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Tutorial
- Yagi Antenna Design
The NBS data for Yagi design
form a good base line for design. These designs are based
on unequal director lengths in general. The difference in
gain between these and equal length director Yagis is negligible
for short antennas and about 0.5 dB for long ones. Lawson
showed that using the average length of the NBS specification
for all elements results in much simpler antennas with equivalent
performance. The NBS design method was thus modified by Poynting
Innovations and is given below :
| Boom length |
0.4 |
0.8 |
1.2 |
2.2 |
3.2 |
4.2 |
| Reflector |
0.482
|
0.482 |
0.482
|
0.482
|
0.482 |
0.475 |
| Reflector
Spacing |
0.2 |
0.2 |
0.2 |
0.2 |
0.2 |
0.2 |
| No. of directors |
1 |
3 |
4 |
10 |
15 |
13 |
| Director |
0.442
|
0.427 |
0.424
|
0.402
|
0.395
|
0.401 |
| Director Spacing |
0.2 |
0.2 |
0.2 |
0.2 |
0.2 |
0.2 |
| G(dBd) |
7.1 |
9.2 |
10.2 |
12.25 |
13.4 |
14.2 |
Yagi Design Details (All
dimensions in wavelengths)
The length of the driven element
can be chosen for the optimum match condition since it does
not affect gain operation much.
Element correction
The element lengths shown above
are for a diameter to wavelength ratio of 0.008. For different
radius elements the same design data can be modified by using
a curve with correction factors as determined by the NBS team.
They used a very complicated process to determine the correct
element lengths due to a clumsy formulation of these curves.
Figure 2 shows a modified curve we derived from their sets
of curves that achieves the same effect quite simply.
 |
 |
| Figure1: Graph showing
the length to be added to Parasitic Elements to compensate
for the effect of the Supporting Boom |
Figure 2: Multiplication
factor for different diameter to Wavelength Ratios of
Director and Reflectors |
If the elements are mounted
through a metal boom, as if often done for mechanical support,
a further correction factor must be applied to get the correct
length. This is due to the slight distortion of the field
at the center of the dipole due to the boom. It is viewed
as some additional capacitance and will thus require the length
to be increased with increased boom diameter. The curve to
do this correction is give in figure 1. When the elements
are mounted on insulators - at least an element radius away
from the boom - this effect becomes negligible.
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