I note some variation in the use of the term 'Radiation Resistance' (Rr)
that suggests that it has different meanings to different folk.

One suggestion is that it is the resistance seen by a transmission line
connected to an antenna that expresses its coupling to distant regions of
space.

If that is the case, Rr would not capture energy that is lost in
reflection from real ground. So, Rr would be the sum of power in the far
field divided by RMS current squared.

If indeed it is the "resistance seen by a transmission line", then the
current above would be the current at the end of the transmission line.

Does the term have an accepted single clear meaning? Is the above
correct?

Some implications of the above are that:
- Rr of a horizontal half wave dipole with zero conductor loss, above
real ground, would have Rr less than R at the feedpoint by virtue of some
loss in waves reflected from real ground;
- Rr of a half wave folded dipole of equal conductor diameters would be
around 300 ohms.

Thanks
Owen

On 07/15/2010 04:14 AM, Owen Duffy wrote:
I note some variation in the use of the term 'Radiation Resistance' (Rr)
that suggests that it has different meanings to different folk.

snip
Hello, and I don't find any ambiguities in any of my various EM and
antenna theory textbooks. FWIW, from the IEEE Standard Dictionary of
Electrical and Electronics Terms:

"Radiation resistance (antenna). The radio of the power radiated by an
antenna to the square of the rms antenna current referred to a specified
point. Note: This term is of limited utility in lossy media."

So if we're looking at free (in vacuo) space the radiation resistance is
simply a "load" resistance component that accounts for where the
radiated power goes. The radiation resistance doesn't include any other
resistive losses in the antenna structure/proximity operating
environment that may also be dissipating source power introduced at the
feedpoint of the antenna. An aerodynamic analogy would be the
distinction between "induced" drag (the price paid for "lift") and
"parasite" drag, which are both components of the total drag.
Sincerely, and 73s from N4GGO,


--
John Wood (Code 5520) e-mail:

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

Owen,

I like the IRE definition, which according to W8JI is: "The total
power radiated in all directions divided by the square of the net
current causing the radiation".

That definition draws a distinction between Rrad and the resistive
component of the feedpoint impedance; it makes the Rrad of a folded
dipole 75 ohms, not 300 ohms; and it avoids some of the errors folk
make in assuming that "folding" a vertical can reduce ground losses.

73,
Steve G3TXQ

On Jul 15, 3:14*am, Owen Duffy wrote:
I note some variation in the use of the term 'Radiation Resistance' (Rr)
that suggests that it has different meanings to different folk.

One suggestion is that it is the resistance seen by a transmission line *
connected to an antenna that expresses its coupling to distant regions of
space.

If that is the case, Rr would not capture energy that is lost in
reflection from real ground. So, Rr would be the sum of power in the far
field divided by RMS current squared.

If indeed it is the "resistance seen by a transmission line", then the
current above would be the current at the end of the transmission line.

Does the term have an accepted single clear meaning? Is the above
correct?

Some implications of the above are that:
- Rr of a horizontal half wave dipole with zero conductor loss, above
real ground, would have Rr less than R at the feedpoint by virtue of some
loss in waves reflected from real ground;
- Rr of a half wave folded dipole of equal conductor diameters would be
around 300 ohms.

Thanks
Owen

Owen
Can I suggest that you look at things differently?
Radiation is created by the acceleration of charge
which effectively that which creates an acceleration of a particle
such as a bullet from a rifle. The reaction to the firing of the
bullet is the recoil
which is considered as the resistance to radiation.
If you expend energy in other places to get to the point of firing
they can only be referred to as losses.
Thus radiation is a point vector and should not be confused by
multiple vectors that create a plasma.
All radiation equations are formed by the use of boundary equations
which requires recognition
of vectors or point charge and NOT by acceptance of the term "waves."
As an illustration, a superconductive material has the minimum
resistance possible to ensure the credibility of Ohms law. This
resistance has nothing to do with "dc resistance" because current flow
is at or near the surface because of the destruction of skin depth
resistance phenomina, which also means the cancellation of magnetic
fields.( Not destruction as the magnetic field is transformed into a
energy bank)
Thus ejection of particles can be considered as purely that of the
displacement current formed.
Expansion of static laws to those of dynamic form
establishes the use of particles with the use of boundary laws
(vectors) together with conformety with Classical laws.
Regards
Art

On 7/15/2010 6:43 AM, J.B. Wood wrote:

"Radiation resistance (antenna). The radio of the power radiated by an
antenna to the square of the rms antenna current referred to a specified
point. Note: This term is of limited utility in lossy media."


