Richard Harrison wrote:
SNIP
Dave wrote:
"That is, it has the same efficiency transmitting or receiving."

I hadn`t given that much thought but it seems to me there may be a
difference. When an antenna is receiving, it is excited by the received
signal, resulting in voltage and current on the antenna.

SNIP: Agree

The antenna
doesn`t care about the source of the signal. If the antenna is
conjugately matched to the receiver, radiation resistance is the source
resistance of the signal feeding the receiver.

SNIP: This resistance is the Radiation resistance of the antenna, i.e.
approximately 73 ohms in a thin 1/2 wavelength dipole.

Half the signal power is
consumed in the source resistance (radiation resistance) and half is
consumed in the receiver.

SNIP: Not quite. Half is RE-RADIATED. [It does not dissipate it
radiates!][See your next statement]. The other half is delivered to the
transmission line sub-system then to the receiver.

The half consumed in the radiation resistance
is re-radiated.

SNIP: Agree

The antenna doesn't know that re-radiation is uncalled
for.

SNIP: I wonder if this statement is the root of our misunderstanding? My
understanding is that the antenna does not have to know anything other
than passively allowing the Laws of Nature [Physics] to operate.

If the antenna is mismatched to the receiver, more than 50% of all
power received is re-radiated, depending upon the severity of the
mismatch.

SNIP: Have to think about what you are trying to say. If the antenna has
received a 10^-12 watt signal and 4*10^13 watts is delivered to the
transmission line and 5*10^-13 watts is reradiated then 1*10^13 watts
is energizing a standing wave in the antenna.


If we have a Class C amplifier feeding power to the same antenna and
enjoying a conjugate match, we can have a source that takes less than
50% of the available energy.

SNIP: Help me understand what you are trying to say.

So, the transmitting antenna system can be
more efficient than the receiving antenna system, it seems to me.

SNIP: I probably disagree. But, I do not fully understand what you are
trying to say in the previous paragraph.

Deacon Dave


Best regards, Richard Harrison, KB5WZI

Dave Shrader wrote:
"Help me understand what you are trying to say."

I`ll elaborate.

Efficiency is output / input.

1/2 or more of the power received by a receiving antenna is re-radiated.

Nearly all of the power received by a transmitting antenna is
transmitted.

Considering the energy available to the antenna, the job done by the
transmitting antenna system as compared with the job done by the
receiving antenna system, the transmitting system is better.

A receiving antenna must be resonant to enable full acceptance of
available energy, and it must be matched to avoid re-radiation of more
than 50% of the energy it is able to grab.
If off-resonance, the receiving antenna has too-high impedance for
significant induced current. Of course, we have such good receivers we
can do without good efficiency.

A transmitting antenna will radiate energy proportional to the current
in the antenna.

Ronold W.P. King says in "Transmission Lines, Antennas, and Wave
Guides":

"---the power (Io squared)(Ro) supplied to a highly conducting antenna
(of Copper), with Ro taken from the curves of Sec. 10, is for practical
purposes all radiated to the more or less closely coupled universe
outside the antenna, while that used in heating the antenna itself is
negligible." This information is on page 113.

Inefficiency is to be found elsewhere from the transmitting antenna
itself. We may use inefficient transmission lines and our wave
generator, the transmitter, may be inefficient. We usually try to keep
their losses low.

It is not uncommon to produce RF in a Class C amplifier with an
efficiency of 70%. With reasonable lines and antennas, nearly 100% of
this power output can be radiated, producing appropriate millivolts per
meter at one mile from the antenna.

This is not completely reversible due to re-radiation of 1/2 or more of
all the power a receiving antenna can grab.

The hope for point to point wireless power transmission is in using
antennas like large dishes, for example, which concentrate power within
such a small angle that the receiving antenna captures all the
transmitted beam. Similarly, all re-radiated power is beamed back to the
transmitting antenna for another trip to the receiver.

Best regards, Richard Harrison, KB5WZI

On Sun, 22 Feb 2004 13:07:46 -0600 (CST),
(Richard Harrison) wrote:
Efficiency is output / input.

1/2 or more of the power received by a receiving antenna is re-radiated.

Nearly all of the power received by a transmitting antenna is
transmitted.

Considering the energy available to the antenna, the job done by the
transmitting antenna system as compared with the job done by the
receiving antenna system, the transmitting system is better.

Hi All,

This is because of a shift in the perception of efficiency. To the
transmitter ALL of matched power is irrevocably lost and to the
perception of transmission wholly in-efficient (absolutely no transfer
of power has been engaged to any mechanical benefit). Radiation
Resistance is as lossy as any resistor, sans the caloric benefit of
work.

From the point of view of the wave impinging upon an antenna, only
half the power is irrevocably lost, half survives (in a Zeno's
paradoxical fashion). Again, absolutely no mechanical benefit is
derived except for a incredibly minute caloric gain (unless you test
your antennas inside a microwave oven) but this does invert the
expectation (some work has been accomplished).

Given efficiency is too often stated in terms of mechanical theory
(adiabatics and such) and then duct-taped to other disciplines
(notable through the egregiously wild claims of fantasy laws of
conservation) it is no surprise that other paradoxes emerge.

73's
Richard Clark, KB7QHC

Richard Harrison wrote:
1/2 or more of the power received by a receiving antenna is re-radiated.

Nearly all of the power received by a transmitting antenna is
transmitted.

Expanding a bit to make the receiving and transmitting systems symmetrical
with respect to power: If the transmitter is linear (like the antenna
is linear), i.e. Class-A, 1/2 or more of the generated power will be lost
in the source. In a linear resonant system, about 1/2 of the power sourced
reaches the antenna and about 1/2 of the received power makes it to the
receiver. It's the old maximum power transfer theorem at work.

