How about a little more explanatin on the 50% reradiates?

--

73
Hank WD5JFR

"Richard Harrison" wrote in message
...
Ken Wood wrote:
"Can you tx on a tv antenna?"

A perfectly matched TV receiving antenna reradiates 50% of all the
energy it intercepts. A mismatched antenna retransmits a higher
percentage.

Best regards, Richard Harrison, KB5WZI

Henry Kolesnik wrote:
How about a little more explanatin on the 50% reradiates?

In a perfectly matched system, half the power is "lost"
in the source. In the case of a receiving antenna, that
power is lost to radiation from the antenna during
receive, i.e. the antenna radiation resistance is the
"source".

Something I have wondered about: If we simply amplify
the receiver voltage through a near infinite impedance
FET with virtually no load on the signal, do we get the
signal information but lose virtually all the power to
re-radiation? (assume high S/S+N)
--
73, Cecil http://www.qsl.net/w5dxp

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Cecil
Good thought. As I understand it a receiver doesn't really need any power,
so if the antenna is 50 ohms and the Rx input is say 1000 ohms most of the
power has to be relflected to the antenna.

--

73
Hank WD5JFR
"Cecil Moore" wrote in message
...
Henry Kolesnik wrote:
How about a little more explanatin on the 50% reradiates?

In a perfectly matched system, half the power is "lost"
in the source. In the case of a receiving antenna, that
power is lost to radiation from the antenna during
receive, i.e. the antenna radiation resistance is the
"source".

Something I have wondered about: If we simply amplify
the receiver voltage through a near infinite impedance
FET with virtually no load on the signal, do we get the
signal information but lose virtually all the power to
re-radiation? (assume high S/S+N)
--
73, Cecil http://www.qsl.net/w5dxp

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Henry Kolesnik wrote:

Cecil
Good thought. As I understand it a receiver doesn't really need any power,
so if the antenna is 50 ohms and the Rx input is say 1000 ohms most of the
power has to be relflected to the antenna.

Make it one megohm and virtually all receive power in
a perfectly matched antenna is lost to re-radiation?
So by astute choice of feedline length we can choose
constructive interference or destructive interference
in the re-radiated signals? :-)
--
73, Cecil http://www.qsl.net/w5dxp

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To the gentleman who posted the original to this thread: QST had an
article on how to convert a particular tv antenna to a vhf t/r antenna.
I can't recall how long ago, perhaps in the fall or summer.

KF5DE

Cecil Moore wrote:
Henry Kolesnik wrote:

Cecil
Good thought. As I understand it a receiver doesn't really need any
power, so if the antenna is 50 ohms and the Rx input is say 1000 ohms
most of the power has to be relflected to the antenna.


Make it one megohm and virtually all receive power in
a perfectly matched antenna is lost to re-radiation?
So by astute choice of feedline length we can choose
constructive interference or destructive interference
in the re-radiated signals? :-)

In article , Cecil Moore wrote:

Something I have wondered about: If we simply amplify
the receiver voltage through a near infinite impedance
FET with virtually no load on the signal, do we get the
signal information but lose virtually all the power to
re-radiation? (assume high S/S+N)

I think so, yes. Electrically, a "driven element" which feeds a
very-high-impedance FET would look like a simple passive element (e.g.
like a parasitic director or reflector in a Yagi). There would be
some small amount of power delivered into the load (the FET's gate
capacitance and its associated losses), and some small amount
dissipated as losses in the element itself, but the rest would be
re-radiated... just as is the case with an unloaded parasitic element.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Cecil,

Check with Melles-Griot on this one.

Hint: Think conservation of energy. Them joules gotta go somewhere.

However, in reality the opposite happens. As the antenna load goes to
infinity the scattering cross section (i.e. retransmission) goes to zero
faster than the capture cross section.

(Plagiarized from Kraus Antennas, 2nd ed.)

73,
Gene
W4SZ

Cecil Moore wrote:


Something I have wondered about: If we simply amplify
the receiver voltage through a near infinite impedance
FET with virtually no load on the signal, do we get the
signal information but lose virtually all the power to
re-radiation? (assume high S/S+N)

Cecil, W5DXP wrote:
"If we simply amplify the receiver voltage through a near infinite
impedance FET with virtually no load on the signal, do we get the signal
information?"

Not as much as possible.

Just as the reactance must be tuned out of an antenna to receive maximum
available power, to get maximum received carrier power, the resistance
of the load in the antenna must be reduced to only that matcing the
radiation resistance. Walter Maxwell, W2DU has had it right all along.
Maximum power transfer requires a conjugate match.

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:

Cecil, W5DXP wrote:
"If we simply amplify the receiver voltage through a near infinite
impedance FET with virtually no load on the signal, do we get the signal
information?"

Not as much as possible.

Just as the reactance must be tuned out of an antenna to receive maximum
available power, to get maximum received carrier power, the resistance
of the load in the antenna must be reduced to only that matcing the
radiation resistance. Walter Maxwell, W2DU has had it right all along.
Maximum power transfer requires a conjugate match.

I happend across this just the other day. On page 1426, "Physics:
Volume Two Electricity, Magnetism, and Light", Ronald Blum/Duane E.
Roller it reads "Note that the optimum power transfer occurs when p=0,
R=Z0, and t=1. [p and t are reflection and transmission coefficients]
In general, for lossy lines, this takes place when the load impedance in
the complex conjugate of the characteristic impedance, just as with
lumped circuits."

Seems to be the only way for p = (R-Z0)/(R+Z0) = 0 and
t = 2R/R+Z0 = 1 to hold true for any Z0.

ac6xg

ac6xg

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:

Cecil, W5DXP wrote:
"If we simply amplify the receiver voltage through a near infinite
impedance FET with virtually no load on the signal, do we get the signal
information?"

Not as much as possible.

Just as the reactance must be tuned out of an antenna to receive maximum
available power, to get maximum received carrier power, the resistance
of the load in the antenna must be reduced to only that matcing the
radiation resistance. Walter Maxwell, W2DU has had it right all along.
Maximum power transfer requires a conjugate match.

But the information in a received signal can be had while
consuming very little of the received signal's power. For
instance, ten feet of wire into my IC-756PRO results in
perfect reception of Neal Boortz on WTAW on 1620 kHz.
--
73, Cecil http://www.qsl.net/w5dxp

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