Whoops! I meant to say "ratio" vice "radio". 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 in news:i1monr$2od$1
@ra.nrl.navy.mil:

On 07/15/2010 04:14 AM, Owen Duffy wrote:
I note some variation in the use of the term 'Radiation Resistance' (Rr)
that suggests that it has different meanings to different folk.

snip
Hello, and I don't find any ambiguities in any of my various EM and
antenna theory textbooks. FWIW, from the IEEE Standard Dictionary of
Electrical and Electronics Terms:

"Radiation resistance (antenna). The radio of the power radiated by an
antenna to the square of the rms antenna current referred to a specified
point. Note: This term is of limited utility in lossy media."


Hmmm. The last statement suggests that, as defined, it is not clear and
unambiguous in the real world because the real world involves "lossy
media".

The "reference to a specified point" suggests that if one gives a value for
Rr, it is necessary to also state the reference point. Is that what it
means?

This is exactly the lack of clarity that is troubling me.

So if we're looking at free (in vacuo) space the radiation resistance is
simply a "load" resistance component that accounts for where the
radiated power goes. The radiation resistance doesn't include any other
resistive losses in the antenna structure/proximity operating
environment that may also be dissipating source power introduced at the
feedpoint of the antenna.

This does not address the issue of ground reflection that I mentioned.

An aerodynamic analogy would be the
distinction between "induced" drag (the price paid for "lift") and
"parasite" drag, which are both components of the total drag.
Sincerely, and 73s from N4GGO,

I am not an aerodynamics type, so drawing that analolgy only helps to
confuse. You might as well use optics!

I know you are trying to be helpful John, but the IREE definition doesn't
seem to clarify the issue.

To put some numbers on my first example, if I have an NEC model of a centre
fed half wave dipole with zero conductor losses, mounted over real (ie
lossy) ground, and feedpoint R at resonance is say, 60 ohms, and total
power in the *far field* divided by I^2 is say, 50 ohms, is Rr 50 ohms? Is
the power "radiated" from such a dipole ONLY the power that makes it to
'distant space', or is radiated power input power less dipole conductor
losses?

The IREE definition suggests that I need also to state that Rr is 50 ohms
at the centre, and the term is is of "limited utility" (not unambiguously
clear?) because of the lossy ground reflections.

If indeed the term Radiation Resistance is only applicable in lossless
scenarios as suggested by the IREE dictionary, what it a clear and
unambiguous language for the real world?

Cheers
Owen

On Thu, 15 Jul 2010 20:01:57 GMT, Owen Duffy wrote:

"Radiation resistance (antenna). The radio of the power radiated by an
antenna to the square of the rms antenna current referred to a specified
point. Note: This term is of limited utility in lossy media."


Hmmm. The last statement suggests that, as defined, it is not clear and
unambiguous in the real world because the real world involves "lossy
media".

Hi Owen,

All of this neatly fits into Broadcast Band transmission where the
current pulse (we are now into the shadow zone of SWR) occurs at the
feedpoint of an antenna that is conventionally a quarter wave tall,
the current can be measured, and the far field power is known. The
matter of "lossy media" has been studied (BL&E) and that variable
reduced by good engineering practices (which brings us back to the
known far field power).

The "reference to a specified point" suggests that if one gives a value for
Rr, it is necessary to also state the reference point. Is that what it
means?

The "reference" is typically the current node. It gets messier with
more complex antenna design.

This is exactly the lack of clarity that is troubling me.

