-.-. --.- 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)