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Yagi Antenna
with Offset Reflectors
Description
of Antenna
The basic configuration of
a Yagi with offset reflectors is shown below.
For the purposes of the design
curves presented below, there are four possible configurations
for the reflectors:
* configuration
1: R2 is not present, only two reflectors
* configuration 2a: sepa
= 0.5sepb
* configuration 2a: sepa
= sepb
* configuration 3: shown
below in the diagram
Design Curves
The following design curves
are developed for Yagi antennas with offset reflectors in
the configurations shown above. The curves illustrate how
the gain and bandwidth of the antenna are affected by changing
thickness factor, which is defined as:
ohm = 2ln(ElementLength divided
byElementRadius)..............1
The curves are generated with
two cases for each configuration:
1. director
length tapers parabolically (away from the reflectors)
2. directors are all
the same length ('no taper')
Figure 1a and 1b show the gain
for the various configurations as a function of thickness
factor for boomlengths of 1 and 2 wavelengths respectively.
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| Figure 1a: Gain vs
Thickness Factor for Boom Length of 1 wavelength |
Figure 1b: Gain vs
Thickness Factor for Boom Length of 2 wavelengths |
Figure 2a and 2b show the bandwidth
of the antenna as a function of thickness factor. These curves
illustrate the gain bandwidth trade-off that must be made
when designing according to thickness factor.
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| Figure 2a: Gain vs
Thickness Factor for Boom Length of 2 wavelengths |
Figure 2b: Gain vs
Thickness Factor for Boom Length of 1 wavelength |
Once the thickness factor is
known, the antenna can either be designed according to a specific
element length or to a specific element radius (by u. The
factors that are constant in the above design curves are as
follows:
* director spacing of 0.125
lambda(spcdir)
* reflector length of 0.5 lambda
* reflector spacing of 0.2 lambda (spcref)
* reflector separation (sepa) of between 0.1 lambda and 0.16
lambda
Example
A Yagi antenna is required
to meet the following specifications:
* operating frequency of 900
MHz
* boom length of 1 wavelength
* minimum bandwidth of 18%
* maximum possible gain
From figure 2a, it can be seen
that all configurations (except configuration 1) meet the
required bandwidth at a thickness factor of 7. The maximum
gain however, is obtained from an antenna of configuration
3 (tapered) and is approximately 11.75dBi.
The antenna can be summarised
as follows:
* Boom length of 1? (333mm
at 900MHz)
* director spacing of
41.7mm (spcdir)
* reflector length of
166.7mm
* reflector spacing of
66.7mm (spcref)
* reflector separation
(sepa) of between 33.3mm and 53.3mm
* bandwidth of 18%
* gain of 11.75dBi
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Note: these design curves
provide only a rough estimate of a particular antenna's dimensions
and performance. In order to fully specify the antenna it
should be simulated, using the values obtained from the curves
if so desired.
Reference:
Fourth Year Design Report, "Antenna Design Wizards: Design
Curves for Yagis with Offset Reflectors" prepared by
Nathalie Tallier for the School of Electrical and Information
Engineering, University of the Witwatersrand, August 2002.
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