Hello,

my mobile setup is composed by a 2 meter vertical whip feeded immediately
close to it by an automatic antenna tuner.

My poor knowledge of antenna systems think that the ATU, choosing the
appropriate LC value, bring the entire system in a resonant condition: true
or false ?

And , second question, because a friend of mine own a MFJ 269 analyzer, how
i can *approximately* have an idea of the efficiency on a frequency F for my
whip ?

Thanks in advance,

-.-. --.-

"-.-. --.-" ha scritto nel messaggio
...
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

-.-. --.-

-.-. --.- wrote:
"-.-. --.-" ha scritto nel messaggio
...
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

-.-. --.-


presuming you mean your whip is 2 meters long (not tuned for the 2 meter
band, and only 50 cm long)

On 40m? Maybe, maybe not. people have used a light bulb as an antenna.

It will almost certainly be inefficient. Here's a short description
(filled with technical detail errors, which will no doubt provoke a long
discussion)..

All antennas have a property called "radiation resistance" (smaller
antennas have smaller radiation resistances)
All antennas also have resistive losses.
There are also losses "after the wave has left the antenna" (e.g. ground
reflections, etc. why salt water marshes are prized for vertical pol).
The latter are not considered here.


To a first order (back, all you nit pickers.. first order), the power
from the transmitter gets distributed between the radiation resistance
and the loss resistance. So, the "efficiency" can be thought of as that
fraction of power that winds up in the radiation resistance, as opposed
to in the loss resistance.

Important he if the radiation resistance is low, that doesn't mean
poor efficiency: as long as you keep the resistive losses low too..
which can be a challenge (assuming you're not carrying a vat of liquid
helium, for instance).

Part of the problem is that the resistive losses aren't just in the
antenna, but also in any substance which is immersed in the antenna's
electric and magnetic fields (like the steel of your car, and the not
particularly good conductivity, but not a perfect insulator either, soil
under the car).


And, then, there are losses in how you get from Tx to Antenna. If your
antenna presents an impedance that is not what the transmitter is
providing, you've got to transform it somehow, typically using Ls and
Cs, etc. (in your autotuner). Those components also have some amount of
loss, although I'll bet it's less than 10% in most situations
(otherwise, the tuner/matching network would melt, and they don't)



Moral of story..
radiation resistance doesn't drop much from a full size antenna until
you get around 1/10th wavelength, then it starts to drop real fast (as
length squared)

For a dipole:
L/lambda = 1/2, R= 73 ohms
L/lambda = 1/5, R = 8
1/10, 2
1/20, 0.5
1/50, 0.08
1/100, 0.02

On 40m, your 2m long whip is like a 4m long dipole: 1/10 (in the above
table) for 2 ohms. Since it's a monopole, it's actually half.. 1 ohm..

That's pretty low..

A bunch of people have measured loss resistances for typical mobile
installations and they get numbers in the 10-20 ohms range, so you're
looking at an efficiency of about 10% of what you'd get with a full
sized 10m tall vertical.
(this isn't far off the "mobile antenna is a 6dB hit" empirical observation)

I'm assuming here that somehow you'll be able to match the 50 ohms in
the feed line to the 10-20 ohms with a lot of reactance at the feedpoint
without too much loss (a reasonable assumption)

On Jun 3, 1:21*pm, Jim Lux wrote:
-.-. --.- wrote:
"-.-. --.-" ha scritto nel messaggio
...
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

-.-. --.-

presuming you mean your whip is 2 meters long (not tuned for the 2 meter
band, and only 50 cm long)

On 40m? *Maybe, maybe not. * people have used a light bulb as an antenna.

It will almost certainly be inefficient. *Here's a short description
(filled with technical detail errors, which will no doubt provoke a long
discussion)..

All antennas have a property called "radiation resistance" *(smaller
antennas have smaller radiation resistances)
All antennas also have resistive losses.
There are also losses "after the wave has left the antenna" (e.g. ground
reflections, etc. *why salt water marshes are prized for vertical pol).
* The latter are not considered here.

To a first order (back, all you nit pickers.. first order), the power
from the transmitter gets distributed between the radiation resistance
and the loss resistance. *So, the "efficiency" can be thought of as that
fraction of power that winds up in the radiation resistance, as opposed
to in the loss resistance.

Important he if the radiation resistance is low, that doesn't mean
poor efficiency: as long as you keep the resistive losses low too..
which can be a challenge (assuming you're not carrying a vat of liquid
helium, for instance).

