The impedance of free space / air is said to be 377 ohms. Impedance is ratio
of E/H. The feedpoint impedance of an antenna is usually 50 or 75 ohms.

Can an antenna ever be regarded as a transducer that transforms a radio wave
from 50 ohms to 377 ohms i.e. provides an impedance transformation? With a
long tapered antenna, the feedpoint is at 50 ohms. Is the end of the antenna
at 377 ohms to launch the wave easily into free space? In this case, antenna
is a travelling wave antenna e.g. broad bandwidth biconical. Does the
impedance gradually change from 50 ohms to 377 ohms over the length of the
antenna?

The impedance of the end of an antenna (open circuit), where it is a high
voltage point, is usually 5K or 10K ohms.

David wrote:
The feedpoint impedance of an antenna is usually 50 or 75 ohms.

The feedpoint impedance of a resonant standing wave
antenna is a result of interference/superposition and
is a virtual impedance equal to

(Vfor-Vref)/(Ifor+Iref)

The characteristic impedance of a standing wave antenna
is actually several hundred ohms.
--
73, Cecil http://www.w5dxp.com

On Mon, 4 Sep 2006 20:36:00 +0100, "David" nospam@nospam wrote:

The impedance of free space / air is said to be 377 ohms. Impedance is ratio
of E/H.

Hi David,

True only in free space.

The feedpoint impedance of an antenna is usually 50 or 75 ohms.

Rarely true, but to the point of your posting, this is not
substantially wrong.

Can an antenna ever be regarded as a transducer that transforms a radio wave
from 50 ohms to 377 ohms i.e. provides an impedance transformation?

A transducer changes energy between domains. That is from electrical
to acoustic, or back again. Or between electrical to photo-electric,
or back again. And so on. What you are describing is transforming
and the appropriate device name would be transformer, not transducer.

To the intent of your statement, yes, an antenna is a transformer.

With a long tapered antenna, the feedpoint is at 50 ohms. Is the end of the antenna
at 377 ohms to launch the wave easily into free space?

The ends of an antenna are no more important than the middle.
Radiation occurs on the basis of the entire radiator, not simply
"some" parts.

In this case, antenna
is a travelling wave antenna e.g. broad bandwidth biconical. Does the
impedance gradually change from 50 ohms to 377 ohms over the length of the
antenna?

In fact it is quite the opposite. The biconical is a classic constant
impedance structure. Thin antennas such as the typical doublet or
dipole are non-linear.

The impedance of the end of an antenna (open circuit), where it is a high
voltage point, is usually 5K or 10K ohms.

This is a characteristic, not an explanation.

73's
Richard Clark, KB7QHC

David:

[snip]
"David" nospam@nospam wrote in message
...
The impedance of free space / air is said to be 377 ohms. Impedance is
ratio
of E/H. The feedpoint impedance of an antenna is usually 50 or 75 ohms.
Can an antenna ever be regarded as a transducer that transforms a radio
wave
from 50 ohms to 377 ohms i.e. provides an impedance transformation? With a
[snip]

The answer is a "considered" yes!

Although the [so-called] term "characteristic impedance", often labelled as
Zo, and units of Ohms are often used to describe a certain characteristic of
a propagation media in field theory, that governs the ratio of the E to H
fields propagating in the media. This "characteristic impedance Zo" is not
the same thing as "driving point or feed point impedance Z" in circuit
theory. Although closely related, circuit theoretic concepts and field
theoretic concepts are different views of electromagnetic phenomena.

Characteristic impedance Zo and the units of Ohms are often used as the
"name" for the square root of the ratio of mu the magnetic permeability is
[u = 1.257E-7 for free space] to epsilon the electric permittivity of a
propagating media [e = 8.85E-12 for free space]

Maxwell's celebrated equations then result in the fact that...

Zo = E/H = sqrt[u/e] = sqrt[(1.257E-7)/(8.85E-12)] = 376.7 Ohms ~ 120pi Ohms

This Zo is not the same as a "feedpoint impedance" Z which is the ratio of
voltage V to current I.