A receiving antenna must be resonant to enable full acceptance of
available energy, and it must be matched to avoid re-radiation of more
than 50% of the energy it is able to grab.
If off-resonance, the receiving antenna has too-high impedance for
significant induced current. Of course, we have such good receivers we
can do without good efficiency.

A properly tuned antenna tuner ensures that the *antenna system* is resonant
for both transmit and receive (assuming the receiver's input impedance is the
same as the transmitter's output impedance). Note that an off-resonant antenna
*wire* is integrated into a resonant antenna *system* through the use of an
antenna tuner. Chapter 7 in _Reflections_II_ explains how even though it might
better have been titled, "My Transmatch Really Does Tune My Antenna" *SYSTEM*.
--
73, Cecil http://www.qsl.net/w5dxp



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Cecil, W5DXP wrote:
"If the transmitter is linear (like the antenna is linear), i.e.
Class-A, 1/2 or more of the generated power weill be lost in the
source."

True, that would be an equalizer between reception and transmitting
system efficiencies of antennas, but Class A isn`t the only way to get
linear amplification, Hi-Fi nuts to the contrary not withstanding. Class
B is often used to combine efficiency with high undistorted output
capability. Class B amplifiers are biased to cut-off so they draw no
current when there is no signal input. A class B amplifier may have 60%
efficiency at full power output, for example. Such an amplifier will
have only about 30% efficiency at 1/2 of its maximum power output.

Turman writes on page 354 of his 1955 edition:

"With the largest signal that the (Class-B) amplifier can be expected to
handle satisfactorily, Emin/Eb will be small, and the actual efficiency
at full power is commonly of the order of 60%."

The receiving antenna can never be more than 50% efficient due to
re-radiation which I don`t seem to be able to explain. Sorry.

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:
The receiving antenna can never be more than 50% efficient due to
re-radiation which I don`t seem to be able to explain. Sorry.

It's because receiving antennas are linear devices which I don't
seem to be able to explain. :-)
--
73, Cecil http://www.qsl.net/w5dxp



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On Sun, 22 Feb 2004 13:07:46 -0600 (CST),
(Richard Harrison) wrote:

A receiving antenna must be resonant to enable full acceptance of
available energy,

Where did you come up with that one?

I suggest you revisit capture area.

Danny, K6MHE

Don, K6MHE wrote:
"Where did you come up with that one?"
(A response to my statement that a receiving antenna must be resonant to
enable full acceptance of available energy)

I`ve tweaked antenna trimmers which dramatically boosted the signal when
reasonance was reached. I`ve seen grounded 1/4-wave structures near a
broadcast station detuned, thus eliminating the distortion they had
caused in the station`s radiation pattern. If they`re not resonant, they
don`t accept enough energy to make any difference in the station`s
pattern.

!/2-wave wires in free-space are resonant. Resonance is defined as unity
power factor, that is, XL=XC. At resonance, reactance is balanced out
and only resistance is left to oppose current in a wire. Usually the
wire has a radiation resistance which is large as compared with its loss
resistance in practical antennas.

At frequencies below first resonance, the ungrounded wire is less than a
1/2-wavelencth. It has a low radiation resistance and a high capacitive
reactance. We can add inductance to tune the wire to resonance.

At frequencies above first resonance, the ungrounded wire is more than a
1/2-wavelength, and if it is not much longer, the wiire has an inductive
reactance. The phase flip-flop at resonance is abrupt and the reactance
is an impediment to the current on either side of resonance. The correct
series capacitor can be placed in series with the roo-long wire to tune
out its excess inductive reactance.

A mechanical analog is the vibrating-reed frequency meter used at power
frequencies. All the reeds are in the power-frequency field. Only the
reed of resonant length has so little opposition to the excitation that
it vibrates freely.

A versatile antenna tuner can insert either inductive or capacitive
reactance in series with an antenna to correct its power factor (tune it
to resonance) so it can accept maximum excitation.

Best regards, Richard Harrison, KB5WZI

On Mon, 23 Feb 2004 08:54:50 -0600 (CST),
(Richard Harrison) wrote:

A versatile antenna tuner can insert either inductive or capacitive
reactance in series with an antenna to correct its power factor (tune it
to resonance) so it can accept maximum excitation.

I believe you are confusing matching with antenna operation (gain and
efficiency). Using a simple dipole for example, you can calculate both
gain and efficiency using NEC. Doing that I have modeled a dipole at
3/8, 1/2 and 5/8-wavelengths (80-meter band). The antenna was modeled
using #14 copper wire in free space. Here is what NEC reported:

Wavelength Gain(DBi) Efficiency

3/8 1.8 96.36%
1/2 (resonate) 2.02 97.22%
5/8 2.29 97.72%

Clearly you should be able to see that only advantage is that the
longer antenna has greater the gain and efficiency. The fact antenna
is resonance or not is not the determining factor.

73
Danny, K6MHE

Dan Richardson wrote:
"The fact is resonance or not is not the determining factor."

Resonance of the antenna system is the determining factor in the
performance of a standing-wave antenna.

This is an amateur group, so you may check the "ARRL Antenna Book". My
19th edition has resonant antennas on page 9-2.

Fig 2 is a series RLC circuit representation of the typical
standing-wave antenna.

Ohm`s law should be noncontroversial (I=E/Z).
To maximize I with a given voltage, Z must be minimized. Z in the series
circuit is the phasor sum of R and X.

R has probably been established firmly in an antenna by its construction
and placement but we can tune the antenna system to make it resonant so
that we eliminate X to get maximum current into the antenna and to
thereby get maximum performance out of the antenna.

Best regards, Richard Harrison, KB5WZI