Caps for RF transmitting loops, that handle power well?

[TimFox]

Again, the real question is how much current will flow through the capacitor.  In a tank circuit, the circulating current is higher than the load current.  For very high tank-circuit current, vacuum is the best and most expensive.  Old clunky mica capacitors can handle current due to their large volume.
For simple flat-plate capacitors, glass works but polyolefins such as polypropylene are easy to cut.  The important thing is adhering the copper electrodes without air gaps between metal and dielectric.  (Polypropylene is hard to glue.)

[coppercone2]

loctite medical primer with loctite plastic superglue works fine for polyethylene, very strong. adhesion promoter not curing agent. nothing special to it, just clean prime on the hard to bond side, inspect with UV if you want, then glue. not sure about pp

dp8805 or dp8810 epoxy might work too but I have not tried it for polypropylene yet. you probobly should scratch it up with silicon carbide sand paper first (very sharp). the 8805 and 8810 are loaded with glass spheres also, Iirc, that should stabilize it and ensure thin proper bond lines.

Loading the glue with ceramic spheres or such may benefit capacitors too to ensure standard size spacing, but I am not sure how that will work out in theory.

[Wallace Gasiewicz]

I built a ten foot dia mag loop and used a vacuum variable cap. The big problem with small loops is that they do not radiate well, The radiation resistance is very low. Therefore you must keep real ohmic resistance to very low levels, less than one ohm. This is impossible without good tuning caps. Even soldering joints is frowned upon, you need to weld or braze or real silver solder. Anything with a normal solder joint will have too much resistance and you will waste power.
There are some air caps that seem to work. Trombone variable caps work and are used by some people who use transmit power into the tens of thousands of watts in vertical antenna tuning. I think they go short is it rains on them. Almost anything will work for RX.
I assembled mine without any solder joints by putting the ends of a Vacuum Variable Cap into the ends of the circular bent 2 inch copper pipe.
Try the Mag loop group on Groups IO https://groups.io/g/MagLoop/topics
Or this group if you want to be a bit different: https://groups.io/g/HelicallyLoadedMagLoop/topics
Feeding small mag loops is another matter, They can be fed with a small loop or a gamma or even a delta match, more fun

About the supposedly huge dangerous voltage, my loop is in the next room and my cat walks by it all the time and her fur still looks OK.
I use it on 20 Meters with about 300 Watts.

Wally

[cdev]

Wow that's a big loop!

 THANK YOU!  This looks like a great resource. I have a thee turn loop made of approximately half inch tubing and Ive been wondering how to make the rest of it and using big huge battery clips for now, not so great but OK for testing, You answered a major question hat I have had.

I do have some silver "eutctic" solder saved just for something like this. Its a bit more brittle than most of my solders when solid.

Cant transmit yet so I have been using my bench multimeter and DE-5000 to measure its equivalant series resistance awith the big clips and my nanovna to try to measure the sharpness of the peak / Q .

This makes me want a new receiver so I can see the effect over a wide bandwidth.

 wonder if I could find a good vacuum cap with a wide low to high capacitance ratio commercially, thats not a one of a kind unit and not too expensive.

Thank you!

[coppercone2]

I would try to make a omega shape with a bushing/collet/fuse holder type thing for the vacuum capacitor on the bottom with a key that inserts into it to adjust its spacing

I wonder if a flaring tool and some slits could make the pipe into a proper electrical contact, but looking at those capacitors I think it should just clip in on the bottom like a cheap two part PCB fuse holder (the two metal bits you solder on)

I think if it was possible to make a good 'fuse holder' for the capacitor it would be very interesting, if its possible with just a press in fit

https://imagesvc.meredithcorp.io/v3/mm/image?url=https%3A%2F%2Fstatic.onecms.io%2Fwp-content%2Fuploads%2Fsites%2F37%2F2017%2F10%2F16021207%2FSCW_007_03.jpg

none of the solutions I see are quite that elegant though, to have a press in fit, its maybe non viable, it all uses bolts i.e.
https://forums.qrz.com/index.php?threads/mounting-vacuum-variable-capacitors-on-mag-loop-antennas.700414/

[Wallace Gasiewicz]

 2 inch Copper and aluminum "pipe" is 2 inches internal dia

2 inch copper or aluminum "tube" is 2 inches outside dia.
 
It's hard to "flair anything without a good tool.
You can however slit the opening of the 2 inch "pipe" to make it easier to insert the VVC. Yhen you can use big hose clamps on the outside to make a tight connections. Some folks use silver paste to make a better connection.You need another hole in the pipe for the tuning rod, this does not have to be isolated since the tuning rod is connected to the end of the cap anyway, however a piece of new fangled PEX pipe can be used to connect the tuning rod to a knob or a tuning motor, to insulate it from the user.

You need to consult the charts to determine the value of your cap. The surplus Russian ones have had good reviews by some folks.
Count on $100 or so for a transmitting VVC. For Rx any good, not corroded air variable cap will do nicely.
Some VVCs come with mounting brackets for connecting. You might get lucky and find one like this. But again, the more connections the worse the performance.

Silver solder is mostly silver, many solders only contain some silver (1-5%)

Anyway. these things are hard to tune. They have sharp tuning and the better they are made, the sharper the tuning.
The more the surface area the more the radiation resistance and better performance. Hence the helically wound loops.

If you have a Vector Network Analyzer, you can look at you antenna's characteristics with just regular solder joints and then with good connections.

The more the turns of this type of antenna the lower the radiation resistance,  I think someone did get an award for making a two turn 80 Meter loop.

Wally

[coppercone2]

I think the flaring tool would need to be custom made on a lathe

[Wallace Gasiewicz]

Harbor Freight sells flaring tools that work somewhat
A muffler shop may be able to flare your ends.

[coppercone2]

it also might be an option to use thick copper pipe for the loop then make a braze to a thin copper pipe of same dimensions to make the holder

[cdev]

Most vacuum variable caps are adjusted simply by turning a rotating part on one of the two ends.

Only a few companies seem to be selling them new, and they are very expensive it seems for what they are.

[Gyro]

Relatively cheap for NOS former Soviet ones on ebay though.

Edit, They must be pretty labour intensive to build though, you've got the large glass to metal seals, metal bellows and screw mechanism acting against a fair amount of static force etc.

Eg. https://www.ebay.co.uk/itm/164070350233  (1nF 20kV 100A).

[Wallace Gasiewicz]

I used a thick copper pipe for my loop.
It worked.
It was very difficult to bend and did kink.
I used a hydraulic bender.

[coppercone2]

Relatively cheap for NOS former Soviet ones on ebay though.

Edit, They must be pretty labour intensive to build though, you've got the large glass to metal seals, metal bellows and screw mechanism acting against a fair amount of static force etc.

Eg. https://www.ebay.co.uk/itm/164070350233  (1nF 20kV 100A).

their main use I think was to determine if we are about to kill each other, so naturally the price drops until another cold war follows ww3 which destroys the advanced technology, then the price will drop when we develop back into current circuits

[Wallace Gasiewicz]

Earlier on this thread, I made an error.
My loop is NOT 10 feet dia
It is 10 feet in Circumference, one piece of 10 foot copper pipe.

SORRY SORRY SORRY


Wally