Re: HF Magnetic Loop Antenna - prototype under test


David Cutter
 


Somewhere I have some small bore PTFE sleeving I used for high temperature work and it was at the time available in various sizes.  If the name comes back to me I'll pass it on.  I also have some rigid tube in about 6mm bore of Whale variety, again, that was available in various sizes from a stockist, so, it can't be that uncommon. 
 
Polyurethane and PTFE have the slipperyest and unglueable surfaces I've come across but 100:1 price difference.
 
For some high voltage work (several kV) I used glass filled paper sheet that was expensive to buy and specialist machining was required, but, again relatively easy to come by.  There's also machinable ceramic that's been around for several years.  Here's an odd ball idea:  buy the cheapest (ie slowest) super glue and dust it with baking powder.  This makes it go off very quickly, becomes very hard but also turns into machinable form (file, drill, etc) which I am told is high voltage proof, ie has a high surface tracking index.  I've got to try this baking powder trick sometime.
 
David
G3UNA

----- Original Message -----
Sent: Friday, January 13, 2017 8:33 PM
Subject: Re: [CrossCountryWireless] HF Magnetic Loop Antenna - prototype under test

On 1/13/2017 2:59 PM, Chris Moulding wrote:

I've just run another high power test after modifying the prototype. This time it got through 30 seconds of a 20m JT-65 transmission at 100W before arcing over.

In both the tests so far I've been mystified by the amount of carbon dust left on the PTFE sheet after the event.

Apparently according to Wikipedia: "tetrafluoroethylene can explosively decompose to tetrafluoromethane and carbon".

It also explains why the flash-overs are so dramatic.

I'm probably running close to the limit with the PTFE sheet I'm using. I'll try doubling it up for the next tests.




This is exactly the problem I had.     I scorched, burned and ignited considerable quantities of polyethylene, surgical rubber and vinyl tubing while coming up with my design.    [Vinyl is absolutely horrible for HF RF applications I discovered.  Polyethylene is the best second only to PTFE. But how do you get Teflon tubing if you are not a defense contractor with an unlimited budget???]



Originally, I used a piece of center conductor/dielectric from RG-213 pushed into a 3/8ths inch inside diameter 10-foot long piece of soft copper tubing.  Here in the US at least, this kind of tubing is sold as "refrigeration tubing" and is used for hooking up the water supply for ice makers in refrigerators. You can routinely get 20-foot coils of this stuff at places like Home Depot for around USD $20.  The inside diameter of this stuff is an EXACT fit for the center-conductor/dielectric removed from RG-213 or RG-8 coax. It must be the solid poly dielectric, not the foam stuff.

Originally, I stuffed the center-conductor assembly  down one side of the circular loop of tubing to form a tubular capacitor, and soldered the stripped end to the other side of the loop.  This would arc over and break down with more than 75 watts CW key down.  I tried using larger diameter tubing and sleeving the coax center in one or two  concentric layers of polyethylene tubing, but that reduced the capacitance per foot so much than even with the entire loop filled with center conductor, I couldn't get enough pF to resonate.  

I then had the inspiration to try sticking the center assembly down BOTH sides of the loop, creating effectively TWO coaxial caps in series, yielding twice the voltage breakdown.  Of course, two caps in series have half the capacitance, so I had to create twice the capacitance on each side.  In turn, this meant having to use FOUR times the amount of center-conductor/dielectric inserted into the tubing.  This is the design in the PDF I attached to the last message in this thread.      It has been working perfectly for over 5 years now in my 30M APRS igate installation beaconing every 10 minutes 24/7 at 100 watts out from my TS-50. 


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