This video shows a magnetic loop I've built which uses a stepper controlled Jennings vacuum variable capacitor along with a Teensy++ controller to automate the tuning and measure SWR plus forward and reverse powers.
The controller is based on a power meter originally designed by TF3LJ / VE2LJX which uses a couple of AD8307 to measure fwd and rev powers.
I've added hardware and code to extend its functionality into an auto mag loop controller.
It can auto tune with powers as low as 4mW or as high at 400 watts.
Briefly, it measures the applied frequency, drives the stepper motor to a calculated tuning point then applies an algorithm to fine tune the loop to a VSWR of less than 1.5, usually less than 1.2.
It does all this in a matter of seconds, much faster for small changes in freq, effectively making the loop appear to have a much greater bandwidth thus overcoming one of the major problems with a magnetic loop - ie narrow bandwidth and difficulty with tuning.
The next steps are to design a proper PCB and write up the whole design for publication.
3 дек 2014
