After using several antenna’s, I want to try out the magnetic loop.
When I spoke with the HAM operators in the neighbourhood they donate all kinds of materials (thanks for that).
So I got de copper loop from Ron (PD2CO), the parts to create the butterfly capacitor, the motor and the steering parts from Peter (PA1CHY).
I bought some relais and PVC and found some other parts in my junkbox
So now I only has to build the loop .
First I made some calculation, I used the loop calculator to calculate which capacity I need for the frequenties I want to use.
On top you see the measurements of the loop and you see the efficiency of the band for this loop.
With this parts we build the butterfly capacitor.
I you use a vacuum capacitor you can use this loop almost everywhere, but there was no vacuum capacitor in my junkbox .
Instead of that I used a Butterfly capacitor, because the tuning of a loop is very sharp you can make a capacitor of 600 pF but then it is very difficult to tune the loop. So I made a capacitor with Less pF it goes from 6 pF to 23.86 pF, so it must be easy to tune the loop.
As you can see in the excel sheet it was only possible in the range of 6 to 24 pF to tune the loop in the 15 and 17 meter band.
So I must put some extra capacity for each sepperate band to make it possible to tune it.
I connect extra capacitors in parrallel to the butterfly capacitor for each band connected via relais.
On the other site you can see the relais and the motor to tune the capacitor.
To activate the relais form a distance I used a 9-pins connector.
And on the top of the picture you see the connector of the motor.
And on the other site of the cable there is the box where I can choose the band to work on.
I connected the 20, 30, and 40 meter band to activate the relais, and when it swiched to the far left there is the position that there are no relais activated and the loop is working in the 15 and 17 meter band.
In the sheet you see wich capacity is activated by each band. The green numbers are activated when you choose the band.
On the top site of the capacitor you see the connectors to the copper loop.
I put the parts together and the results where fine, first I used the coupling loop to feed the antenna, this works ok but I must bend the coupling loop when I was using 40 meters and that’s effects the other bands. There was a compromise where it was ok for all bands. But the next morning it was all different ??
I don’t like that because if I want to use this antenne on a mast in the garden it must Always have a good swr, it was very instable.
Originally there was a gamma match on this antenna so I put this back again, but then I had another problem, I could tune it by sliding the connector, then the swr for 15, 17, 20 and 30 meter band were ok, but 40 meter band never gets better then 1:1.5.
To tune the 40 meter band I had to slide the connector further away to get a 1:1 swr reading.
I got the idea to make two connectors in the gamma match but the first one with a relais (normally closed) and when I switched to the 40 meter band to activate the needed cappacitors it also activated the relais on the gamma match to open the connection. And it works .
Thats why you see 3 connections on the picture, the last one is just for stability, the second one is the 40 Meter band and the first one with the relais wich is normally closed, and opened when the 40 meter band is activated.
Now we must tune the loop, for this we use software in combination with an AD convertor to steer the motor.
As you see we can turn the motor to the left and right in 3 different speeds, fast, medium an slow.
This Daqfactory software give commands to the Labjack AD converter.
You see the convertor build in an old housing of an MFJ tuner, I used this housing because there is an SWR bridge in it and an SWR meter.
The next mission is to tune this loop automatic and here we can use the signals from the SWR bridge.
Here you see the Magnetic loop on the mast with the rotor in place.
Here you see in detail the PVC tube with the butterfly capacitor inside it.
And in detail the gamma match with the relais
I made some screen prints to let you see the results, on the 20 meter wspr print I was sending with 5 watts and I only was listening on the 40 meter.
Great results I think because the conditions where not very good and the efficiency was only 80% on 20 meter and 28% on 40 meter.
Now I can rotate and tune the magnetic loop, and it is working fine, but stil I must tune the loop by hand.
I wanted to create an autotune solution.
The first “problem” we saw that the minimum step of the capacitor was nearly enough to tune it precisely.
First we tried an PWM module to influence the DC motor, but that burned out .
I found an arduino wich i got out of the legacy from Ed SK (PD9ND) and sniffed the internet and found several solutions to tune my loop.
I replaced the DC motor for an servo to tune the loop.
My friend Peter (PA1CHY) created a nice construction for the servo !
Here the Arduino Uno is still on the breadboard and connected to the 20×4 Display.
And in the video my first tests on an tuner connecte to my dipole antenna.
This works great on my dipole antenna but on my magnetic loop it was not precise enough.
It tunes in the neighbourhood but not always exact 1:1 .
I replaced the DC motor for the servo.
I placed the servo with capacitor in the loop,
Check the movie to see how it rotates.
Now it was time to contruct the arduino in a casing.
So the display , wires and knobs are not falling out of the breadboard.|
I made some pictures of the inside.
The plan is to make it a fully automatic system, so I made the following schematic to follow.
Now the important part of all is can I tune the antenna with the arduino ?
As I say I have found several options and ideas on the net. I used parts of software and combined it with my own solution of software. I made the ruf version and my son corrected the code .
Because each antenna system is different I wil show you my solutions.
I want a system where I can tune the antenna manualy and a full automatic system.
In this video you see the full automatic option.
I started the arduino en sets my transceiver on the 40 meter band.
You will see that the system starts on the 15 meter band and fully automatic search and find the 40 meter band.
In the next image you see the needed servo poistions by band.
|Cross section circle||1,25 meter||4,00||6,20|
|Copper round||22 mm||Range|
|freq||Frequentie||Effiency||Bandwidth||kV||pF||VAR.||15||Relay – 1||47||Relay – 2||130||Relay – 3||Relay – 4|