On Feb 27, 12:54 pm, "KE5MBX" wrote:
I'm planning to get a Yaesu FT-857D and operate mobile HF on as many
bands as possible. I see two options so far for my Jeep Wrangler- a
102" whip with a tuner or a screwdriver antenna. Which is the better
way to go?


Do you need fast tuning from band to band?

Can you see the antenna from where you operate?

Do you want to operate while actually driving, or just when you're
parked.

I had a screwdriver on the roof of my VW Passat for a while. It was
tedious to adjust because I didn't have feedback about where it was
set. I then moved it to the front bumper, and it was a lot easier to
do band changes (you can quickly see when you're in the right
region). Worked greate for 10,12,15,17, and 20, because you can tune
to a visible mark, and then just check SWR. However, it's still a bit
tedious to tune for 40 and 75, because it has a pretty sharp
resonance, so if you QSY very far, you have to readjust the antenna.

There are some newer screwdriver antennas with automatic controllers
that make life much easier. They can count turns, or actually look at
the reflected power and adjust up and down for the match. However, you
still have the "tuning speed" issue.. that motor only moves so fast.

And, no matter how you look at it, you're only adjusting one parameter
(series L), which limits the best possible match you can get.
Granted, with the lossiness of a typical screwdriver, and the usual
clever 4:1 transformer with leakage C, you can do pretty well. better
than 2:1 from 10-40 is no problem and you can probably do somewhat
better.

A whip and auto tuner at the base of the antenna, on the other hand,
is virtually instantaneous to tune and requires no manual intervention
on the part of the operator. Squeeze the pickle and you're matched
and ready to go. I suspect that the loss in the tuner is not
materially different than in the variable loading coil, particularly
when you consider the mismatch from only having one adjustable device,
and besides, the dominant efficiency issue is going to be from having
a physically short radiator on lower frequencies, which is the same
regardless of how you tune it.

I didn't have very good luck with a single whip on a SGC tuner, (as
far as getting a match on all bands) but when I improvised a dualband
whip by wrapping another wire around the first whip (say, 25-30 ft of
AWG 16 hookup wire) and connecting them at the base, it works quite
well. (this is similar to what's inside the SGC whips and other
similar wideband whips... essentially it's like having two verticals
in parallel, with widely varying resonant frequencies... when you're
close to the resonance of one, the other just looks like a lumped L
or C in parallel, and the tuner takes it out)

On Feb 27, 6:34 pm, wrote:
On Feb 27, 12:54 pm, "KE5MBX" wrote:


And, no matter how you look at it, you're only adjusting one parameter
(series L), which limits the best possible match you can get.
Granted, with the lossiness of a typical screwdriver, and the usual
clever 4:1 transformer with leakage C, you can do pretty well. better
than 2:1 from 10-40 is no problem and you can probably do somewhat
better.

Not all screwdrivers are the same though.. Some are pretty close to
a bugcatcher as far as efficiency. Some are horrible due to excess
coil loss. In general, a screwdriver will usually be one notch down
from
a bugcatcher, which I consider king of the hill. The bugcatcher is
often better due to it's often better coil location. Most screwdrivers
have the coil fairly low, which limits them as far as current
distribution.
I don't use transformers to match per say.. I usually use an
inductor,
and this inductance can be included with the main lumped loading coil.


A whip and auto tuner at the base of the antenna, on the other hand,
is virtually instantaneous to tune and requires no manual intervention
on the part of the operator. Squeeze the pickle and you're matched
and ready to go. I suspect that the loss in the tuner is not
materially different than in the variable loading coil, particularly
when you consider the mismatch from only having one adjustable device,
and besides, the dominant efficiency issue is going to be from having
a physically short radiator on lower frequencies, which is the same
regardless of how you tune it.

The screwdriver is likely to be much more efficient than the tuner/
whip.
The efficiency is most certainly not the same for a given size whip.
A tuner/whip is likely to be 10-12 db down from a good bugcatcher,
or high quality screwdriver.
The current distribution through the whip effects the efficiency
greatly.
If you feed a whip with a tuner, maximum current is at the tuner.
If this tuner is in the car, maximum current is in the car with you.
A center fed antenna will be a good bit more efficient than a normal
base
fed for a given short height.
The tuner/whip can do semi ok on the higher bands, because it doesn't
take much there to begin with. On the low bands, they will be fairly
horrible.
Anyway, as far as the OP, the screwdriver would be much better, as
long as it's a decent screwdriver. Some screwdrivers have lousy coil
design, some don't.
MK