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Jim Lux · June 7th 10, 05:34 PM posted to rec.radio.amateur.antenna

wrote:
On Jun 3, 3:41 am, "-.-. --.-" wrote:
"-.-. --.-" ha scritto nel ...

Hello,
my mobile setup is composed by a 2 meter vertical whip feeded immediately
close to it by an automatic antenna tuner.
Missed that the expected frequency of the system is between 14 and 30 MHz,
but just curious if i had any chance to work 40 meters

-.-. --.-

It's possible.. But feeding a whip with a tuner usually does not
make
for an efficient mobile antenna. Not only are many/most tuners more
lossy than say using a loading coil on the whip, but current
distribution
suffers.

Can you give measured data for the losses? (or reasonably high fidelity
model data). I don't think the losses are *big* in either case, so you
might be looking at the difference between 5% loss and 7% loss, which is
negligible in the overall scheme of things.

I'd be willing to bet a six pack of frosty cold beverages (not the
antenna) that it's not a 75% loss vs 10% loss situation..

Maximum current will be at the tuner which is not desirable.

yes and no. I don't think, on a short antenna (2 meters long, here, but
let's say up to 3 meters) the difference will be significant;especially
when viewed in the context that the whip is next to a big giant metallic
object. They're ALL short compared to a wavelength, so the difference
in ideal gain is going to be somewhere between 1.64dBi (infinitely short
dipole) and 2.15dBi (half wavelength dipole)... And, given it's a
(mostly) vertical antenna, for which you have no real control over the
propagation path, who's to say that the fatter lobe on the infinitely
short dipole might not be better than the slightly skinnier one on the
half wavelength one.

(yes, it's a monopole..same idea though)

The location of the loading coil has a large effect on the current
distribution
and efficiency of the antenna.

Quantify "large"...

Is it bigger than 3 dB? (100%)
Bigger than 1 dB? (25%)

You're already taking a 5-6 dB hit just by having it bolted to a car
driving down the road. While I wouldn't say you should capriciously
throw away performance, you're already in a compromise situation.
Having less wind drag or a broader operating band might be a bigger
advantage than a dB or two.

Where you have it is about the worst
possible
place.
I have lots of people ask me about running whips matched with tuners..
I pretty much have a standard reply.. No! Not on my watch!
Chortle..
My mobile antenna is center loaded in the driving config.

And what's your operating bandwidth? Can you tune anywhere in the 40m
or 80m band? What's the efficiency of your system when you're not right
at the "sweet spot"...

The efficiency of the autotuner system is pretty constant across the band.

Now, it's possible all one needs to do is check into the nets at fixed
frequencies within a few kHz.. In which case the fixed tune system works
fine (assuming you've tuned it while actually driving... )

If you operate "mobile" (as opposed to portable, parked by the side of
the road), the autotuner takes care of the substantial change in antenna
impedance as the wind pushes it back. Or, one could mount it in the
center of the roof and guy it.. (been there, done that)

.. Even higher
if
I add the 3 foot lower mast, but that's only when parked. In the
parked config,
my loading coil is 8 ft above the base of the whip. "14 ft tall whip"
And yes, you can tell a pretty good difference from the normal driving
config,
with the coil at 5 ft above the base. "11 foot tall whip"

measured difference?