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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.
----- Original Message -----
Sent: Friday, January 13, 2017 8:33
Subject: Re: [CrossCountryWireless] HF
Magnetic Loop Antenna - prototype under test
On 1/13/2017 2:59 PM, Chris Moulding
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
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.
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.
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
Stephen H. Smith wa8lmf (at) aol.com
Node # 14400 [Think bottom of the 2-meter band]
_______ Windows 10
Off-The-Air APRS Activity Maps
APRS on 30 Meters HF