Z = V/I Ohms.

That said it should be recognized that any radiating antenna is "immersed"
in a propagating media, usually free space, and the u and e of that media do
have an important affect on the "characteristic impedance, or surge
impedance" of the antenna which will in turn affect the driving point or
feedpoint impedance of the antenna.

For instance it is well known that the resonant feedpoint impedance (Ratio
of V to I) at the center of a half wave dipole in free space is 73 Ohms. If
that dipole were placed in another medium other than free space with
correspondingly different u and e, the driving point impedance of the dipole
would definitely be affected. So would it's resonant frequency, etc...

And so in that sense, an antenna may be considered to be a transducer and
not a transformer. Antennas may then be viewed as transducers that
transduce the circuit theoretic variables of electric currents and voltages
flowing in and between conductors into field theoretic variables of electric
and magnetic fields flowing through a propagation media. And... indeed
there is a "reaction" between the u and e of the media in which the antenna
is immersed and the currents and voltages flowing in and on the antenna.

The 73 Ohm driving point impedance of a free space half wave resonant dipole
[in the ideal case this is the radiation resistance] is a direct result of
the u and e of the free space in which the antenna is immersed.

If all other things were held constant and the values of u and e of the
medial were changed [i.e. move the antenna from free space where u/e=377 to
be under water where u/e=x??? the driving point impedance of the antenna
would most certainly change!

[snip]
long tapered antenna, the feedpoint is at 50 ohms. Is the end of the
antenna
at 377 ohms to launch the wave easily into free space? In this case,
antenna
is a travelling wave antenna e.g. broad bandwidth biconical. Does the
impedance gradually change from 50 ohms to 377 ohms over the length of the
antenna?
[snip]

No! Not really.

Surprisingly, the actual surge or characteristic impedance Zo of a single
wire antenna in free space, considered as a one wire transmission line
placed high over a ground plane [the earth] is actually in the neighbourhood
of several hundred Ohms... say 600Ohms or so.

The exact value of Zo is easily calculated by well known transmission line
formulas, that assume TEM mode propagation on the line, and this Zo
basically depends upon the height over ground and the diameter of the wire.
This is not a driving point impedance but is a "surge impedance". The
driving point impedance of the single wire transmission line depends upon
where and at what frequency it is "driven" by a source.

Of course because this single wire is quite distant from it's return path
[ground] this single wire transmission line is "leaky". That is it radiates
and loses, or dissipates, power to some extent, as opposed to what it might
do if it were placed very close to the ground where there was a nearby field
"cancelling" current flow. [a microstrip transmission line for instance].

We know that if this single wire transmission line high above the earth is
driven by a source it will exhibit a driving point impedance that depends
upon its length relative to the wavelength of the driving voltage or
current. [73 Ohms resistive if it is a 1/2 wave, some other in general
complex Z if it is not 1/2 wave.

[snip]
The impedance of the end of an antenna (open circuit), where it is a high
voltage point, is usually 5K or 10K ohms.
[snip]

I believe that you are referring to the driving point impedance of an end
fed half wave dipole which is certainly high and in that neighbourhood.
This is not a characteristic or a surge impedance.

And so in summary...

An antenna may be thought of as a transducer between a circuit theoretic
electro-magnetic venue and wave propagation in a propagating media, but the
relationship between the circuit driving point impedance and the
characteristic impedance of the media is quite complex and is certainly not
a simple linear relationship such as found in a transformer or other device.

As far as I understand there is no practical application that has ever
required anyone to quantitatively determine the exact relationship between
the Zo of a propagating media and the driving point impedance Z of an
antenna that is immersed in that media.

In my opinion such a determination would be a very difficult
experimental/engineering exercise. The experimenal problem is one of how
does one vary the Zo of a media while measuring the effect on the Z at the
driving point? Here's a thought experiment...

