Fixing a Chinese "QRP Manual Days" Antenna Tuner Kit 

I am glad to have assisted! :)

Thank you!

You're welcome

Thanks for your solution!

Your suggestion makes sense.

I'm personalizing my kit now and I just thought the same. I use an aluminum case 10x10x5 cm (4x4x2 inches) with an additional switch for a general bypass and five others to switch between T/L/Π network

Я рад, что помог тебе!

Yes, the corrected T-match does tune high-impedance and end fed antennas.

Yes, I have seen the LED light with as little as 0.1 watt.

Hi Rob. No need to rewind the toroid. Just connect the end with one turn to the capacitors. Then either position "A" or "L" would be minimum inductance depending on how you wound it. My position "A" corresponds to one turn, or minimum inductance. My schematic diagram shows the far end of the coil connected to ground. T-network tuners with air core inductors usually wire the switch that way to short the unused coil turns and avoid parasitic resonances in the free end. There is some evidence that shorting the unused turns of toroid core inductors may cause core heating and power loss. I plan to check this in my tuner by comparing its performance with and without that ground connection to the inductor. Lifting that connection off ground is a trivial matter.

@@kp4md Thanks. I've soldered it up now. I just found out the LED and BNC nuts are missing from the kit, so i have to go off and source those. Another question; I have a 40M QRP transceiver. Once i have this built can i simply attach a length of wire for 40M band, like for EFHW 84' long end fed and a 17' long counterpoise, to a BNC plug and use this tuner to tune it (no unun or anything else needed)? If that's the case maybe i can replace the antenna BNC with another style that allows me to screw down wire.

If the coil end at switch position "L" is not connected to ground as in the original Chinese circuit diagram, the unused turns of the T network coil behave as an autotransformer. When the rotary switch selects few coil turns, a high voltage is induced at the free unused end of the coil. To test this effect, I temporarily connected a toggle switch between the unused end of the toroid coil and ground to compare the tuner insertion loss with it either grounded or floating. My observed results shows that tuner functions as desired over the 2-28 MHz frequency range when the lose end of the coil is grounded. When the unused end of the coil is disconnected from ground (floating), the tuner exhibits increasing insertion loss and fails to achieve an impedance match at frequencies above 14 MHz. For this reason, I connect the "bottom" of the T network inductor to ground to short out its unused turns.

@@Rob-ml7ps Yes, you can use the tuner that way and use any type connector on its output.

George, you correctly noticed that change. Here is the reason: If the loose end of coil end at switch position "L" is not connected to ground as in the original Chinese circuit diagram, the unused turns of the T network coil behave as an autotransformer. When the rotary switch selects few coil turns, a high voltage is induced at the free unconnected end of the coil. To test this effect, I temporarily inserted a toggle switch between ground and the "bottom" end of the toroid coil at the "L" position switch contact 12 to compare the tuner insertion loss with it either grounded or floating above ground. The graph comparing the observed results shows that tuner functions as desired over the 2-28 MHz frequency range only when the loose end of the coil is grounded. When the unused end of the coil is disconnected from ground (floating), the tuner exhibits excessive insertion loss (>10 dB) and fails to achieve an impedance match at frequencies above 14 MHz. For this reason, I short the unused turns by connecting the "bottom" of the T network inductor to ground. My data is posted at photos.app.goo.gl/zY3cQaQDLFs2QU7U8 I would appreciate comparing with your findings. 73, Carol

I just asked the same question. Is the end, ground? Did you figure this out?

Having both the bottom end of the coil and the center contact of the switch to ground seems strange: you basically create a transformer with the secondary winding short circuited. The 4STUNER schematic reverses the connections, but also leaves one end of the coil open. I'm also not sure about if A having only one winding from signal to ground is any useful. 10 windings as minimum may be too much, only one seems very little even at 10 meter frequency. My gut feeling says: start with 3 or 4, and step up 2 windings per step until about D, make steps of 3 windings until H and increase the steps to 4 windings. This brings the total number of windings to 38, about the same as the original, but in my opinion with a more sensible distribution. (adding 4 windings to a coil of 20 or 2 windings to a coil of 6) update: I ended up to use 2, 2, 2, 2, 3, 3, 3, 3, 4, 4, 4, 4 windings (36 in total) windings and the first results look promising. More after I finish building my (tr)usdx.

I did ground the tail end. Mine works well. If you look at what the multi position switch does it makes sense. It keeps as someone else pointed out parasitic problems at bay. You can wind it anyway you like that's the stuff hams do. I just needed it to work and it does quite well. The center knob was a challenge but I got it

If you carefully watch the KP4MD Video you will discover that the diagram is backwards and the turn closest to the tee is somewhere around ten turns! That is wrong! It is something like one turn and then so on two ect. After the last turn of the ten turns it goes to ground. Each click gives you more and more inductance. If I had blindly followed the instructions it would not have worked and also the primary and secondary windings are backwards printed on the board. Many folks did not end up with a functioning LED if they followed the instructions that came with it. Again thanks KP4MD I think there were others who caught the LED mistake. I watched a number of videos before I built mine. I have been tempted to re-do the torroid again as I found a nice open non plastic rotary switch that won't melt when you solder it, also at one point I found brass extensions for the rotary capacitors that were made for just such a problem as the knobs. Mine cracked when I tightened them.

@@andyleatherbarrow7322 Yes, the tuner exhibits high losses above 14 MHz with the end floating off ground. See photos.app.goo.gl/zY3cQaQDLFs2QU7U8

Hi, George. There are a total of 36 turns on the T106-2 core. The numbers on the switch diagram are the number of turns between adjacent taps. The one turn at the top of the coil is between the first switch contact and the junction of the two variable capacitors.

@@kp4md Thank you Can I ask a favor? Could you re draw for me showing the turns and where the taps are? as the drawing that came with this is very confusing? Thanks George

@@georgealbertacanadaeh1038 See the photos on arkadiam.github.io/transmissionlines/atu/qrp-antenna-tuner/

@@georgealbertacanadaeh1038 Scrape the enamel insulation off the ends of the wire loops in order to solder the taps to the switch contacts. Use the minimum possible soldering time to avoid melting the plastic switch body.

@andy leatherbarrow My schematic diagram shows the bottom end of the coil connected to ground. T-network tuners with air core inductors often short the unused coil turns that way to avoid undesired parasitic resonances in the free end of the coil. Some have reported that shorting the unused turns in this way may cause heating and power loss in toroid cores. When I disconnected the unused end of the coil from ground (floating), my tuner exhibited excessive insertion loss and failed to achieve an impedance match at frequencies above 14 MHz. See photos.app.goo.gl/zY3cQaQDLFs2QU7U8

@@kp4md Thank you so much. I wound the toroid yesterday. My kit came with no documentation whatsoever, so your vid has been invaluable. 73 G8ZZF

Ignacio, the letters "A" through "L" in order represent the increasing or decreasing values of inductance, depending on how you wire the switch. The letters help you to reset the inductor for a particular frequency. I posted the calculated inductance values in my photo album at photos.app.goo.gl/zY3cQaQDLFs2QU7U8