I'm thinking of taking the insulator and center conductor from coax and
sliding it in and out of a copper tube, maybe with threaded rod to hold
position and facilitate fine adjustment. I'm figuring on copper tape on a
hula-hoop as the loop, with the cap mounted directly to the hoop, and
copper foil tying the sides of the cap to the loop so I don't have to
halve the capacitance with a dual-plunger reimplementation of the
butterfly. What kind of capacitance per inch can I expect? Would a 36
inch diameter loop at 5 watts make enough voltage to puncture rg-58?
rg-8?
If it'll take a lot of sliding, I'll have to switch to dual-plunger
to avoid excessive conductor length and flexing.

I plan to mount it on a backpack, using dowels, or maybe PVC, to elevate
it for a foot or so seperation from the metal pack frame - cap at
top(another reason to keep the cap light). Will tuning change a lot
depending on whether the pack is worn? I really don't want to have to add
motorized tuning so I can tune it while worn... weight, don't you know?

One other possible deviation - I'm considering carrying it flat. My width
will be unchanged, but it'll lower height by 18" or allow another 18" of
elevation, and give me an omnidirectional horizontally-polarized pattern,
right?

At 5W, I'm not concerned about heating, and even any RF burns should be
manageable, but I want to be as efficient as I can be within the
constraints of /PM operation.

If I can successfully build this, I'll try to document it well, and make
the design available to others. At the very least, maybe we'll get some
cool ideas.

73,

tim

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?

Tim,

The voltage rating on coax is a lot higher than what you would calculate
based on max power. I can't find the numbers right now, but seem to recall
that RG58 was rated at 750 V or more. The big stuff is good for over 2000V.
The reason you don't want the end of the wire exposed is that it could lead
to arcing or corona. Probably not at low power, though. Sorry, can't answer
your other question.

Tam/WB2TT

Jim, NN7K wrote:
"---that center conductor of RG-8 was used as Power supply for lots of
AMPS! 3 and 4 KV in some of those."

Nothing wrong with that if the right RG-8 is used. RMS Max V varies from
600 V to 3700 V (Belden 9914). Peak a-c or d-c max V should be 5232 V
for the Belden cable. This is 1.414 X the rms volts. No need to
speculate. Coax tables abound. One is found in the ARRL Antenna Book.

Best regards, Richard Harrison, KB5WZI

On Wed, 06 Aug 2003 08:14:22 -0500, Richard Harrison wrote:
the Belden cable. This is 1.414 X the rms volts. No need to speculate.
Coax tables abound. One is found in the ARRL Antenna Book.

Good point - If I can get a dos or windows system up and running, I'll
model it with that magloop4.exe program. Regardless, I'd intended to pull
the center conductor back a half inch or so after stripping the cable to
the insulator. I actually intend to use rg-58 - smaller, lighter, and can
set the outer conductor closer to the center, for (I hope) higher
capacitance. I'm now considering mounting the sides of the cap to the
hula hoop with the hoop actually split there, so the hoop is stretched and
compressed as the capacitance is changed, to avoid tight flexure of
conductors or sliding contacts.

My biggest concern is dialectric puncture. I'll be running a maximum of
5W.

tim

Tim- at 5 watts, doubt that you could even puncture RG-174 (that the coax
about the thickness of 1/8 inch), that is used for test , and multiplex drop
cables! Jim NN7K




"Tim sent:
My biggest concern is dialectric puncture. I'll be running a maximum of
5W.

tim

"Tim Conway" wrote in message sethatanddropthis.com...
Good point - If I can get a dos or windows system up and running, I'll
model it with that magloop4.exe program.

Hi Tim,
Reg's programs run well in a DOS window of a Win2k system. Any system
from 386 to Pentium III should work for you.

BR from Ivan

On Wed, 06 Aug 2003 23:45:27 -0700, OK1SIP wrote:

Reg's programs run well in a DOS window of a Win2k system. Any system
from 386 to Pentium III should work for you.

Thanks, Ivan. I'm sure they do. I just don't have a working
microsoft-based system at the moment. I'm trying to get WINE up and
running, but my role as "Mr. Mom" keeps me from any long projects. It
took me two weeks to build a simple two-coil mobile HF antenna, just
because I can work on it for only a few minutes at a stretch.

One of these days, I'm going to have to move beyond just talking about it
and try to build one of these suckers.

73,

tcc