Immerse an antenna in a liquid media with a given u and e in an anechoic
tank then drive the antenna with a generator while measuring the driving
point impedance (V and I) and then pour or mix in some other liquid with
different u and e and observe the change in Z.

Would that work?

It could also be accomplished numerically on a computer by using a program
[like the NEC programs] based on solving Maxwell's partial differential
equations iteratively.

As far as I know no one has ever attempted to do this... and notwithstanding
the possibility for "invention" or "discovery" I might ask, why would one
want to do this?

Hey it might make a good Ph.D. or M.Sc. thesis... but what is the practical
application?

For all practical purposes, the characteristic impedance of the media in
which antennas are immersed never changes!

Who cares how Z varies when Zo varies?

Thoughts, comments?

--
Pete k1po

Peter O. Brackett wrote:
. . .
In my opinion such a determination would be a very difficult
experimental/engineering exercise. The experimenal problem is one of how
does one vary the Zo of a media while measuring the effect on the Z at the
driving point? Here's a thought experiment...

Immerse an antenna in a liquid media with a given u and e in an anechoic
tank then drive the antenna with a generator while measuring the driving
point impedance (V and I) and then pour or mix in some other liquid with
different u and e and observe the change in Z.

Would that work?

It could also be accomplished numerically on a computer by using a program
[like the NEC programs] based on solving Maxwell's partial differential
equations iteratively.

As far as I know no one has ever attempted to do this... and notwithstanding
the possibility for "invention" or "discovery" I might ask, why would one
want to do this?

Hey it might make a good Ph.D. or M.Sc. thesis... but what is the practical
application?
. .

It would be one of the easiest degrees ever attained. NEC-4 allows
setting the primary medium to any (reasonable) value of conductivity and
permittivity, so you can have the answer in seconds with a free space
analysis. Alternatively, you can bury the antenna deep in NEC-4's ground
medium and define the ground characteristics for your test.

I did a short consulting job a while back for some people interested in
transmitting RF for short distances under water. Immersing the antenna
eliminates the substantial signal loss incurred by reflection at the
air-water interface when the antenna is out of the water. And antenna
system design requires knowledge of the antenna feedpoint Z. I've seen
numerous papers in the IEEE publications about antennas immersed in
other media such as a plasma, and know that antennas buried in the
ground are used. So it's of considerable practical interest.

Roy Lewallen, W7EL

Roy, David:

[snip]
It could also be accomplished numerically on a computer by using a
program [like the NEC programs] based on solving Maxwell's partial
differential equations iteratively.

As far as I know no one has ever attempted to do this... and
notwithstanding the possibility for "invention" or "discovery" I might
ask, why would one want to do this?

Hey it might make a good Ph.D. or M.Sc. thesis... but what is the
practical application?

It would be one of the easiest degrees ever attained. NEC-4 allows setting
the primary medium to any (reasonable) value of conductivity and
permittivity, so you can have the answer in seconds with a free space
analysis. Alternatively, you can bury the antenna deep in NEC-4's ground
medium and define the ground characteristics for your test.

I did a short consulting job a while back for some people interested in
transmitting RF for short distances under water. Immersing the antenna
eliminates the substantial signal loss incurred by reflection at the
air-water interface when the antenna is out of the water. And antenna
system design requires knowledge of the antenna feedpoint Z. I've seen
numerous papers in the IEEE publications about antennas immersed in other
media such as a plasma, and know that antennas buried in the ground are
used. So it's of considerable practical interest.

Roy Lewallen, W7EL
[snip]

Well thanks for that input Roy, that's very interesting and of course a
useful application.

But for the "thesis" idea I was thinking of something a little more
"challenging", e.g.

Clearly the antenna "driving point impedance" Z = R +jX is a complex
function f (Zo) of the "characteristic impedance" Zo, where in general Zo =
Ro +j Xo = sqrt[u/e] of the medium in which the antenna is immersed.

