Remote control 'L match' Aerial Tuning Unit.

Although I used an SGC230 auto ATU very successfully to tune my 'inverted L' HF antenna it went against the grain to use a microprocessor controlled ATU when operating old valve radios.

mounting on post

The SGC230 is a commercial unit but I wanted to use a homebrew ATU. If I could persuade one of my homebrew transmitters and receivers to work at the same time this would provide a completely homebrew 'station', something I regard as an amateur radio ideal.

I decided I had to make my own ATU and wanted to locate it remotely in the garden (like the SGC230 had been) allowing it to tune the inverted L right at the feed point. A tunable and multiband remote ATU either requires a microprocessor system to control it (as with the SGC230) or a simpler system, some remote control arrangement with a control box in my 'shack'.

The SGC is a 150 watt capable tuner but I also wanted to make an ATU that would permit me to test and use a couple of recently overhauled linear amplifiers I have. Each one is easily capable of producing the 400w pep legal limit power for UK radio amateurs.

A good friend of mine provided two very similar small geared motors operating from around 12V DC. I also found a large 25 uH 'roller coaster' inductor and a 500 pF variable capacitor with moderately wide plate spacing that looked up to the job.

A low pass 'L match ATU' uses a series inductor element and a capacitor in parallel with either the input or the output port (depending on what the load to be matched). The L match configuration was perfect for my application as I had two motors and one L and one C. The more flexible T match configuration would require a second capacitor and a third motor to implement.

Unlike the T match, an L match will only provide a perfect match with one unique value of L and C. This requires the L to (ideally) be a continuously variable item and not a switched unit as is sometimes used on T match ATUs.

One of the geared motors turns the roller coaster inductor, the other a variable capacitor.Both motors run at approximately 60 RPM but the motor driving the capacitor is further geared down to slow the tuning. Two 6:1 'slow motion' ball drive reduction units are cascaded to give a 36:1 reduction over all. With the extra gearing on the capacitor both motors take approximately 20-30 seconds for a complete movement.

close up view

In the interest of simplicity (i.e. actually getting this project finished :) there are no remote position sensors in the unit, tuning is by watching the SWR rise and fall in the shack...  When the roller coaster inductor reaches one of it's end stops the current in the motor increases so the operator can tell that the limit has been reached if a metered PSU is used.

The capacitor will rotate through a full 360o and so has no remotely discernable maximum and minimum positions. Apart from the maximum and minimum capacitance any other value will be found in two positions of the rotor which takes a bit of getting used to when tuning!

Control of the ATU is via a 4 button control box located in the shack. Two buttons control the inductor position, these are up and down. The capacitor controls simply turn the vanes one way of the other. The capacitance may increase or decrease (depending on the position of the vanes) for a given direction.

The unit will match my 84ft (approx 26m) inverted L wire on the 160-20m (1.8-14 MHz) bands without reconfiguration. Two match above 20m it is likely the capacitor will have to be moved from the antenna side of the inductor to the transmitter side of the inductor. To date this has not been tried as the primary bands of interest are all below 20m.

I have made provision for reconfiguration by using a 4mm banana plug and two sockets in the unit although this can not be done remotely. Use of a relay at this point would require a high voltage unit (probably a vacuum relay which I do not have) if more than say 50 watts is used because the high impedance of the aerial on some frequencies will result in a very high RF voltage across the inductor.

The components were assembled on a piece of 6mm Perspex (acrylic sheet) which is a good insulator. Once the L and C had been mounted and wired, mounting brackets were made up for the two motors and also the reduction drives on the capacitor input shaft.

what it looks like in the garden

The assembled sheet was then bolted onto a wooden frame and 'weather proofed' by being covered by an upturned plastic kitchen rubbish bin. This keeps the rain off in the summer and (so far) has not proved problemtic due to condensation in the wet British winters.

Six core telephone cable is used to link the motors two the control box two of the cores being paralleled with the two cores used to drive the motor controlling the L as this takes much more current than the motor driving the C.  The small control box has two relays and a combination of diodes and push buttons to allow the voltage to each motor to be off or driven either way.

To date the unit has performed well and has shown no signs of distress when driven with 400w pep of RF. Any arcing over will manifest itself as a sudden increase in reflected power visible on the SWR meter as well as possible damage in the unit...

Tuning the unit when moving up and down a single band is easy, tuning from one band to another requires patience and a memory of where you left the L and the C last time you used the ATU :)

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