plastic off cut from an old credit card works well)) or else it will be terminal for the capacitor.

CARE: I had someone ask me

"I am almost finished with it but cannot find anywhere where to solder the end of the enameled wire that makes the inductor.  Your instructions say the end nearest the ten windings attaches to the wire between the capacitors I believe.  But where does the other end attach to?  To ground?
I can't believe it remains unattached."

so for the benefit of all...

Look at the circuit diagram and then re-read the whole of the steps to winding the toroid .... and then think about it. Position 1 should be the 10T tap, position 12 on the switch should be ALL the windings, so the end of the wire. You spread the winding over the torroid and have the switch select different L values. If you have filled all the switch positions and have wire left you have added 1 too many tap points. Either reorganise them or use the tuner without the extra L 1 turn or whatever will give you and cut the bit of wire off at the 12t switch position (you really should reorganise, don't rewind the whole thing just desolder one of the taps move it away from the switch (use a bit of tape so it doesn't short circuit or anything and move the others along one position (near the 12th position end) then solder the wire end to the 12th position. I would make sure you have ALL the windings sorted before soldering to the switch.


G1KQH said...

W5CYF has done two excellent videos constructing this ATU:

The $20 eBay Antenna Tuner Kit Assembly and Review - Part 1 :

Part 2

73 G1KQH

Sverre Holm said...

One thing worth mentioning that happened to me, is that the two screws for mounting each of the variable capacitors were too long. The result was that they interfered with the moving plates of the capacitor and I have probably ruined both of them. There was no warning of this in the manual, so beware anyone who buys this kit

Martin said...

Hi, I was just wondering if you had any comments on how well it worked and with what antenna configurations?


M1KTA said...


The tuning is very sharp.

I've tried it on a couple of different dipoles 15m, 20m and 40m.

Verticals and long wires again it worked but not on 80m, did not seem to have enough C.

g4fre said...

I had the same experience as Sverre Holm. The capacitor mounting screws were way too long and destroyed the variable capacitors. Luckily the local emporium had replacements. The build instructions were strange in the extreme, I ended up building mine from the photos


Roy Appleton said...

It appears that the file on your box account is no longer available. Do you have another source for the schematic?

Kevin E. Ivey said...

Any idea what "QRP Manual Days Antenna Tuner Tune DIY Kits" means? What is a "Days?"

I can understand the usual bad translation and can't figure out what they are trying to say...


M1KTA said...

The circuit I have added as an image.

I have no idea what "Days" in the translation meant as far as the original author is concerned, probably meant Days. I suspect the title should be something like ... QRP Field Day Antenna tuner.... might make more sense.

Kevin E. Ivey said...

I'm pretty sure "Days" is supposed to be "DIY." The Chinese translation bot choked on the acronym...

RH said...

Mine works, but the LED gets brighter when the SWR goes low and vice versa, i.e. the wrong way around. I've checked and I have the in out connections correct to the circuit board and the secondary toroid the right way around. Not the LED itself, 'cause if it was the wrong way around it wouldn't conduct/light.

I've obviously done something wrong, which must be simple, but I'm having a senior moment and it isn't coming to me.

Hans (BX2ABT said...

The original name for this kit is quite simply:手動天調套件
The three parts/words are:
天調=antenna tuner
Problem is that when they translated it they separated 天 and 調. The character 天 means "heaven", but also "day", while 調 means "tone" but also "tune".

So, there you go. Simply a hiccup during translation. Blame it on Google. 73 de BX2ABT

Thomas said...

At current prices, the price of this tuner is close to the cost of the parts, making it a good deal. And the case is high quality.
The tuner is essentially identical with the tuner featured in the QST article:
"A QRP Console" DeMaw, QST, Sept. 1970, p. 23-25.
The SWR meter is a creation of Tayloe.
It's probably better to build this tuner according to these two sources: Same parts.
At least the builder will have good directions.

Kevin E. Ivey said...

Great reference! Thanks!

No kidding about having readable instructions!


Sverre Holm said...

I looked up the 1970 QST article by DeMaw. There are similarities: It contains two variable capacitor and a variable inductor. But the DeMaw article has them in a pi-arrangement, while the Chinese tuner has them in a T-arrangement. So they are not really similar. But were you perhaps suggesting that it would be a good idea to rearrange the Chinese tuner into a pi?


M1KTA said...

Sverre/Thomas.... Thanks both for comments. I am not suggesting any deviation from the Chinese tuner on these pages, I included the build instructions in a slightly clearer form as some were challenged with the Chinese instructions. This page is all about how to build THIS one not create your own. We all know how to obtain details from other references for Pi and T tuners using a couple of poly-variable caps and an inductor, there are probably 100's of variations in the home-brew space this page is not about that. I am sure I might have a page that talks about other tuner variations.

For those that need/want a cheap tuner this seems to fit the bill.



Alan said...

My kit came with countersunk screws for the variable capacitors so I counter-sank the holes and possibly butchered the capacitors. With the 2 sections in parallel each capacitor measures a total of 42pF, much less than I would have expected. Maybe I've been provided with fm rather than am capacitors? Has anyone else measured theirs?
ps the coils is 16.3uH end to end

Justin Goonting said...

hi everyone..i am a new builder and new to HF QRP. Ive tried to assemble the kit based on the instructions and reference to this blog and youtube. however, while installing the selector switch to its knob, the selector switch popped and a small ball bearing came off and it seems not to select anymore. I have manage to find a replacement on ebay. For the capacitors, the small nut which goes between the knob and the capacitor is too large for the screw size provided therefore i cant seem to connect the knobs to the caps. Any ideas on this? I would like to also inquire, what are the specifications of the two variable capacitors as i would want to purchase replacement. (specs and capacitance).

It would be very helpful if I can get these information from you experienced HAMs.

Thank you very much.
Justin Goonting

M1KTA said...

About the toroids as someone asked..

The schematic shows the two clearly see the main section image

The grey/shiney or black FT37-43 about 1/2" diameter is in the VSWR

The RED T106-2 about 1" diameter is in the ATU.

Unknown said...

Been using one of these that I build for backpacking etc for 2 years. Don't go over 10 watts.!

Possumn said...

I found a slight problem with the knob screws not staying secure in the variable capacitors, so I added a very small drop of blue (removable) lock-tite on the threads to secure them better.

On the capacitor measurements, I read 98pf to 170 pf from center to either outer tab, When outer tabs are joined I read 245pf to 151 PF.

I like the kit just for the parts, and found your article and notes great reading.

I modded my kit to use the SWR indicator with a 49 to 1 end fed half wave balun instead of the switched coil, and kept one cap in the box to adjust it with. And put the switched coil assembly in another box for use as a second tuner.

Possumn said...

I had a slight problem with the knob screws not being secure on the caps, so I added a very small drop of blue 9removable) lock-tite to hold them better.

On the variable caps, I read 173 PF to 97.7 PF from center to either outer tab, and when the outer tabs are joined, I read, 245 PF to 151 PF, using my Knight K-240C LCR meter.

I found your site informative and the notes great.

I like these kits and modded mine to use the box and SWR indicator assembly with a 49:1 (half wave end fed) balun and kept one cap to adjust it with.

The switch and coil assembly I put in another box with the second cap to use as a second antenna tuner.

Unknown said...

Hi Guys,
Please I 1 of the qrp tuner but the red led indicator does not show or work some where I heard that there is a mod to the board but do not know please if there is please do let me know wopuld like it to work and the daigram of it,
many thanks,

Dom M1KTA said...

Go to and read about the QRP Delight.