# Electrically short antenna

Discussion in 'Electronic Basics' started by thejim, Jan 17, 2006.

1. ### thejimGuest

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?

2. ### Fred StevensGuest

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.

3. ### David HarmonGuest

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?

4. ### John LarkinGuest

A short antenna looks like a capacitor.

John