Making variable air capacitor for a mag. loop antenna

[apelly]

And I have some questions.

Background:
Over here cdev suggested I fool around with a mag. loop antenna.

10 minutes research later I see that a decent vacuum cap is quite expensive indeed.

I ponder a while, then think "An air cap is pretty much all flat or thin laser cuttable stuff. Certainly < $100 + labour" So I draft up a parametric one in FreeCAD. Here's a clue what it looks like:



It's not quite finished yet, but the idea is it fits inside a piece of pvc pipe with some end caps (not in pic). You choose the kV rating, pipe diameter and max capacitance and it shows you the dimensions and number of plates, spacers, rods and stuff to fit. Then it's off to the laser cutter with the DXFs.

Questions:

• Where can I find a reliable table of dielectric strengths & constants on the web?
• Preliminary research suggests it might be useful, from a size reduction perspective, to fill the pipe with mineral oil. I don't think I trust some random brand to give me what I think mineral oil is. Has anyone experimented with this?

[cdev]

Yes, yes..

"Butterfly capacitors" are the very best kind of cap for use in a balanced mag loop.

It looks like a butterfly cap.

People have used all sorts of methods to make these capacitors..

Another design used Coke soda cans.

I have a table of dielectric strengths.. I'll find it. give me a few minutes..

You will enjoy this a lot more if you build it yourself.

Another search term...
Trombone capacitor

[cdev]

This is a start!

http://4hv.org/e107_files/public/1490831097_61521_FT179355_dielectric_strength_of_insulating_materials_-crc_handbook_of_chemistry__physics_.pdf

[apelly]

It looks like a butterfly cap.

Yup. Thank you internet.

I have a table of dielectric strengths.. I'll find it. give me a few minutes..

Cheers mate.

You will enjoy this a lot more if you build it yourself.

Hope so. It's sunk a few hours so far.

Another search term...
Trombone capacitor

Don't be mischievous. Actually, they showed up in my earlier research, but I was hoping to get a wider range with a butterfly. I'm still not sure what the minimum will be. Even with no actually overlapping area the plates are still closish. Hopefully it turns out close to nothing.

And thanks for the link.

[apelly]

Note to self:

Silicone oil. Polydimethylsiloxane. http://www.clearcoproducts.com/pdf/low-viscosity/NP-PSF-5cSt.pdf

Dielectric strength: 375V/mil, which I think works out to about 14.5kV/mm
Dielectric constant: ~2.5

Non flammable.

But where to get it? I see an online hobby shop with a .nz domain, but what's likely to be in their silicone?

[cdev]

What about distilled water?

You could likely find a decent vacuum cap for a reasonable price if you look for a while in an organized way.

[apelly]

You could likely find a decent vacuum cap for a reasonable price if you look for a while in an organized way.

What is your budget for the cap?

No idea. I was hoping < $30 originally. I hadn't realised how big it would be. Now I'm out to < $200, and probably that much again for the rest of the antenna. Seems a little pricey.

I haven't actually finished the math yet, so I don't know what I need. I'm working on bits of that now.

Bigger values will let you tune lower.  Then you could use a stepper motor to control it, repeatably. That has the potential to be really convenient and fast.

I haven't got costs yet, but it looks like building my own should be a similar price, acceptable quality, and super flexible. A stepper is certainly part of the plan. Again; I haven't finished the math yet, so I'm not sure if a reduction gear is useful either.

Also, some time ago I tried a search for (whatever chinese term is for "variable capacitor" on Taobao). I forget what term I used (I dont speak Chinese so I think I got it from a site that no longer exists that translated taobao or Chromium auto translation. )

but when I found it I found tons of Chinese, large army surplus transmitting capacitors, really cheap. $11 pops into my mind. Might be worth trying to find.

Good tip. There are some on Taobao here if you scroll down a bit. Looks like somewhere between (US)$30 & $100. I'm not sure what their shipping cost is likely to be.

Here's one of the things I searched for: (I think the forum will bork the utf8 though) ?
It's a google translation of "variable vacuum capacitor"

edit: yep. total bork. anyone following on will have to do the translation themselves.

[cdev]

Have you seen this new Jeri Ellsworth video about constructing a loop, and its thread here?

https://www.eevblog.com/forum/chat/jeri-ellsworth-new-series-of-videos-on-youtube/

Also, comment on Alan W2AEW's comment on that video, yes, the coupling method makes a huge difference in how a loop performs, as far as Q goes.

This web page has collected a lot of info on magnetic loops, including different coupling methods in one place, its a good reference to the different things one can try.

https://www.nonstopsystems.com/radio/frank_radio_antenna_magloop.htm

[apelly]

Have you seen this new Jeri Ellsworth video about constructing a loop, and its thread here?

Yes. Noticed it the other day. Looking forward to the rest of it. Thanks for the heads-up though.

This web page has collected a lot of info on magnetic loops, including different coupling methods in one place, its a good reference to the different things one can try.

https://www.nonstopsystems.com/radio/frank_radio_antenna_magloop.htm

Also found that. It's one of the better ones I've seen.

