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Electrically short antenna

T

thejim

Jan 1, 1970
0
I read in a book the following:
An Electrically short antenna means that is shorter than one half
wavelength which also means that is not resistive.
I have 2 Qs.
1.When it says "shorter than one half wavelength" it means the physical
length?
2.Why being electrically short means it is not resistive?
 
F

Fred Stevens

Jan 1, 1970
0
thejim said:
I read in a book the following:
An Electrically short antenna means that is shorter than one half
wavelength which also means that is not resistive.
I have 2 Qs.
1.When it says "shorter than one half wavelength" it means the physical
length?
2.Why being electrically short means it is not resistive?

1) The physical length is shorter than one half the wavelength of the
signal
2) An antenna or transmission line which is not terminated with its
characteristic impedance will have an input impedance which has phase
angle and will be an impedance rather than a pure resistance.

fred.
 
D

David Harmon

Jan 1, 1970
0
On 17 Jan 2006 09:05:45 -0800 in sci.electronics.basics, "thejim"
1.When it says "shorter than one half wavelength" it means the physical
length?

Mainly, but if it says electrically short then I think it means
having the electrical characteristics normally associated with an
antenna physically shorter than the resonant length. Something
other than physical length that affects resonance could be figured
in.
2.Why being electrically short means it is not resistive?

If the antenna is a resonant length for the frequency in question,
then its inductance reactance and capacitance reactance cancel out
leaving only the resistive components as seen by the transmitter.
The current neither leads nor lags the voltage at the feed point.

If shorter, it is capacitive + resistive, current leads voltage.
If longer, inductive + resistive, current lags voltage.
Or is that the other way around?
 
J

John Larkin

Jan 1, 1970
0
On 17 Jan 2006 09:05:45 -0800 in sci.electronics.basics, "thejim"


Mainly, but if it says electrically short then I think it means
having the electrical characteristics normally associated with an
antenna physically shorter than the resonant length. Something
other than physical length that affects resonance could be figured
in.


If the antenna is a resonant length for the frequency in question,
then its inductance reactance and capacitance reactance cancel out
leaving only the resistive components as seen by the transmitter.
The current neither leads nor lags the voltage at the feed point.

If shorter, it is capacitive + resistive, current leads voltage.
If longer, inductive + resistive, current lags voltage.
Or is that the other way around?

A short antenna looks like a capacitor.

John
 
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