On Mon, 27 Sep 2010 03:26:52 -0700 (PDT), wrote:

I preferred my plastic bugcatchers so I could
use a higher coil location for more efficiency and better current
distribution.

Hi Mark,

That was my point about the screwdriver being run as a elevated
vertical dipole.

In point of fact, that is how it is designed to work (after a
fashion). The bumper mount (or car chassis attachment) is the
grounded lower element, then we have a coil of variable length,
followed by the upper element with the feed hidden inside the lower
element.

I don't see that radials add anything to this. Their radiative
contributions all cancel anyway if they are straight out, and they add
towards approximating a vertical element if they droop. If,
perchance, you wanted short radials (straight out or drooping), then
it would only require more inductance to match the capacitive
reactance. Push the tune button and walla! (as the french would say),
more turns of the coil out there. And for being a sleeve dipole, that
coil would be quite high - current distribution would come out in the
wash.

73's
Richard Clark, KB7QHC

Richard Clark wrote:

That was my point about the screwdriver being run as a elevated
vertical dipole.

So why not just use two screwdriver antennas with each antenna being used
as half of the "dipole"? Since Don Johnson was mentioning this being
done in the 90's by apartment dwellers, it's obviously been tried before
and may even work. It likely won't be the most efficient at getting the
most RF out but likely will work.

I note incidentally that homebrew screwdrivers would drastically lower
the cost of this experiment.

Art Clemons wrote:
Richard Clark wrote:

That was my point about the screwdriver being run as a elevated
vertical dipole.

So why not just use two screwdriver antennas with each antenna being used
as half of the "dipole"? Since Don Johnson was mentioning this being
done in the 90's by apartment dwellers, it's obviously been tried before
and may even work. It likely won't be the most efficient at getting the
most RF out but likely will work.

I note incidentally that homebrew screwdrivers would drastically lower
the cost of this experiment.

Sure, you could do this, but there's really not much advantage over a
single screwdriver and a fixed element opposite it. Either way, you're
basically putting an adjustable inductor in series with the feed.
Whether it's symmetric is almost immaterial, especially for a short
(relative to wavelength) radiator.

Short antennas look capacitive. Putting in a series L compensates for
that, but doesn't do anything about the low resistance at the feedpoint.
What most mobile screwdrivers do is put a 4:1 transformer at the
feed.. that gets a nominal 12.5 ohms up to 50, the inductance takes care
of the reactive component, and you're "good enough".

In fact, it's typically even more clever, the 4:1 transformer has a fair
amount of leakage capacitance, so at higher frequencies, it looks more
like a 1:1, which works out nicely, since the feedpoint R is closer to
50 ohms with a longer (in terms of wavelength) radiator.

More details and measurements he
http://home.earthlink.net/~w6rmk/ant...crewdriver.htm


I think there are better ways to do a space limited antenna.

A guy here in Tampa has a screwdriver mounted to a metal guard rail on his 15 story condo overlooking Tampa Bay. He gets out very well from his Tampa QTH.