This implies the more complex designs following (or not following)
what you reject as "rules of thumb." Or at least their appearance.
I'm sure there are long and elaborate academic treatises that explain
the current node current measurement in relation to the known radiated
power. I haven't read any of them that I can glibly quote here.

This does not address the issue of ground reflection that I mentioned.

I will return to your original and comment to that:
Some implications of the above are that:
- Rr of a horizontal half wave dipole with zero conductor loss, above
real ground, would have Rr less than R at the feedpoint by virtue of some
loss in waves reflected from real ground;

There are two mechanisms hiding in one description. The ground is
lossy - period. The ground is reflective - period. These are two
different issues in regard to radiation resistance. The Rr is not
ground loss although the measure of Rr may be corrupted by Rground.
That is a problem of separating out the variables. Others have
described that. The reflection from ground may upset the measure of
Rr as well, but if that does not upset the total power, and the
current node can be measured, then you still have a way to quantify
Rr.

- Rr of a half wave folded dipole of equal conductor diameters would be
around 300 ohms.

I thought someone else preceded this discussion with Tom's
explanation. Maybe it went unread, or unrealized. So, in other
words: A folded dipole/monopole is a current transformer. That
transformation ratio is driven, in large part, by the ratio of the
diamters of the conductors. You have acknowledged as much in your own
specification of equal sized conductors. Having said that, the
transformer is also transforming the Z of the load (Rr + Rground) by a
square law. The usual sense of current node has been lost in a more
elaborate design, but the transformation of it returns us to the usual
Rr.

73's
Richard Clark, KB7QHC

Hi Steve,

steveeh131047 wrote in news:8a4ba365-af4e-40fc-abaf-
:

Owen,

I like the IRE definition, which according to W8JI is: "The total
power radiated in all directions divided by the square of the net
current causing the radiation".

That definition draws a distinction between Rrad and the resistive
component of the feedpoint impedance; it makes the Rrad of a folded
dipole 75 ohms, not 300 ohms; and it avoids some of the errors folk
make in assuming that "folding" a vertical can reduce ground losses.

If I take a half wave folded dipole immersed in some environment where
the ambient noise temperature is T, and attach a load directly to the
feedpoint, the load power will be maximum when it is about 300 ohms
rather than about 75 ohms, and the noise power density due to ambient
noise would be K*T*300 W/Hz rather than K*T*75 W/Hz. If Rr is the
(virtual) resistance due to coupling of the antenna with distant space,
then surely this example suggests that Rr is 300 rather than 75 ohms.

(If I performed the same experiment with a plain half wave dipole, the
load power will be maximum when it is about 75 ohms, and the noise power
density due to ambient noise would be K*T*75 W/Hz.)

To some extent, my question comes to is Rr at a 'defined' point, and is
that point the interface between non-radiating feed line and the
radiating structure.

Yes, you could argue as Richard states that a folded dipole includes an
integral transformer, but then, the generalised outcome of that argument
is that all antennas are impedance transformers, Rr could be some agreed
fixed quantity located at a great distance in space, or even more
confusing, an non-shared arbitrary value, and that key to it all is some
new concept of the antenna impedance transformation ratio.

The notion that Rr is defined at a virtual point that is between any
deemed integral impedance transformation and the rest of the radiator,
rather than defined at the physical feedline / radiator interface doesn't
seem too sensible.

Owen

Radiation resistance is pretty much what the writer wants it to be.
Consequently, it has to be explicitly each time it's used whenever an
ambiguity might arise. It's simply a resistance whose "dissipation"
(absorbed power) is the amount radiated. Most writers would probably
argue that power lost from the near field to nearby lossy objects such
as ground never got radiated, and therefore the corresponding resistance
should be considered loss rather than radiation resistance. The presence
of nearby ground, however, can also change the value of the remaining
resistance due to mutual coupling and alteration of the current
distribution, so a particular antenna doesn't have a single inherent
value of radiation resistance independent of environment.