Part of the problem is that the resistive losses aren't just in the
antenna, but also in any substance which is immersed in the antenna's
electric and magnetic fields (like the steel of your car, and the not
particularly good conductivity, but not a perfect insulator either, soil
under the car).

And, then, there are losses in how you get from Tx to Antenna. *If your
antenna presents an impedance that is not what the transmitter is
providing, you've got to transform it somehow, typically using Ls and
Cs, etc. (in your autotuner). *Those components also have some amount of
loss, although I'll bet it's less than 10% in most situations
(otherwise, the tuner/matching network would melt, and they don't)

Moral of story..
radiation resistance doesn't drop much from a full size antenna until
you get around 1/10th wavelength, then it starts to drop real fast (as
length squared)

For a dipole:
L/lambda = 1/2, R= 73 ohms
L/lambda = 1/5, R = 8
1/10, 2
1/20, 0.5
1/50, 0.08
1/100, 0.02


I checked this out one time and my memory is a bit hazy. Using my
computer program which ofcourse could have a glitch in it, I found
that the gain hit a max at the very low resistance value and then
backed off as the resistance became close to zero. This blip occurred
during the last fraction of an ohm
just prior to closure of supplying gain figures.
At the time I put this down as the point where the skin depth
penetration was minimal and thus we had reached a point where the
applied current was totally to provide gain after which it dropped
dramatically. I looked at this as a progression for the current flow
in the surface of the element where it progressed to the surface and
the flow
progressed above the surface but below the particle encapsulation.
Further progression created losses and thus the gain blip dropped.
Resistance never dropped to zero thus ohms law
was not declared invalid. If somebody could duplicate the above via
another program it may prove interesting. Keep- dropping the applied
frequency until you reach about 0.7 ohms and then start recording.

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. Maximum current will be at the tuner which is not desirable.
The location of the loading coil has a large effect on the current
distribution
and efficiency of the antenna. 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.. 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"

On Jun 3, 11:03*pm, wrote:
But feeding a whip with a tuner usually does not
make for an efficient mobile antenna.

A 11.5 foot (~3.5m) whip driven by an SG-230 autotuner was measured to
be 12 dB down from the top-rated bugcatchers and screwdrivers at one
of the CA 75m mobile shootouts back in the 1980's.
--
73, Cecil, w5dxp.com

Cecil Moore wrote:
On Jun 3, 11:03 pm, wrote:
But feeding a whip with a tuner usually does not
make for an efficient mobile antenna.

A 11.5 foot (~3.5m) whip driven by an SG-230 autotuner was measured to
be 12 dB down from the top-rated bugcatchers and screwdrivers at one
of the CA 75m mobile shootouts back in the 1980's.


that's a pretty big difference.. (12 dB implies a factor of 16.. that's
like most of the Tx power being dissipated somewhere, and that sounds
like "component melting" levels)

Have you a link to the data and test methodology?

On Jun 7, 11:36*am, Jim Lux wrote:
* Have you a link to the data and test methodology?

I summarized the data from three CA 75m mobile shootouts at:

http://www.w5dxp.com/shootout.htm

I don't recall a test methodology being published. The test receiver
consisted of a ferrite loop antenna in the far field feeding a lab-
grade RF voltmeter. The power incident upon the 75m mobile antenna
system was assumed to be forward power minus reflected power on the
coax to the antenna system, measured using two Birds. The receive
results were normalized accordingly. I may have left out a detail or
two.

The SG-230 plus 11.5 whip at -12 dB was equal to a 75m hamstick. I
entered both the top-rated (0 dB reference) antenna and the (-12 dB)
autotuner+whip on the same vehicle. When I "superposed" all of the
three results, I assumed 0 dB for each top-rated antenna and let the
rest fall where they might. That may or may not have been a reasonable
assumption.

I suspect the SG-230 is designed to dissipate 100 watts (using large
#2 material powdered-iron toroids). During one shootout episode, I
forgot to attach the antenna to the mobile mount. The SG-230
faithfully tuned to close to a 1:1 match on the input - with a near-
infinite SWR on the output. It was a damp foggy day and the mobile
mount arced. That taught me not to mount the SG-230 unobserved in the
attic. :-)
--
73, Cecil, w5dxp.com

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?

In the admittedly very few looks I've had at mobile "shootout" results,
there seems to be more of a correlation between vehicle size and field
strength than antenna and field strength. This comes as no surprise,
since the vehicle is usually a comparable or even greater part of the
radiating system than the titular antenna, and its coupling to ground
has a large impact on the efficiency.

Roy Lewallen, W7EL