Construction of mine has slowed. I'm still sourcing material to make a cap. At this stage it's looking like I should probably just grab one from taobao.

[timelake]

ref: https://www.eevblog.com/forum/rf-microwave/making-variable-air-capacitor-for-a-mag-loop-antenna/

I hope you and yours are well.

I was pleasantly surprised to see your post recently because I was also thinking to have a butterfly cap built for a loop that would slide into a pvc pipe then filled with mineral oil.   

But I didn't get much out of the comments so I thought to ask you directly for any updates?

[apelly]

I thought to ask you directly for any updates?

Still unbuilt. I got distracted, as usual. One day I'll sort it out.

[ebastler]

I bought an air-gap capacitor from this small German company a few years ago. Sold as a kit. Unfortunately their website is German only:
http://schubert-gehaeuse.de/drehkondensatoren.html

(Click the "Mehr..." link on that page to see a list with models and pricing.)

[coppercone2]

how are you attaching the plates to the shaft?

I am curious how you are building it. I assume you put it with some kind of spacers and solder it all at once to a copper shaft?

are those ball bearings?

[apelly]

how are you attaching the plates to the shaft?

The stators are squeezed with spacers and nuts on threaded rod.

The rotor is on a plastic shaft, clamped at each end.

I am curious how you are building it. I assume you put it with some kind of spacers and solder it all at once to a copper shaft?

Brass and plastic spacers as required.

are those ball bearings?

There are a couple of skate-board bearings. For no good reason.

[radiolistener]

Since magnetic loop antenna is too small compared to half-wavelength, it has high Q to be effective. It means that it accumulates a lot of energy in the reactive near field region. As result, it is very sensitive to any energy loss on bad contact, bad conductivity material or dielectric loss. The magnetic loop antenna efficiency is very dependent on each milliohm. As well, it has high energy loss on any conductor or earth which falls into the near field region.

The main problems with variable capacitor for magnetic loop antenna is voltage (the gap between plates should allow to work with many tens of kilovolts in order to work with transmitter) and sliding contact on the rotor (which has bad contact).

The gap between plates can be increased by remove odd plates. The issue with sliding contact can be solved by using dual-section variable capacitor and use two capacitors in series coupled by rotor. But the best results can be achieved with vacuum variable capacitor and copper tube for antenna body material. It will minimize energy loss within antenna.

But if magnetic loop antenna is planned to use with transmitter, there is need to take into account that this is not safe for health. Since it has high Q and small size, it has extreme high electric and magnetic field strength near the antenna. So, it is not safe for people health to be near the antenna during transmission even with 10 W transmitter.

Generally, it is good antenna, but it not safe to use it with powerful transmitter. This is even more relevant, since magnetic loop antenna often considered as a small replacement for full size antenna. So, many people thinking it can be placed on balcony, or just near the table with transceiver. But this is a deceptive impression. If you place it near the home, it will suck in various types of noises from your home in receive mode and you'll be exposed with extreme high level of electric and magnetic fields in transmit mode.

In order to detect thermal losses on magnetic loop antenna, FLIR camera can be used.
Here is nice example from G8JNJ site:



Here is how to build rotor-coupled variable capacitor to solve issue with sliding contact:

[quadzillatech]

just thought i would mention it,a while ago i built a loop in my loft for 80m,it was 6.8 meters of 15mm copper water pipe in a hexagon,for the cap i milled a chamber in a 10cm dia piece of delron and used a broadcast cap,i filled the lot with hydraulic oil  with a perspex lid and a sealed roller bearing on the shaft,may be an idea if you have access to a mill and lath etc,cheers m3vuv 73

[tkamiya]

Is the safety factor any different from using mobile whip and transmitting with equal power?  It also has high-Q and high-voltage at the tip.  Considering this is for HF, I tend to think risk is not as great.  That's provided people don't touch it while transmitting.  TOTAL magnetic flux can't be any larger than any other antenna. 

[radiolistener]

Considering this is for HF, I tend to think risk is not as great.  That's provided people don't touch it while transmitting.  TOTAL magnetic flux can't be any larger than any other antenna.

No. The key points here is antenna size and wavelength.
Any resonant antenna accumulates some amount of RF energy in its reactive near field region.
The size of reactive near field region depends on antenna size and wavelength.

Antenna can be considered as impedance match circuit between feeding coax line and free space.
Free space has impedance 376 Ohm. Feeding coax line output has impedance about 50 Ohm.
In order to transform impedance, antenna needs to accumulate some amount of RF energy in the antenna system (antenna + antenna tuner).

For full size antenna this RF energy is distributed in a large volume of space.
For small size antenna this RF energy is distributed in a small volume of space.

For small size antenna the same amount of RF energy will be concentrated in a small volume of space.
It means that this small volume of space will have extremely high field strength.

When you look into small antenna, you're think that it is more safe than a large one. But it is not!
This is deceptive impression. This small antenna radiates the same amount of energy as a large one.
But the small antenna uses much more powerful field strengths (both E and H) in order to radiate the same amount of energy.

For example, let's check E and H field strength for different size antenna with the same 100W transmitter at 14 MHz.
Safe levels marked with black mark (E=130 V/m and H=60 A/m).

