Antenna design question
 

Richard Clark wrote in
:

On Fri, 24 Oct 2008 09:48:43 -0400, Michael Coslo
wrote:

I've been working with mobile antennas for the past several months, and
I might be going astray, because I keep thinking about increased
bandwidth as a partner of lowered efficiency. Not likely the case here.

Hi Mike,

Only if you think of the vehicle body as the fat radiator and the
"mobile antenna" as a tuned radial. Unfortunately, that tuned radial
restricts the capacity of the fat radiator to achieve wide band
operation. Wide bandedness is a function of the complete system.
Unfortunately most look at the "radiator" and miss the necessity of
the counterpoise which demands an equally "large" contribution.

Agreed. while messing with my installation, I spent much more time with
bonding all my vehicle - did you know a Suzuki Vitara has an actual frame?
Many dozens of braid straps. It was all worth it though.

- 73 de Mike N3LI -

Antenna design question
 

On Oct 24, 4:31 pm, K7ITM wrote:
On Oct 24, 12:11 pm, wrote:

And, "end capacitance effect" is a poor model for what's really going
on. It's been used as an "explanation" for the observation that an
antenna that is slightly shorter than half a wavelength is resonant(as
in has no reactive component at the feedpoint). The problem is that an
infinitely thin dipole is resonant at less than 1/2 wavelength, and in
that case, there's no real "end" to have an effect.

?? I have been under the impression that in the limit as the
conductor radius goes to zero, the resonance does go to a freespace
half wavelength. You have to make the antenna _really_ thin to get
anywhere near that, though. Even a million to one length to diameter
ratio won't do it.


Half wavelength of zero radius has a feedpoint impedance of about
73.1+j42.5 ohms
(see, e.g. page 639 of Orfanidis's online electromagnetics book
(chapter 16), which gives a one page derivation)
(http://www.ece.rutgers.edu/~orfanidi/ewa/)

Roy's right about the spiral not converging to 377+j0.. I misread the
graph.

Jim

Antenna design question
 

Roy Lewallen wrote:

...
377 ohms is the impedance of free space, which is the ratio of E to H
fields of a planar TEM wave. People keep looking for this value when
dealing with antenna impedances, apparently due to the common
misconception that antennas are "transformers" of some sort to "match"
the impedance of free space (an E/H ratio) to a transmitter impedance (a
V/I ratio).
...
Roy Lewallen, W7EL

I believe the "misconception" is that the E/H fields induced by the
injection of an electromagnetic signal into the media (ether) ARE the
properties of the ether; I don't believe they are.

Rather, these fields are only the effect of the affect ...

Regards,
JS

Antenna design question
 

"R. Fry" wrote in :

"Michael Coslo"
I'm still left with the increased bandwidth phenomenon. None of the
above would seem to account for this.
____________

The reactance of a conductor with a relatively large cross-section
changes slower with a change in frequency than one having a small
cross-section.

Therefore its impedance bandwidth remains below a given limit over a
greater frequency span than one of a smaller cross-section.


I understand all that. I am however getting the feeling that we don't
understand just "why" that happens.

- 73 d eMike N3LI -