David
I reviewed your page to find out how the capacitor was made
and how it was driven. Unfortunately I could not figure it out!
One thing I thought about as I read the page was the soldered
joints and its added resistance. It would be interesting if some time
you could use some aluminum foil preferably with adhesive backing
to go from one end of the loop to the other in a continuous strip to
gauge
what difference the jointing made on the resonant frequency.
Adhesive tape can be found at any hardware store in the heating and
cooling
ducting section.
Regards
Art

Problem is, I have no idea how to afix a 16 foot vertical antenna to
the top
of my little four cylinder car!

Why do you need the extra large whip? A normal mobile antenna would be
quite
efficient on those bands. Thats what I'd do, if it were me...Not to
mention
my mobile antenna is good for 80-10...Give you something to do at night
when
the high bands are dead..MK

"art" == art writes:

art David I reviewed your page to find out how the capacitor was
art made and how it was driven. Unfortunately I could not figure
art it out!

The capacitor is made in a coaxial fashion with 3/4" polyethylene
"PEX" tubing as the dielectric, and copper pipe as the plates. The
inner copper tube (1/2" copper repair coupling) is the stator, and the
outer tube (3/4" copper repair coupling) the rotor. The rotor slides
over the joint between the dielectric and the stator (3/4" copper
pipe) which have the same outer diameter.

The capacitance is adjusted manually, by sliding the rotor from side
to side. I am relying on the tight fit and the large amount of
overlapping copper area between the rotor and its contact to keep
losses low. Also, because the sliding contact is at the high voltage
end of the loop, I presume resistive losses aren't as bad as they
would be at the bottom where voltage is low and current is high.

art One thing I thought about as I read the page was the
art soldered joints and its added resistance. [...]

Gee, how resistive is the solder? I know it's not as good a conductor
as copper, but the joints fit pretty well. I'm guessing the amount of
solder width in the electrical path is probably on the order of one
millimeter for the entire antenna. Given that the copper loop's DC
resistance is (in theory) around 40 milliohms, it's hard for me to
imagine that a solder "fuse" (if you will), connected in series, would
add much. But I'm prepared to learn something new -- it happens all
the time!

When I was first experimenting with the design, I had been worried
about the union joints, but they have something like 1cm^2 of surface
contact and the copper squishes together as the joints are tightened.

In any case, your suggestion about applying a continuous conductor and
checking for its influence is well taken. At least, I don't have a
horrible problem or the bandwidth would be too large and/or something
would be getting hot.

David
N5IZU

David

I have had good success with a vertical wire of whip that is longer that
a 1/4 wave on the lowest frequency with a variable capacitor at the base
for tuning. I use a sectional military vehicle whip that is about 20'
long mounted on a wooden broom handle that is pushed into the ground.
The capacitor is connected between the centre of the coaxial feed and
the bottom of the whip. Then it is just a case of connecting a couple of
radials for the band that you want to work and tuning the cap for
minimum VSWR. I have worked all over the world on the higher bands using
my FT-817 with 5W. it takes only a couple of minutes to set up and is
very efficient. The whole antenna weighs about a pound and will fit
inside a rucksack. I have used it with 100W even with a receiving
capacitor, as the cap is at a low voltage point there is no worry of
flashover.

If you want more information please email me and I will send you a picture.

Jon G2FHF



David B. Thomas wrote:
For portable operation in the 14-30 MHz range, I use a small, handheld
transmitting loop. Here is a web page describing it and how to build
one from inexpensive materials. The page also links to other pages
with good information on loops.

http://dt.prohosting.com/hacks/antenna/magloop.html

I chose the loop because it is actually possible to operate while
walking around. Obviously if you are able to erect a larger antenna
at your operating site, you would be able to get better performance.
My loop is about 30% efficient at 14 MHz, theoretically delivering
about one S-unit less than a half wave dipole.

David
http://www.qrz.com/n5izu

David
Thanks, it was the term "rotor" that threw me.
On the joint resistance question, from what I have heard over the years
you may well be surprised when you add the aluminum !
Art