Magnetic Loop - Part 2
Perfect SWR when the loop is tuned.(160m)
Finished you think?
Well, not quite.
The weight of the vacuum cap with mounting plate, motor drive, etc. was too much for this Y-construction.
The cap-assembly + enclosure needed support.
So I decided to go for a +-construction...
to be continued...... PA0KV
Busy day today... may 29, 2009....
Creating the +-joint was easy using a piece of Trespa and bending some aluminum strip.
The white stuff in
the middle of the epoxy tubing is glue.
Cross section of horizontal & vertical pole.
The new assembly..
The vacuum cap has to be 'all weather proof' ...
... here the 160 mm PVC enclosure with glued PVC support
to fit on the epoxy tube.
The wires for cap-motor and slide-pot meter are lead through the vertical tube of the +-joint.
In some way this feels better than leading them through the copper loop.
I can't get it out of my mind that leading the wires through the copper loop has some bad influence on the loop's
Another thing was keeping my mind busy.
How to mount the cap-assembly into its
enclosure without drilling
holes in it?
Holes would make it less 'all weather proof'.
Well, after another cup of coffee and some
I came up with this....
To be continued...
11 june 2009 ..
This is the idea... turning the cable-tensioner will push the doorstop upwards and the base-plate downwards.
Okay, let's put it up... Two high tech equipment in one picture.....
Final position. Shack view on the loop
Now I'm testing it... first rapports from PA-land on 80 and 160 where very good.
Designed with the loop-calculators on the web it should tune on 160, 80 and 40M.
But in practice the loop has to be a little bit smaller in diameter to tune it on 40M.
Too much L in the circuit in this setup.
On 80M, tuned 1:1 with the vacuum cap, bandwidth is +/- 5.5 kc, swr 1.5:1
On 160M, "" "" +/- 1.1 kc, swr 1.5:1
The vacuum cap is a 5KV type so 100 watts is save.
In compare with my Inv-V on 80m there is little or no difference in send and received reports.
(Local European contacts.)
But the Loop is much more quiet and no splatter from nearby stations.
73's, Twan - PA0KV
Time to add another chapter to this loop story :-)
The Loop is working fine, but two things are bothering me.
1- The loop was meant to span from 160 to 40 meter. Unfortunately the loop can't be tuned to 40m.
2- The tuning via a DC motor with some up and down buttons is a hassle.
To solve problem 1. the loop has to be made smaller. Less 'L' so I get more headroom in the high band (40m).
I have plenty of 'bottomroom' on the lowest band (160m).
To solve problem 2. I need a very accurate and easy to control Vacuum Cap driver. A stepper motor has both!
But, for a stepper motor a control program is required.... And such a program for this application or not available.
I started searching the web for 'off the shell' stepper motor controllers.
Most of them have no smart logic on board and all the control you have to build (program) yourself.
But I found a board called "Motor Hawk" which has USB control and is delivered
with a dll for all the actions towards a stepper motor.
It comes with a CD with the dll and very useful program examples in C++
and Visual Basic.
Here the small controller board, developed and sold by PC-Control in the UK.
It's a 4-phase Bi-Polar (or hybrid) stepper motor (or two DC motors) controller
and can handle up to 36V-2A/fase.
There are also 8 digital inputs and 5 digital outputs available for general use,
with 4 of those inputs configurable as automatic limit switch inputs for motion control applications.
Here the updated vacuum C control with the stepper motor.
It's a lot simplified. No end-switches, no position potentiometer.
The stepper motor is a gift from a friend, PE1BWK.(Tnx Sjel!) The tooth belt and tooth pulleys
are not hard to find on the web.
Here you see the board in it's enclosure.
At first I wanted to mount the controller near the Loop, so the motor wires remained short. But then I would need a 15 meter long USB cable. Not recommended. In addition, the controller also needs a DC power supply** of a few amps to feed the stepper.
So I decided to keep the controller in the shack and connect the stepper motor by a 4x 1.5 mm² cable.
**) My stepper, a Minebea 23LM-C303-05, needs 3.5V-1.4A/phase.
I had an old Philips power supply, 12Volt-3Amp, perfect for the job.
With 12V, two shunt resistors in the motor leads are needed, generate a lot of heat. :-( So I calculated the voltage drop across the 15 meter long leads, added the voltage drop of the board and
adjusted the output voltage of the supply accordantly (+/- 7 Volt).
Digital switching produces RFI. Something we do not like. :-(
This board is no exception. I installed a 100nF ceramic capacitor between each motor output and ground (0V).
The wires are wrapped five times through a ferrite toroïd. Close to the stepper I also used a toroïd core.
The board is mounted in an alloy box (with lid) and grounded to the box with 100nF. The box itself is grounded via the mains ground.
As a result, RFI is reduced by 95%.
(more testing to do...)
To write a controller program for this board it was "back to the classroom" for me.
Although I had some programming experience,....(IBM mainframe systems programmer).........., that was ten or more years ago!
It took me a while.... I studied visual basic from MS...
... with lots of try and error.
Finally I managed.
Control screen (tab). Band selection for 160-80-60-40m. Each band has 8 memories.
The display shows the chosen memory frequency. Tuning by steps to fine tune for 1:1 SWR.
Init screen (tab). Program first needs to initialize the stepper board. (Searching the USB devices connected.)
With the stepper settings: stepper motor power range 0-250, stepping rate in ms and default tuning steps when Up and Down buttons
Stepper power can be forced on (max) or off (zero) for test purposes.
The program uses 1 control file and 4 memory files (xxx.cfg). They are read at initialization when a stepper control board is found.
Files are saved by the Actuation button and by the memory Store buttons.
I also made the loop smaller. It's now 3,4 meter in diameter and is tunable from 1.75 to 7.3 Mhz.
I created a rar file containing the program, the config files and a manual. To download here> MagLoCon.
Any questions? Mail me!