On Jun 7, 1:23*pm, Roy Lewallen wrote:
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.

Which is why, in this case, it is well to note that the 0 dB top-rated
antenna and the -12 dB antenna were mounted on the same vehicle
(mine).
--
73, Cecil, w5dxp.com

Roy Lewallen wrote:
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

that seems quite plausible. A bigger vehicle essentially means a
physically larger antenna (think of the whole system as a dipole fed off
center, and a fan on one side but not the other.

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 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

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 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

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

*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 radiation resistance present at the base of an electrically short,
linear, monopole (whip) antenna of various ODs can be calculated
rather accurately using equations found in various antenna engineering
textbooks

So if the base current entering the whip itself can be measured, and
whether or not a loading coil was used at the base to resonate* the
antenna system, and regardless of the loss in the r-f ground used by
the whip -- then for practical purposes the power radiated by the
antenna system will be the product of the square of that base current,
and the radiation resistance of the whip.

* However "resonated" antenna systems using a short whip do not have
the same radiation performance as those using a naturally resonant 1/4-
wave monopole. Resonance just means that the capacitive reactance at
the feedpoint of the short whip has been exactly offset by the
inductive reactance of a loading coil, which can allow for the most
efficient power transfer into the radiator.

However the radiation resistance of the whip is not changed by this
process -- it is still very low compared to a naturally resonant 1/4-
wave monopole. Therefore the radiation efficiency of such a short,
loaded, whip antenna system cannot approach that of a naturally
resonant 1/4-wave monopole unless the losses in the loading coil and r-
f ground are nearly zero.

Loading coils do not replace the "missing degrees" of an electrically
short radiator, as far as its radiation resistance is concerned. The
radiation resistance of a whip depends only on the electrical length/
OD of the whip, itself.

RF

Richard Fry wrote:
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 radiation resistance present at the base of an electrically short,
linear, monopole (whip) antenna of various ODs can be calculated
rather accurately using equations found in various antenna engineering
textbooks . . .

This is true only if you don't confuse the idealized textbook models
with real antennas. But most of us are unfortunately stuck with using
the latter. In general, the impedance you calculate with the idealized
models doesn't match that of real world antennas. It works pretty well
for AM broadcast installations, where the length and large number of
radials make the impedance relatively independent of ground
characteristics. But this doesn't describe the typical amateur monopole
antenna, either ground or mobile mounted.

An approximation to input resistance can be made by adjusting for an
abbreviated radial system, but this gets increasingly unreliable as the
number of radials decreases. The best readily available modeling program
allowing the inclusion of a buried ground system, which uses the same
well-established equations as textbooks, is NEC-4. It, however, suffers
from a serious shortcoming in doing this calculation -- it assumes that
the ground is homogeneous to an infinite depth. Real ground is typically
stratified, and skin depth at HF is as much as several tens of feet, so
the representation of real ground is very poor. There are many cases
where a single "equivalent" value of homogeneous ground doesn't exist
which gives the same results as actual measurement. I've made very
careful measurements of a simple vertical monopole with various numbers
of buried radials whose impedance couldn't be matched with NEC-4 using
any ground parameters, and I believe this to be a common occurrence. In
no case would I depend on a computer model, let alone an even more
simplified textbook model, to predict the resistance of a real monopole
having an abbreviated ground system with enough accuracy to reasonably
estimate the efficiency.

As a side note, Brown, Lewis, and Epstein's sparse radial results can be
matched reasonably well with NEC-4, but it does require a fair amount of
ground constant adjustment for various numbers and lengths of radials.

Mobile mounted whip antennas fare even worse relative to simple textbook
models. I don't have any experience with comparison of computer models
with actual measurement. Those results should depend on the care with
which the model is constructed and the amount of influence the ground
has on the impedance.

Roy Lewallen, W7EL

On Jun 4, 9:53*am, Roy Lewallen wrote:
Richard Fry wrote:
*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 radiation resistance present at the base of an electrically short,
linear, monopole (whip) antenna of various ODs can be calculated
rather accurately using equations found in various antenna engineering
textbooks . . .

This is true only if you don't confuse the idealized textbook models
with real antennas. ...

For the sake of discussion, below are two pastes from the same NEC
model using the demo version of EZNEC v. 5.0 -- which rather well
support my earlier post that the radiation resistance (NOT the
impedance) of an electrically short monopole is a function of its
electrical length, and not the loss resistance of the r-f ground and/
or the loading coil.

CASE 1 = Zero loss resistance and reactance in the r-f ground, and
zero loss resistance in the loading coil:

EZNEC Demo ver. 5.0

1650 kHz 3 meter monopole 6/4/2010 10:50:57 AM

--------------- SOURCE DATA ---------------

Frequency = 1.65 MHz

Source 1 Voltage = 0.08578 V at 35.09 deg.
Current = 0.4986 A at 0.0 deg.
Impedance = 0.1408 + J 0.09888 ohms
Power = 0.035 watts
SWR (50 ohm system) 100 (25.17 ohm system) 100


CASE 2 = Same model as above, except with a total of 25 ohms loss in a
loading coil and r-f ground, and no reactance in the r-f ground:

EZNEC Demo ver. 5.0

1650 kHz 3 meter monopole 6/4/2010 10:49:40 AM

--------------- SOURCE DATA ---------------

Frequency = 1.65 MHz

Source 1 Voltage = 0.9386 V at 0.22 deg.
Current = 0.03729 A at 0.0 deg.
Impedance = 25.17 + J 0.09579 ohms
Power = 0.035 watts
SWR (50 ohm system) = 1.987 (25.17 ohm system) =
1.004

EZNEC calculated the radiation resistances of these two cases to be
0.14 ohms and 0.17 ohms, respectively -- fairly close, but not exact.
Perhaps Roy could comment on the reason why their agreement using NEC/
EZNEC is not better.

Those wanting a good resource for the measured results for monopoles
of less than 1/8 electrical wavelength might try to locate the paper
by Carl E. Smith and Earl M. Johnson titled PERFORMANCE OF SHORT
ANTENNAS, published in the October, 1947 edition of the Proceedings of
the I.R.E.

The equation for the radiation resistance of short antennas given in
that paper is independent of the resistive losses in any loading coil
or r-f ground system.

RF