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Author Topic: Antenna Tuner Position Indicator  (Read 14003 times)

Posts: 19

« on: January 30, 2016, 08:27:38 AM »

I have a designed a remotely-mounted double L-network tuner for an open-wire feedline.  There will be two ganged roller inductors and a vacuum variable capacitor, turned by stepper motors.  Since each of these are multi-turn components, I need some sort of position indicator to tell me their angular position as well as how many rotations have taken place.  It would be helpful if the indicator would save the position and turn count when power is removed.

Does anyone know of a solution?  My knowledge of microcontrollers is zero, so the most basic solution would be helpful.

Posts: 631


« Reply #1 on: February 07, 2016, 07:54:18 PM »

I'm building the same system for myself.
I am using two Collins 180L inductor units, and a motorized vacuum cap
Positioning will be displayed by to small machine shop DRO units, the digital encoders are coupled to a shaft the rotates once per 25 of the inductor.
These are rotational angle encoders with a resolution of 0.1 degree. Zero can be set anywhere I want is, but I'll set it just above the end limit of the inductors.
I use 13.8vdc from my standard radio psu, and run it into a PWM DC motor driver module so I have variable speed with good high torque even at slow rpm.
I bought the displays from the Allendale group, in the UK. $60 each shipped. They run on a single coin battery in each unit.
Found them on eBay. Found most of my other parts there too, including the PWM module.

Posts: 19

« Reply #2 on: February 08, 2016, 03:24:53 AM »

Thanks for the reply - this is very helpful.  I'll check these out on eBay.   73 ...

Posts: 105

« Reply #3 on: August 31, 2016, 07:27:44 AM »

I'm an analog guy, so my first reaction is to use a multi-turn pot in a resistor divider and an A/D converter reading the position.  You can get them in 3, 5, and 10 turns with 1/4" and 6mm shafts.

Posts: 5

« Reply #4 on: February 08, 2017, 08:42:52 AM »

Hi Joe,

By now you may have found a solution to your problem, but I think, it might be useful for others, what I would like to share here.

I built a magnetic loop antenna using one of those large vacuum capacitors tuned with a stepper motor. I ran into the same problem as you, how does the system "know", where the stepper motor was left off the last time before power was turned off.

I decided to solve this by saving the current stepper position in the EEPROM, as I tell the Arduino to go through the shut-down process. Well, so much so good, but what if it is turned off by pulling the plug. I found a solution to this as follows:

I use a buck converter to power the Arduino. On its input, I have 19.5V, which is also powering the stepper driver, but on its output, I obtain 9V for the Arduino. I put a small circuit between the buck converter's output and the micro-controller: a 10 Ohm/2W resistor and a rectifier diode shunted with a 50,000uF capacitor. The resistor is needed to protect the buck converter from the large inrush current. The diode is making sure that current is not going to flow back to the buck converter when it is off. The capacitor holds the voltage for long enough (1-2 secs) for the Arduino to save the current stepper motor position.

One more thing, the Arduino is monitoring the voltage at the input of the buck converter, and whenever it drops from 19.5V to 15V it triggers an orderly shut down, saving among others the position of the stepper motor.

On start up, the first thing it does is, it reads the EEPROM to find the stepper motor's last position. Of course, I assume that there is no loss of steps over time and calibration is only needed very infrequently. I am not using a closed loop system with an encoder to have positional feedback.

I hope it helps someone. Wish you all happy home-brewing.

Best regards and 73,
Steve, n9eu

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