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Transcript
```Spencer Streeter
ECE 3300
Portfolio - 9/29/06-10/04/06
Portfolio - 9/29/06
Antenna parameters:
input impedance
“The impedance presented by an antenna at its terminals or the ratio of the voltage to current at a pair of terminals or
the ratio of the appropriate components of the electric to magnetic fields at a point”. An antenna’s input impedance
is generally a function of frequency. Thus the antenna will be matched to the interconnecting transmission line and
other associated equipment only within a bandwidth. The impedance is important for determining antenna
efficiency. All power sources have internal impedance. Maximum power is transferred to an external device when
its impedance matches the source internal impedance.
resonant frequency
Typically an antenna is tuned for a specific frequency, and is effective for a range of frequencies usually centered on
that resonant frequency. For example, an antenna tuned for 440MHz has a resonant frequency of 440MHz.
bandwidth
is defined as “the range of frequencies within which the performance of the antenna, with respect to some
characteristic, conforms to a specified standard”. The considered characteristic can be input impedance, pattern,
beamwidth, polarization, side lobe level, gain, beam direction or radiation efficiency. The bandwidth can be
considered as the range of frequencies on either side of a center frequency. For broadband antennas, the bandwidth
is usually expressed as the ratio of the upper-to-lower frequencies (e.g. 10:1). For narrowband antennas, the
bandwidth is expressed as a percentage of the frequency difference (upper minus lower) over the center frequency of
the bandwidth (e.g. 5%). All the characteristics of an antenna do not necessarily vary in the same manner, or are
even critically affected by the frequency, so the specifications are set in each case to meet the needs of the particular
application.
radiation pattern is the directional function characterizing the relative distribution of power radiated by an antenna.
Also known as the antenna pattern.
Antenna pattern – (pg. 380) describes the far-field directional properties of an antenna when measured at a fixed
distance from the antenna. In general is a three-dimensional plot that displays the strength of the radiated field of
power density as a function of direction (zenith angle  and azimuth angle )
Gain
"Gain" as a parameter measures the directionality of a given antenna. An antenna with a low gain emits radiation
in all directions equally, whereas a high-gain antenna will preferentially radiate in particular directions.
Specifically, the Gain, Directive gain or Power gain of an antenna is defined as the ratio of the intensity (power per
unit surface) radiated by the antenna in a given direction at an arbitrary distance divided by the intensity radiated
at the same distance by an hypothetical isotropic antenna:
G

4R 2Smax
4R 2 Smax P Sant
 D  

Pt
P Siso
Gain accounts for ohmic losses in the antenna material, directivity (D) does not.
The gain of an antenna is a passive phenomena - power is not added by the antenna, but simply redistributed to
provide more radiated power in a certain direction than would be transmitted by an isotropic antenna. If an antenna
has a greater than one gain in some directions, it must have a less than one gain in other directions since energy is
conserved by the antenna.
How would you measure them?
Input impedence can be measured by using a network analyzer.
Resonant frequency can be found by using a network analyzer to find a “null”.
Bandwidth can also be found using a network analyzer to find the width of the gap at 3 dB which ultimately defines
the bandwidth.
1-2
UNIVERSITY OF UTAH DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING
50 S. Central Campus Dr | Salt Lake City, UT 84112-9206 | Phone: (801) 581-6941 | Fax: (801) 581-5281 | www.ece.utah.edu
Spencer Streeter
ECE 3300
Portfolio - 9/29/06-10/04/06
Figure 1 - Bandwidth
Reciprocity allows you to compute the radiation pattern in the transmission mode. (pg.374). Another way to
measure the radiation pattern would be to measure the power at each angle as you turned the antenna 360º’s.
Gain could be measured by taking the maximum power found of all the power measurements made at each angle
and dividing by the minimum power found of the same power measurements.
Portfolio - 10/2/06
What is instantaneous power and time-averaged power?
Instantaneous power is the product of instantaneous voltage and instantaneous current.
For instantaneous incident power or power as it arrives to the load we have…
P (t) 
i
V0
2
cos 2 t     (W)
Z0
For the instantaneous power reflected by the load (negative sign signifies –z direction of travel) we have…

P r (t)  
2
V0
2
cos2 t      r  (W)
Z0
Time-averaged power is instantaneous power averaged over one time period.



Pav  P  P 
i
av
r
av
V0
1
Pav  V˜  I˜ * 
2
2
2Z 0
Portfolio - 10/2/06
No portfolio for today.
1  
2
(remember that
z  z*  z )

2-2
UNIVERSITY OF UTAH DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING
50 S. Central Campus Dr | Salt Lake City, UT 84112-9206 | Phone: (801) 581-6941 | Fax: (801) 581-5281 | www.ece.utah.edu
```