On Fri, 01 Aug 2003 12:31:24 -0400, Tarmo Tammaru wrote:

Tim

First off, if the the inside diameter of the tube is about the same as the
inner diameter of the coax shield that you took off, then the capacitance
per inch would be the same as what is specified in cable charts. For
instance, RG58 is something like 29 PF/foot.

I wouldn't bother with fancy threaded rods. Slit the end of the tube, and
fasten a small hose clamp down on it to keep things in place.

Lastly, what you describe is how CushCraft seems to build their gamma match,
and they take hundreds of watts. They adjust the capacitance by sliding the
insulated wire in and out of the tube. This will have limited range; so, to
get it in the ballpark you would have to keep cutting pieces off the end.
For good measure, melt some insulation on the end of the wire so it is not
exposed.

Tam/WB2TT

Thanks, Tam. I wasn't sure if there would be a difference going from
braid to solid.
On the cutting: I'm going for a really broad range of capacitance, which
is another factor for the threaded rods, as they can hold things steady
even if I have the coax nearly all the way out of the tube. I know such a
cap could handle a lot of power, but remember, a magloop runs at a very
low internal impedance, which means really high current AND voltage. I'm
not worried about dialectric melting, just puncture. Most magloop designs
I've seen call for 3kv breakdown voltage for 50 watts. 50 ohm coax
handling 1000W is carrying about 4.5A at 224V, RMS, so what is that, about
320V peak? Let's say they rate it for 100% safety factor at the legal
limit, so a round 1000V? Yeah, I guess you're right, about any coax
should handle my measly 5W, though I don't think voltage scales linearly
with power in a magloop... maybe with the square? That'd put me a bit
under 1000V. Heck, the worst that could happen is that I could lose my
only rig, and maybe get a nice RF burn, right?

So, what about radiation pattern and body tuning?