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Tunable Vertical

This page will discuss the construction of a tunable fixed vertical antenna using an adapted "Screwdriver" as the tunable element and I make the usual promises to keep this page up to date.

The content of this page is presented as is, and the author makes no claim to the fitness of this design for any particular purpose, you use this information at your own risk.

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27 Feb 2010 - Introduction I want to put up a vertical antenna.

Weekend of 27-Feb-2010.

I want to put up a vertical

I want to put up a vertical antenna but I would like to have it remotely tunable with the tuning done at the antenna base rather than at an antenna tuner in the shack and to achieve this I am going to have to build an adjustable coil so I did some surfing about the internet and decided I would make the adjustable component of the Screwdriver Antenna and use that tunable element.

Existing components

This isn't the first time I have played around with this. Myself and Brian Smith VK3NBS did some work last year producing the inner tube and coil.

Existing components from last experiment with VK3NBS.
This image shows the inner conductive tube, the movable coil and the cheap 12Volt drill that will be used as the drive for the tuning coil.

The Drive

What I had trouble deciding on was the drive and after messing around with stepper motors, then a high torque DC motor was the drive for this tuner. Thinking about how much control I want versus the cost of achieving that control. Ideally you want a high torque drive with low momentum because a drive you can stop accurately is the key to repeatability.

However this weekend I went to my local hardware store and purchased the cheap 12Volt drill and decided to used the drive out of that.

This is not ideal but I will use:

Optical shaft encoder @ 4 or 8 ppRPM on the screw.
PWM motor drive.
Possibly a breaking resistor.

Lets pull the drill apart.

This is the drive train from the drill, it has a clutch attachment on the front of it which will be removed later.

The drill was quite easy to pull apart, no machine screws, the drill was a 12Volt "Ozito" Bunnings Special I purchased new for AUD$30.00.

The clutch comes apart easily enough, you can take the two screws from the font plate and it will fly apart on its own accord dropping steel balls as it goes.

These drills have a clutch which appears to be implemented by using steel balls to lock a ring gear
within the transmission. As the ring gear tries to turn under load it lifts the balls out of the race while a spring tried to keep the balls in.

The clutch in Figure 2 was disabled using two screws from a computer fan mount, they fitted perfectly.

After the drill was pulled apart, I needed to make two sleeves so that the drive could be mounted within some 50mm HD polly tubing.

Drive mounted in two wooden sleeves. The sleeves were made with a hole saw out of 12mm plywood.

Bottom End Stop

As we have no clutch, we need to have a positive end stop, the difficulty was in finding a switch that could cope with the potential thrust of the moveable coil.

This is the end stop, conduction between the two springs by a conductive plate on
the bottom of the coil will be used to stop the motor.

After some head scratching the solution was quite simple, basically make the bottom of the coil conductive and then use two springs as contractors.

How it works

End stop schematic, shows how a relay is used to open circuit if end stop is reached.

If current is flowing from FwdRev (+) to FwdRev (-) then it is conducted by the relay and blocked by the Diode (D). If the Switch (SW) is contacted then the relay switches the circuit open. At this point the end stop has disengaged drive, now if the current is reversed the Diode (D) conducts allowing the motor to drive the other direction and moving the coil away from the end stop.

While it looks clunky its reliable and hard to fool!

Thats it for this Saturday..