Last year I used a magnetic loop antenna for QRP /P. It worked very well but the major downside was the tuning. Standing next to the loop and tuning manually was not much fun as the human body detunes a loop. So the method had to be tune, stand back, look at antenna meter, tune again, stand back etc etc. It also meant that a QSY was very tedious.
This year I want a remote tuning system. I tried the suggestion by YO3GGX. It uses an NE555 and the motor from a radio control servo. The electronics, position pot and 180 degree end stop are stripped from the servo to allow for continuous rotation. I tried the circuit and it worked but the motor could not provide enough toque at a speed low enough for accurate tuning. Mag loops have a very high Q so the tuning is sharp.
The next idea was to try a servo test circuit that also uses a 555 but with a standard 3 wire connection to the servo. Rotation is limited to 180 degrees. This looks like it just what I need. The servo turns the capacitor easily and tuning is by turning the knob on a pot, the servo follows the movements. Schematic is here
The only issues to sort out are coupling the servo to the tuning cap and the resolution of the system i.e. are the tuning steps small enough to be able to get perfect tuning of the loop.
A word of warning about buying cheap servos. I found 4 units on ebay which were supposed to be Futaba s3003. They were about 25% of the UK price. They are NOT made by Futaba and are cheap Chinese copies. I have since ordered a real Futaba S3003! You get what you pay for.
Update 27 Feb 2013
The new servo tune box works very well. The loop will tune on 20, 17, 15 and 12m. The tuning is very sharp but I think the use of a 10 turn pot and turns counting dial will help sort that out and give tuning points for each band which should make band changing quicker. With the loop on a first floor landing inside the house, which has walls 1m thick, I heard YT and LZ stations both at 599+ Next will be a trial outside where the loop can be easily compared to the long wire.
Update 01Mar 2013
The genuine Futaba servo arrived and a new tune box was made today. The performance is better, much quieter and finer steps when tuning. Initial tests with a new FG-01 anatenna analyser shows that the box will tune 20m-10m. The SWR is slightly high on 20m but I think that can be cured by adjusting the coupling loop. Now for some test with the loop out in the clear, just need a fine day with some sunshine.
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Update 10 Mar 2013
The control box was completed today and tested with the loop inside the house. The walls are about 1M thick and the house is built into a bank. Quick comparisons with the inverted L showed signals were about 6-8db down which is ~ 1 S point. The weather is bad today with more snow and a windchill of -6C so no chance of an outside test. Forecast for Monday is windchill of -10 to -12C so more testing will have to wait for a few days!
The tuning is much better with the 10T pot and a decent servo but can still be tricky especially on 20m. The next step (groan) is to play with an Arduino controlled stepper motor. The motor board is built but still waiting for the Arduino. The idea is to develop a tune box that has push buttons for forward/reverse and a speed control. Much more complex than using a servo but it should give really fine control of the tuning. Watch this space.
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