What determines an antenna's impedance
 

I know in a ground plane the angle of the radials can, but what about a
simple, multiple element yagi?
How does one measure what the impedance is or will be?

wrote in message
oups.com...
I know in a ground plane the angle of the radials can, but what about a
simple, multiple element yagi?

The spacing of the elements and thei angles they're mounted out (a yagi
consists of a regular old dipole with a reflector and a bunch of directors
added).

How does one measure what the impedance is or will be?

The equations for designing a Yagi are rather involved, but you certainly
could use them if you're dead set on it. Otherwise, you use an antenna
modeling program such as NEC to predict the impedance, and an antenna
analyzer to measure the impedance of an actual antenna. (Internally, an
antenna analyzer is measuring the SWR [and relative phase] of a small RF
test signal applied; it can convert this back into an impedance.)

Any antenna will be either "inductive" or "capacitive", depending on its
length as relative to the frequency its designed for. If it is short for the
frequency, it is usually capacitive, and if it is long for the frequency, it
will exhibit inductive qualities. The combination of these inductive and
capacitive factors, added vectorially, results in the "Z", or impedance , of
the antenna. Impedance is actually sort of a lossless form of resistance.
More info on these factors can be found in the arrl antenna handbook,
available from the arrl headquarters in Newington, Connecticut.

"Z" was a factor in these discuassions and arguments I have been having with
people about how their antenna's active element was "grounded' over the
years. for DC purposes, it may be grounded, but they fail to take into
account that the antenna's impedance to ground is actually higher than that
when RF energy is applied to it... The antenna is usually "grounded", so to
speak, through a reactive element that produces quite a high impedance
between ground and the active antenna element at RF. If it didn't , the
antenna would send and receive nothing, since all signals WOULD get shorted
to ground ! But, it seems the gist of this argument, which any novice ham
operator is more than familiar with, escapes them....

wrote in message
oups.com...
I know in a ground plane the angle of the radials can, but what about a
simple, multiple element yagi?
How does one measure what the impedance is or will be?

Zombie Wolf wrote:
Any antenna will be either "inductive" or "capacitive", depending on its
length as relative to the frequency its designed for. If it is short for the
frequency, it is usually capacitive, and if it is long for the frequency, it
will exhibit inductive qualities. The combination of these inductive and
capacitive factors, added vectorially, results in the "Z", or impedance , of
the antenna.

Simply put, the feedpoint impedance of an antenna is the ratio of
the total voltage to the total current at the feedpoint. In a
standing-wave antenna, like a center-fed dipole, the total voltage
is the vector sum of the forward voltage and reflected voltage.
The total current is the vector sum of the forward current and
the reflected current. The reflected current undergoes a 180 deg
phase shift at the tips of the dipole but the reflected voltage
does not. For a resonant 1/2WL center-fed dipole, the feedpoint
impedance is:

Zfp = (|Vfor|-|Vref|)/(|Ifor|+|Iref|) = low

i.e. Vfor is 180 degrees out of phase with Vref and Ifor is
in phase with Iref.

For a (resonant) one-wavelength center-fed dipole, the feedpoint
impedance is:

Zfp = (|Vfor|+|Vref|)/(|Ifor|-|Iref|) = high

i.e. Vfor is in phase with Vref and Ifor is 180 degrees out
of phase with Iref.
--
73, Cecil http://www.qsl.net/w5dxp


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Stryped wrote:
"I know in a ground plane the angle of the radials can, but what about a
simple multiple element yagi?"

The feedpoint impedance of a resonant 1/2-wave dipole is about 67 ohms
resistive (page 120 of Kraus` 3rd edition of "Antennas"). The impedance
of a 1/4-wave antenna perpendicular to ground is about half the dipole`s
impedance (page 568 Kraus` 3rd edition).

Drooping radials place a ground plane in a transition between a 1/4-wave
perpendicular to ground and a 1/2-wave antenna. Impedance follows suit,
ranging from 30 ohms with no droop to 60+ ohms with complete droop. In
between, a match for 50-ohm coax can probably be foind.

An antenna element alone in free-space has a particular self-impedance.
Aide by side elements are coupled. Current flowing in one ekement
induces voltage in other elements nearby. This affects the feedpoint
impedance of the driven element because it is feeding an array. Another
name for coupling is mutual impedance. It is measured as the voltage
produced in one element bivided by current in the driven element. It`s
reciprocal. Either element can be driven and the other can be parasitic.
The mutual impedance is the same either way. The feedpoint impedance is
a complex sum of all the array impedances.. It is often tedious to
calculate. Kraus shows the mutual impedance story on page 452 of his 3rd
edition of "Antennas".

Cecil, W5DXP gave a good explanation of impedabce in a standing wave
antenna. He showed how a high impedance at the open-circuit end of the
antenna is converted to a low voltage and a high current 1/4-wave back
at the feedpoint. Their quotient is the low self-impedance at the
feedpoint.

Best regards, Richard Harrison, KB5WZI