Is a Screw Driver antenna nothing more than an antenna tuner you mount
to a vehicle with a piece of wire attached to it?

On 10/3/2010 6:40 PM, Professor wrote:
Is a Screw Driver antenna nothing more than an antenna tuner you mount
to a vehicle with a piece of wire attached to it?

The first screwdriver antenna was homebrewed using a cheap cordless
screwdriver to adjust match ...

But, a real screwdriver antenna is one you have to get out and adjust
with a screwdriver every time you go over a big bump ... or it rains ...
or a bit of corrosion happens ... straight face

Regards,
JS

On 04/10/2010 02:40, Professor wrote:
Is a Screw Driver antenna nothing more than an antenna tuner you mount
to a vehicle with a piece of wire attached to it?

No.

--
Conor

I'm not prejudiced. I hate everyone equally.

On Sun, 3 Oct 2010 18:40:58 -0700 (PDT), Professor
wrote:

Is a Screw Driver antenna nothing more than an antenna tuner you mount
to a vehicle with a piece of wire attached to it?

Sort of.

After you accept that as the objective the clean up involves:
Physical details ... rugged beyond belief, small enough to be
acceptable... maintainable...weatherproof... under daily hurricane
conditions... affordable materials

install issues... different vehicles require a lot of flexibility in
mounting.

I have a project underway where I expect to have a Homebrew T-Match
tuner at the base of a vertical antenna at my QTH. I plan to drive
the mechanics with three $10 electric screwdrivers. Locating the
tuner at the antenna will allow the feed line to operate at a low SWR
which will keep feed line losses to a minimum and rf out of the shack.
Since the contraption will be in the garden shed most of the difficult
aspects of a vehicle mounted screwdriver antenna will be out flanked.

If you sit down and plan it through I expect you will conclude that
the only reasons that you might have to homebrew a vehicle mounted
Home brew Screwdriver antenna is that you have excess time and
materials or you think you can improve on the commercial offerings.

Sometimes my only real motivation is that I have to homebrew an item
before I can fully appreciate the commercial offerings!

BTW, an SGC-237 tuner and a 28 foot whip will tune all of the HF ham
bands automatically and fast! That set up will cost a lot more than a
commercial screwdriver but you can consider that as an ideal
arrangement and compromise your way back from there.

Good Luck,

John Ferrell W8CCW

On Oct 4, 9:12*am, John Ferrell wrote:
BTW, an SGC-237 tuner and a 28 foot whip will tune all of the HF ham
bands automatically and fast! *... you can consider that as an ideal
arrangement ...

The feedpoint impedance of a 28 foot whip on 3.8 MHz is ~5-j620 ohms
(according to EZNEC) which means considerable tuner/ground losses
(~9dB) = not ideal IMO, compared to a 1/4WL monopole. If one wants
reasonable performance on 80m, best to stick with not much less than
3/16WL = ~46'.

The Take-Off-Angle of a 28 foot whip on 28.4 MHz is ~43 degrees = not
ideal IMO, compared to a 5/8WL monopole. If one wants reasonable
performance on 10m, best to stick with not much more than 5/8WL =
~23'.

There's no single monopole length that is optimum from 3.5 MHz to 29
MHz. Just about the best compromise we can reach with an autotuner at
the base, IMO, is the 46' monopole for 80m-20m operation or the 23'
monopole for 40m-10m operation.
--
73, Cecil, w5dxp.com

Cecil Moore wrote:
On Oct 4, 9:12 am, John Ferrell wrote:
BTW, an SGC-237 tuner and a 28 foot whip will tune all of the HF ham
bands automatically and fast! ... you can consider that as an ideal
arrangement ...

The feedpoint impedance of a 28 foot whip on 3.8 MHz is ~5-j620 ohms
(according to EZNEC) which means considerable tuner/ground losses
(~9dB) = not ideal IMO, compared to a 1/4WL monopole. If one wants
reasonable performance on 80m, best to stick with not much less than
3/16WL = ~46'.

The Take-Off-Angle of a 28 foot whip on 28.4 MHz is ~43 degrees = not
ideal IMO, compared to a 5/8WL monopole. If one wants reasonable
performance on 10m, best to stick with not much more than 5/8WL =
~23'.

There's no single monopole length that is optimum from 3.5 MHz to 29
MHz. Just about the best compromise we can reach with an autotuner at
the base, IMO, is the 46' monopole for 80m-20m operation or the 23'
monopole for 40m-10m operation.


Which is why the whips that SGC sells (and others use) are typically two
monopoles in one.. I have fairly good luck with a 9ish foot CB whip with
about 25-30 feet of wire roughly spiraled up it. The precise length
isn't real critical, as long as you don't wind up with an anti-resonance
in a ham band making it hard to match.