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



-----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
http://www.newsfeeds.com - The #1 Newsgroup Service in the World!
-----== Over 100,000 Newsgroups - 19 Different Servers! =-----

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



-----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
http://www.newsfeeds.com - The #1 Newsgroup Service in the World!
-----== Over 100,000 Newsgroups - 19 Different Servers! =-----