Mag Loop Antenna info. 2E0ERO's version.

VK3ZYZ

Moderator
There has been some discussion around Mag Loop Antennas recently.
This is a Youtube video that just poped up and I think it is a neat design...
It is not using a variable vac cap, just a "standard" mechanical one that the guy makes. Have a look at these photos.MagLoopCap.jpg

And for those who are on Facebook (not me), there is a group..

And a lot of the parts are 3D printed :)
I may have a play later.
 
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VK3ZYZ

Moderator
What does anyone think of getting a stack of plates laser cut to make some caps?
Here is my first try.
Variable Capacitor.png

It may need more clearance.
I'm in the process of getting a rough estimate on 1K sets in 1.5mm Aluminium.
Maybe the club could be interested in a kit project?
 
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VK3ZYZ

Moderator
1620195561498.png
A geared stepper motor like this for under $10 can be used to drive the cap, and a multi turn pot for feedback so remote tuning would work well.
Here is another good Arduino project!
 

VK3ZYZ

Moderator
I think for my design above, if the between plates gap is 2mm, each section (with 1 rotor plate between two fixed plates) will be about 10pF max.
 

VK3ZYZ

Moderator
Kevin dropped a variable cap around so I could get a good look at one, and it just so happens the plates are almost the same size as my drawn ones!
VariableCap1.jpg
It has 20 fixed plates and 21 moveable ones.
Capacitance is from 34 to 304pF according to my meter.
The plates are 1.35mm thick and like plates are spaced about 6.3mm apart so the plate spacing is around 2.5mm.
The max Voltage I can test to is 1KV and that is ok.
VariableCap2.jpg
So that is an indication of what we can expect in capacitance.
 

VK3YNV

Moderator
You can work on 3KV per mm according to what I've read, so 2.5mm spacing would be 7.5KV not sure what immersion in transformer oil would do to the capacitance and breakdown volts.
 

BillC

Active member
Hi Denys, just an idea for the plate design of your capacitor as drawn , maybe a generous radius on the outer corners of the semi circular moving plates would help to resist arcing , say 5 mil. Also how would a solid dielectric disc of suitable plastic go ,placed between the plates with some overlap extra to the dimension of the plates . I do have a roll of thin plastic abt. .75mil . I will try to find it and get some to you for a test , if you are interested . Cheers.
 

VK3ZYZ

Moderator
Caprounded.png
Like this Bill?
I was thinking of some insulation between the plates to increase the voltage.
But I will need to get plates cut first some time in the future....
 

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BillC

Active member
Hi Denys, that design looks great, I am sure that the rounded profile would help to stop the formation of an arc . RF is funny /peculiar stuff it likes to creep around and find a weak spot , I'm sure you have more exp. with high power RF than I . I do have a high voltage transformer here ex vacuum gauge tester it supplies about 5 KV. , you could test basic insulation with that but it would not replicate high frequency RF . For testing perhaps try your high voltage spark coil try driving at 7 mhz and stand well back WISH WE ALL HAD MORE TTTT. TIME.
 

VK3YNV

Moderator
Some design ideas. The capacitance depends on the area of the plates and the spacing as well as the dielectric constant.

C = (Area * Dielectric Constant) / spacing

So to increase the capacitance you can either increase the Area, reduce the spacing, Or increase the dielectric constant.
Teflon has a dielectric constant of ~4, transformer oil is about 2? 4.5? It also increases the breakdown voltage, provided the oil isn't contaminated.
The trick is to choose a dielectric material, that has a high dielectric constant, high breakdown voltage, and low loss.
Losses depend somewhat on frequency as well, so you need the loss tangent data at the frequency being used.
A good way to measure the loss would be to measure the temperature of the oil at full transmit power and see how much it heats up.
 
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BillC

Active member
Good on you Ray . I, being the devils advocate am wondering about the long term stability of oil I know that some oils absorbs moisture , it is difficult to keep moisture out of housings etc. I suppose if moisture was mixed with the oil a few minutes of operation might heat everything up and dry the oil . The system could be self cleaning . However mineral oil has an S.G . of about .89 so water could mix in it , and synthetic T/Fmr. oil has an S.G. of about 1.5 so water could float on top of the oil . Nice rain here tonight .
 

BillC

Active member
Hi Denys. I watched the vk5sfa presentation on loops , very interesting those chaps have certainly done a lot of work on loops. It looks as though you might have to change the capacitor design to a butterfly as there could be too much current for a commutated rotor, up to 15 amps at 400 watts according to the designer. cheers .
 

BillC

Active member
Fantastic video, We could cut a trial set of a couple of plates to measure the capacitance then add the numbers for the required result. I,m heading out to split firewood now with the twin ram splitter , seeyou.
 

VK3ZNA

New member
On the DX Zone website, there is an interactive programme, real time, to calculate the Magloop parameters by VK3CPU. There are 5 sliders to adjust variables and a screen to show the various parameters as adjusted.
Cheers
Geoff VK3ZNA
 
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