Clearly this function f(Zo) is the "transducer" function that David [The OP]
was seeking.

It's clear of course that f(zo) will also be a function of complex frequency
p = s + jw.

Now expressing this functionality as:

Z(p, Zo) = f (p, Zo)

One might ask [the thesis candidate, (grin)] to derive/discover and tell
us...

What. precisely, is the functional form of this complex "transducer"
function f that takes Zo to Z [377 Ohms to 73 Ohms! (grin)]

Is f(Zo) a simple linear function? [e.g. like a transformer turns ratio as
the OP David had asked] or perhaps... A non-linear function? or maybe... A
differential or integral relationship?

What?

Except for a few isolated niche applications, such as those you mentioned
having consulted on, I can't really think of any practical applications that
demand knowledge of the functional form of "f".

Which is likely why this subject is not mentioned in antenna textbooks or
widely discussed. i.e. No one ever needed to know it and so no one worked
out this relationship or even investigated it... Simply a matter of supply
and demand (grin)! We have Ohm's Law and other such well known
relationships such as V = IR because there was a demand by "scienticulists"
(grin) to know these relationships to do real Engineering, i.e. build stuff
they needed out of stuff they could get.

The discovery of the functional form "f" of this relationship might perhaps
be at least suitably hard for a Master's Thesis, a good challenge, and I
believe that it is suitably "academic", since there is very little use for
it (grin).

What exactly is "f (Zo)"?

Thoughts, comments.

--
Pete k1po
Indialantic By-the-Sea, FL

Peter O. Brackett wrote:
What? ... What exactly is "f (Zo)"?
Thoughts, comments.

Peter, I for one, have missed your style.

Consider the following:

I(s)
+--------------------------------------------open
|
V(s) 1/4 wavelength, Z0=600 ohms
|
+--------------------------------------------open

Given: The ratio of V(s)/I(s) is 50+j0 ohms. Can you
solve for f(Z0)?
--
73, Cecil http://www.w5dxp.com

David wrote:
The impedance of free space / air is said to be 377 ohms. Impedance is ratio
of E/H. The feedpoint impedance of an antenna is usually 50 or 75 ohms.

Can an antenna ever be regarded as a transducer that transforms a radio wave
from 50 ohms to 377 ohms i.e. provides an impedance transformation?

I'd like to take a somewhat different tack on handling this question.
It is a wave versus photon perspective. It seems to be very easy to
slip into the thought mode where we view radio transmissions as a wave
phenomena in some sort of space media. After all, most of the math we
use for electromagnetic signal propagation work quite well assuming
that. Maxwells math still works wonderfully even today, a century after
Einstien described photons. Maxwell died more than a decade before
Einstien published his seminal papers though.

The real model of the radio operation is for there to be alternating
electrostatic and magnetic fields surounding an antenna when it is
driven by an RF power source. Through some yet poorly understood
physical mechanism, either envolving the acceleration of electrons in
the antenna's conductor or from the alternating E and H fields, photons
are flung off the antenna.

I interpret this to mean that what we call the "near field" around an
antenna is the volume around an antenna where the electrostatic and
magnetic fields are at an energy level significantly above natural
background levels. The "far field" I interpret to mean where RF energy
exists as a photon flux.

To describe an antenna as a transducer is probably correct
symantically. It does convert between electrical energy in the form of
RF current and voltage and photons. This is both for transmitting and
receiving.

The 377 ohm thing though is a function of the releative intensities of
the electrostatic and magnitic fields surrounding an antenna. It is a
constant like pi or e. It is an emperically measured characteristic of
our universe. It does not, however mean that an antenna transforms a
feed impedance to this impedance. It simply tells us what to expect for
when we feed an RF current into an antenna. It is much like knowing
that a 1 foot diameter wheel will travel 3.14159265... feet for every
revolution.