Electric field for full size half wave dipole:


Magnetic field for full size half wave dipole:


Electric field for magnetic loop (square with 1x1 m from copper tube 20 mm + capacitor, Q=2000):


Magnetic field for magnetic loop (square with 1x1 m from copper tube 20 mm + capacitor, Q=2000):


As you can see, it's more safe to be near full size antenna, than to be near small size antenna.
Just compare max E field strength 1700 V/m for full size antenna and 15000 V/m for small size antenna.
The same issue with max H field strength 3.8 A/m against 60 A/m.

As you can see, bigger antenna is safer than smaller 

[cdev]

Its my understanding that the voltage on a very high Q (thats the way to make them work the best!) magnetic loop used for transmitting can get very high, literally thousands of volts.

There are actually lots of ways to make a capacitor that can handle that, the only problem is most of them are not going to be as easy to tune/adjust as one would like. But some are.

Jeri Ellsworth has a youtube video showing the first step of construction of what looks like a nice magnetic loop design but I don't know if I was able to find the subsequent episodes. Anyway, it looks ideal for a home station, if you can deal with the need to retune it as you move around.  A mag loop is great with an SDR because its also as good of a preselector as you are EVER going to see.

A really really good one. Virtually nothing other than a little slice of RF around your desired signal is going to get through.

[radiolistener]

Its my understanding that the voltage on a very high Q (thats the way to make them work the best!) magnetic loop used for transmitting can get very high, literally thousands of volts.

yes, since it is small, it has low radiation efficiency. So, there is need high voltage and high current in order to radiate the same amount of energy. The part of energy which is not radiated will remain in the antenna system with help of antenna tuner (capacitor for magloop), it will be summed with a new energy from next wave cycle and it will leads to higher amplitude. At some point input energy and output energy (loss+radiation) will be balanced. In general Q means how many wave cycles is needed for such balance.

But there are two cons:

1) High Q means low bandwidth. The antenna resonance is sharp and needs to be tuned very frequently. This is very
uncomfortable, especially since tuning is too hard because variable capacitor is placed on the antenna. And we cannot place variable capacitor near transmitter, because it will leads to high loss in the feed line.

2) High Q means high field strength, high voltage and high amperage. As result it leads to high loss on any subject which will be placed inside near field region or any resistance in the antenna. The same issue in receive mode, any subject within near field region will eat power of weak signal or add noise.

[shved]

Apart from obvious issues, most frustrating thing about loops is tuning.
I'm now prototyping remote controlled(ATAS) mag-loop antenna for Yaesu and some other gadgets.
Photos of the loop and stuff: https://flic.kr/s/aHsmHPwA1M .

[cdev]

That looks interesting. Would like to hear more about it.

Apart from obvious issues, most frustrating thing about loops is tuning.
I'm now prototyping remote controlled(ATAS) mag-loop antenna for Yaesu and some other gadgets.
Photos of the loop and stuff: https://flic.kr/s/aHsmHPwA1M .

[CaptDon]

Here is the biggest secret of mag-loop antennas, tune for peak
noise on the receiver!!!!! You get what you get on transmit, maybe
as high as 2:1 VSWR!! The two-way contact will be much better
went peaked on receive noise and NOT lowest VSWR. The tuning
is so critical you won't believe it, just 2 or 3 pf difference out of
a hundred or more is like night and day!! If your tuning mechanism
has uncontrolled backlash and 'pops' back and forth by about 10pf
you will learn to hate mag loops in one night!!! The MFJ 'Super-Loop'
performs poorly when tuned for lowest VSWR!! You will see a 10-15db
drop in receive signal. You can't talk to them if you can't hear them!!!

[Kalvin]

You could probably get away with a smaller variable capacitor if you constructed fixed capacitor(s) from a coax(es) to be placed in parallel with the variable capacitor. You will have to connect/disconnect the fixed capacitors when you change bands, but probably you could come up with some kind of high voltage and low loss switch for easier band change.

[cdev]

I've seen all sorts of things used for variable capacitors, and the ways they ensure they dont have the loss inherent to the brushes are also very creative. It can be very simple. I once had a big long piece of flat plastic from a window blind. I was able to make a magloop out of that by covering one side with a piece of aluminum foil. - when folded into a circle you could tune it over a pretty wide range in the core HF region by adjusting the proximity of the two ends from one another by holding the loop down and shut with a big book and changing the pages where the plastic pieces were inserted. (It worked great indoors but would have been a bad choice for outdoors because of wind loading)

I think because it was a single continuous pice of aluminum foil that was wide so this worked out well as far as the skin effect.

A piece of glass between the two would allow a fairly high voltage and transmitting, while at the same time being very low loss.. You could also use a more traditional capacitor like a vacuum variable. the wideness of the loop is supposed to be a big aid to more efficient operation. Ive also read a web page of somebody who was having great success using foam pool floats spirally would with wide copper foil tape like a candy cane. This was supposed to work really well plus be super light. It creates a physically smaller loop that has enough inductance to work on much lower frequencies but still be fairly efficient.