As for the location where radiation resistance is defined, I believe
it's common in AM broadcasting, for example, to refer the radiation
resistance of a monopole to a current loop (maximum). If this is a
different location than the feed point, the resistance (neglecting loss)
at the base will be different from the loop radiation resistance. The
ratio of base radiation resistance to loop radiation resistance will in
fact equal the square of the ratio of loop current to base current. So
radiation resistance measured at the base can be "referred" to the loop
by scaling by this ratio. (The power "dissipated" by radiation
resistance referred to a loop or any other point has to equal the
"dissipation" of the radiation resistance seen at the base or any other
point. So Rr has to differ to keep I^2 * Rr constant as Rr is referred
to points having different values of I.) The radiation resistance can be
referred to any point on the antenna, so the writer has to specify what
point is used. But one point is as acceptable as another. It's vital,
though, when using radiation resistance, that the current at the defined
point is used for calculations. And loss resistance must also be
referred to the same point if efficiency calculations are to be made.

Some authors, for example Kraus, consistently refer the radiation
resistance to the feed point. But Kraus doesn't explicitly apply the
term "radiation resistance" to a folded dipole. There's nothing at all
wrong, however, with declaring the radiation resistance of a folded
dipole to be ~300 ohms. The power radiated is the current measured at
the feed point, squared, times that resistance. It's equally legitimate
to declare the radiation resistance of a folded dipole to be that of an
unfolded equivalent, or ~75 ohms. If you do, though, you also have to
work with the current of the unfolded dipole to make the power come out
correct.

A common mistake when dealing with folded unipoles, made by at least
several prominent people who should have known better (and marketing
people who probably do know better but find it advantageous to be
incorrect), is to refer the radiation resistance to the feed point but
the loss resistance to the unfolded equivalent. This results in an
erroneous efficiency calculation that incorrectly attributes an
improvement due to folding. As I said, you can refer the radiation
resistance to either, but if you want to calculate efficiency, you have
to refer the loss resistance to the same point and having undergone the
same transformation. And when you do, you find that folding fails to
produce the often-claimed efficiency improvement.

Roy Lewallen, W7EL

Thanks Roy.

I note you observe similar variation in usage as I note.

Yes, consistency in an application is more important than a common meaning
of the term, but a common meaning of the term assists simpler
communication.

Regarding say, a base fed folded monopole and efficiency calculations, if
the connection to ground is though of as having some actual value Rg, since
the current flowing in Rg is twice the feedpoint current, consistent
development of the circuit model will reveal the correct efficiency as:

Rr/(Rr+2Rg)

where Rr is the sum of power in the far field divided by feed point current
squared. You don't need to fudge Rr to get the result, proper allowance of
the power due to the actual current in Rg provides the correct result.

Kraus (Annennas for All Applications) effectively defines Rr as part of his
development of the concept of a pair of conductors transitioning from a
non-radiating transmission line to an antenna to free space radiation.

He does say "... the radiation resistance Rr, may be thought of as a
"virtual" resistance that does not exist physically but is a quantity
coupling the antenna to distant regions of space via a "virtual"
transmission line."

It is his use of "distant regions of space" that suggests in the case of
ground reflection, it is the remaining total power in distant free space
after lossy reflection that is used to calculate Rr. The power lost in
reflection would be a component of feed point R, but not Rr.

He also states a little earlier "... the antenna appears to the
transmission line as a resistance, Rr, called the *radiation resistance*.
It is not related to any in the antenna itself, but a resistance coupled to
the from space to the antenna terminals." This seems fairly clear to me
that he defines radiation resistance to be at the transmission line /
antenna interface.

Both of these statements by Kraus are simple, but would seem to be capable
of application to real antenna systems. I can't immediately think of
exceptions (game on???).

In Kraus's language, ground reflections might reasonable be considered part
of the 'antenna' since they influence its pattern and loss, and loss in the
ground reflections is due to resistance "in the 'antenna' itself" and so
excluded from Rr.

Is there anything in Kraus's statements that is wrong, or my
interpretatiohn of them.

Owen