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

-.-. --.-

On Thu, 03 Jun 2010 10:08:15 +0200, -.-. --.- rearranged some electrons to
say:

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,

-.-. --.-

The tuner will provide an impedance match to your transceiver (50 ohms).
It won't make your antenna resonant on 20m (or any other band, other than
2m).

On 06/03/2010 04:08 AM, -.-. --.- wrote:
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 ?
Hello, and that's sort of the ultimate goal. The tuner is designed to
match the antenna type(s) to the source (transmitter) and transmission
line design (characteristic) impedance.

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 ?

The problem here is an impedance analyzer can't distinguish between
resistive losses (antenna, ground, tuner (if considered part of the
antenna)) and the radiation resistance of the antenna. To determine
efficiency you'd have to make some field strength measurements (usually
performed with a calibrated field strength meter) in order to determine
how much of the power going into the antenna terminals is being radiated
into free space. The only "approximation" would be to measure the
resistive part of the antenna feedpoint impedance and then subtract from
this the radiation resistance obtained from calculation. Sincerely, and
73s from N4GGO,
--
John Wood (Code 5520) e-mail:

Naval Research Laboratory
4555 Overlook Avenue, SW
Washington, DC 20375-5337

On Jun 3, 3:08*am, "-.-. --.-" wrote:
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 ?

If you perform a frequency sweep with an antenna analyzer connected to
the tuner input and the impedance goes from R-jX ohms to 50+j0 ohms to
R+jX ohms, the purely resistive frequency meets the definition of the
*system* resonant frequency. Unfortunately, under those extremely
lossy conditions, "system resonance" means almost nothing.
--
73, Cecil, w5dxp.com

On Jun 3, 5:15*am, david wrote:
On Thu, 03 Jun 2010 10:08:15 +0200, -.-. --.- rearranged some electrons to
say:





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,

-.-. --.-

The tuner will provide an impedance match to your transceiver (50 ohms). *
It won't make your antenna resonant on 20m (or any other band, other than
2m).- Hide quoted text -

- Show quoted text -

I think "CQ" meant the whip was 2 meters long. That would be 1/4 wave
on 38 MHZ if I did the math right. This is the 8 M band? The tuner
might get it on 20 M, but 40 M will be a stretch.

Gary N4AST

-.-. --.- 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)

J.B. Wood wrote:

The problem here is an impedance analyzer can't distinguish between
resistive losses (antenna, ground, tuner (if considered part of the
antenna)) and the radiation resistance of the antenna. To determine
efficiency you'd have to make some field strength measurements (usually
performed with a calibrated field strength meter) in order to determine
how much of the power going into the antenna terminals is being radiated
into free space. The only "approximation" would be to measure the
resistive part of the antenna feedpoint impedance and then subtract from
this the radiation resistance obtained from calculation. Sincerely, and
73s from N4GGO,

A good deal of, if not the majority of, the loss in a short antenna
system is in the matching system components. So even if this method
allowed you to get a reasonable estimate of the efficiency of the
antenna itself(*), you still wouldn't know what fraction of the
transmitter power is getting radiated, since you can't tell how much is
lost in the tuner.

(*)My limited experience in doing careful antenna measurements leads me
to be very skeptical of the ability to determine antenna efficiency even
very roughly by a single impedance measurement. I think comparison of
measured bandwidth to lossy model results is probably the best indicator.

The bottom line is that the impedance meter won't tell you much about
the efficiency of the antenna or system. About the only practical way
available to most amateurs is comparison of received signal strengths
between the antenna and a known reference antenna, using a step
attenuator to measure the difference.

Roy Lewallen, W7EL

On Jun 3, 7:41*am, Gary wrote:
On Jun 3, 5:15*am, david wrote:



On Thu, 03 Jun 2010 10:08:15 +0200, -.-. --.- rearranged some electrons to
say:

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,

-.-. --.-

The tuner will provide an impedance match to your transceiver (50 ohms).. *
It won't make your antenna resonant on 20m (or any other band, other than
2m).- Hide quoted text -

- Show quoted text -

I think "CQ" meant the whip was 2 meters long. *That would be 1/4 wave
on 38 MHZ if I did the math right. *This is the 8 M band? *The tuner
might get it on 20 M, but 40 M will be a stretch.

Gary N4AST

Gary I hope your interpretation is correct. I used to operate a
homebrew 2m 5/8 WL on 6 10 and 15 by shorting out the coil and using a
tuner. Worked fairly decently for local rag chewing.

Jimmie

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.