23 June, 2022

Reversing the inductor in the Chinese QRP antenna tuner makes all the difference

Like many I was disappointed with the Chinese antenna tuner kit "QRP manual days" back in 2014 and I never really got it to work. 

That was even after I had replaced the two polyvaricon capacitors. 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 ruined both of them. There was no warning of this in the manual, so beware anyone who buys this kit.

Despite fixing this, it didn't really tune anything. I later learned that it probably works for low bands like 80 m as it is, but I didn't notice that. I thought maybe the inductor was of inferior quality, but never really investigated it, so it sat unused on the shelf for 7-8 years.

Recently, however I learnt that there is an error in the instructions which makes all the difference: The inductor should be reversed. In this way, one can get the small inductance value consisting of only a single turn at position A of the rotary switch. Prior to this fix, the inductor starts with 10 turns at position A, and this is too much except for the lower bands like 80m. 

This is spelled out in this video by KP4MD, Carol, from 7 July 2021 starting from about 10 minutes. The particular frame that shows this is reproduced here also.


What I did was to desolder all 12 or so connections from the inductor to the rotary switch - be careful not to melt the plastic around the switch contacts. I actually did that and had to disassemble it in order to get it back in working order. Then I unwound the wire from the toroid core and rewound it in the opposite direction. And then resoldered it back.

That made the trick, now it tunes 20 m also!


2 comments:

  1. Hello Sverre, Yes, the coil must have the minimal number of turns at the TOP.
    When I designed my low weight holiday transmatch, I considered that, the inductance L increases with the square of the number of turns.
    I double the number of turns, for the next lower band.
    Doubling the number of turns increases the inductance by four times.
    The frequency is 1 divided by the square root of the inductance. Square root (1/4) = 1/2 So the frequency will be half. Hi.
    For the next position on the rotary switch the frequency will be half.
    My bands are: 100 (112) MHz, 50 (56) MHz, 28 MHz, 14 MHz, 7 MHz. The highest band (112 MHz) runs up to 145 MHz)
    Now I only use 5 position on the switch. Hi.
    I take great care on 145 MHz, with the lowest number of turns, for high voltages between the two ends of the coil.
    https://a29.veron.nl/hams/pa1b/qrp-holiday-transmatch/
    Click on the schamtic to see the number of tuns on the coil.
    73, Bert PA1B

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    Replies
    1. Hi Bert. Nice to hear about your experience, and yes, your design confirms my findings exactly!

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