Now, a point worth noting is that while RF current in a conductor
produces photons, photons produce RF current in conductors. That, of
course, is why antennas are able to operate for both receiving and
transmitting. That is also why radio signals bounce. Photons are
absorbed by objects such as wires or dirt and RF currents are produced.
Those currents, in turn, generate new photons. The conductivity and
dielectric constant of the absorbing material determine the amplitude
and phasing of the current and thus the primary direction of emission
of the new photons.

So... Yes the antenna is a transducer. No, it does not transform 50
ohms into 377 ohms. 377 ohms refers to the eletrostatic and magnentic
fields as they exist in the near field of an antenna or conductor. It
does not refer to what is going on electrically in the antenna
conductors.

Anyway, that's my take on the subject.

Gary
N0GW

On 10 Sep 2006 14:53:25 -0700, "N0GW" wrote:

Hi Gary,

A number of comments:

I'd like to take a somewhat different tack on handling this question.
It is a wave versus photon perspective.

Photons are no less wave oriented than RF - nor more. Also, RF is no
less "corpuscular" (Einstein's term) than photons.

The real model of the radio operation is for there to be alternating
electrostatic and magnetic fields surounding an antenna when it is
driven by an RF power source. Through some yet poorly understood
physical mechanism, either envolving the acceleration of electrons in
the antenna's conductor or from the alternating E and H fields, photons
are flung off the antenna.

Hardly "flung off" and even less, "poorly understood."

I interpret this to mean that what we call the "near field" around an
antenna is the volume around an antenna where the electrostatic and
magnetic fields are at an energy level significantly above natural
background levels.

The background levels of EM fields are predominantly in the
milli-micro-nano Kelvins of temperature. Nearly everything produces
an energy level significantly above that.

The "far field" I interpret to mean where RF energy
exists as a photon flux.

Photons don't exist in the near field? A flux is simply the bulk of
them; like one electron flowing, or a trillion, is current.

The 377 ohm thing though is a function of the releative intensities of
the electrostatic and magnitic fields surrounding an antenna.

It is a function of permittivity and permeability which exists even if
the fields are not there.

It is a constant like pi or e.

Unless you happen upon something other than a vacuum.

It is an emperically measured characteristic of our universe.

It is empirically measured, but that does not create its value.

Now, a point worth noting is that while RF current in a conductor
produces photons, photons produce RF current in conductors.

How much current is created by your antenna in sunlight?

That, of
course, is why antennas are able to operate for both receiving and
transmitting. That is also why radio signals bounce. Photons are
absorbed by objects such as wires or dirt and RF currents are produced.

An 10 meter doublet of 1mm wire is exposed to 5 - 10 W of power. How
much can you capture and bottle due to this production of current in
conductors?

Those currents, in turn, generate new photons.

Photons are generated on the basis of an electron in an excited state
falling to a lower ground state, not by current flow.

The conductivity and
dielectric constant of the absorbing material determine the amplitude
and phasing of the current and thus the primary direction of emission
of the new photons.

Photons do not follow any particular channel of radiation, not unless
you have a very large lens (few antennas do).

So... Yes the antenna is a transducer. No, it does not transform 50
ohms into 377 ohms. 377 ohms refers to the eletrostatic and magnentic
fields as they exist in the near field of an antenna or conductor.

In fact, in the near field of an antenna, there is nothing that
resembles 377 Ohms of Z.

73's
Richard Clark, KB7QHC

On Sun, 10 Sep 2006 16:01:34 -0700, Richard Clark
wrote:

In fact, in the near field of an antenna, there is nothing that
resembles 377 Ohms of Z.

The page at:
http://home.comcast.net/~kb7qhc/ante...pole/index.htm
dramatically reveals that the near fields fluctuate wildly from 377
Ohms, and I have restricted my analysis to those values falling at
roughly 100 Ohms or 1000 Ohms (the hot spots marking the feed point
region and the tips of the dipole).

Other antenna design's modification of the 377 near field around them
can be observed at:
http://home.comcast.net/~kb7qhc/ante...elds/index.htm

73's
Richard Clark, KB7QHC