RE: NEC-LIST:Band width of Yagi

Hi Zvi,

A number of years ago, we used a genetic algorithm to design a wide band
Yagi with low sidelobes. It was published in the April, 1997 Antennas and
Propagation Magazine.

The encoding of the algorithm was done as follows:

                        length of element #1 (reflector)
                        length of element #2 (driven element)
                        spacing between elements #1 and #2
                        length of element #3 (first director)
                        spacing between elements # 2 and #3
                           length of element #4 (second director) and etc.

 The antenna was simulated at 243 and 223 MHz, near the edges of the desired
frequency band. We assumed that if the antenna performed satisfactorily at
these frequencies, it would probably be acceptable over the rest of the
band. We chose a sample population of 175 chromosomes, 30% of which was
saved from generation to generation; parent selection was based on the
fitness-weighted roulette-wheel.

Although the feed was over a finite ground plane, we decided to initially
use a conventional reflector element in the design since modelling a finite
ground plane using NEC adds a prohibitive amount of computer time. After an
optimal configuration was obtained we conducted a thorough computational
analysis for the whole frequency band from 219 to 251 MHz at increments of 2
MHz. This is to ensure that the antenna is truly broadband. This was done
initially for the Yagi with a reflector element and then repeated with a
1.17 meter ground plane.

    We then fabricated the antenna and measured E- plane patterns. Since
the antenna pattern range did not operate satisfactorily at frequencies
below about 800 MHz and since the full scale antenna would have been quite
large we decided to work with a sixth scale model having a center frequency
of 1410 MHz. The Yagi elements were made of .8mm ( 1/32 in.) copper rod.
These elements were inserted into 1.27 cm (1/2 in.) PVC pipe. It is
estimated that the Yagi was built to an accuracy of about +/- .5 mm. The
Yagi with a reflector element was first measured over the frequency range
from 1310 to 1510 MHz at 10 MHz increments and then the measurements were
repeated using a 19.5 cm square ground plane in place of the reflector
element.

As expected, the genetic algorithm produced a configuration that was quite
different from one that would have been obtained using conventional
methods. Typical Yagi designs have directors that are about .4 lambda in
length and .35 lambda in spacing; the lengths become slightly shorter and
the spacings become slightly larger the further the distance from the driven
element. The genetic Yagi had 13 elements, plus the ground plane, with a
boom length of only 1.11 lambda. The directors varied in length from about
.25 lambda to .4 lambda with an average spacing of less than .1 lambda. A
conventional 14 element Yagi would have had a boom length about 3 times as
long.
.
The performance of this Yagi was computed at 2 MHz increments over the band
from 219 to 251 MHz, a bandwidth of 13.6%. The initial results were for a
Yagi having a reflector element and no ground plane for frequencies of 219,
235 and 251 MHz. We were cautiously optimistic that replacing the
reflector element with a 1.17 meter ground plane would certainly reduce the
backlobes and might also lower the sidelobes. With this change the side and
backlobe levels for both planes were greater than 25 dB down from 223 to 243
MHz, the most important part of the band, and were over 20 dB down over the
rest of the band. The E- and H-plane half-power beamwidths ranged from 51
to 55° and 64 to 69° respectively, slightly larger than desired 50° but
certainly acceptable. The antenna gain ranged from 10.4 to 11.0 dB over the
frequency band. This gain is approximately 1 dB lower than that for a Yagi,
with the same boom length, that is optimized for maximum gain. The VSWR’s
were less than 3.0 from 227 to 245 MHz; they were however higher at the ends
of the band.

E- plane patterns and VSWR were measured for the genetic Yagi over a finite
ground plane for the frequency range of 1310 to 1510 MHz at 10 MHz
increments. The pattern range was about 2600 feet in length. The test
antenna was housed in a bay that is 20 ft.x 20 ft.x 20 ft. with absorbing
material that was good to about 800 MHz. The VSWR measurements were made
with an HP8510 Network Analyzer. The measured VSWR’s are less than 3.0
over most of the band and have a maximum value of 3.7 near the ends. The
measured gains are slightly less than 10 dB, however if the reflection
losses are taken into account the corrected values for a matched antenna
approach the computed gains.

Please let me know if you need any additional info.

Ed Altshuler
-----Original Message-----
From: zvi frank [mailto:zvi_f_at_mti-group.co.il]
Sent: Sunday, June 16, 2002 3:08 AM
To: NEC LIST (E-mail)
Subject: NEC-LIST:Band width of Yagi

HI All,
         This is probably an old question but what is the bandwidth of a
YAGI. As far I know it is very small . Is there away of broadening the
bandwidth ? How much can one get ? For low Gain YaGI's (3 or 4 elements) Is
the bandwidth wider ?
By Band width I mean both the VSWR and Gain perameters
     Thanks in advance for all comments regards,

Zvi

--
The NEC-List mailing list <nec-list_at_gweep.ca>
http://www.gweep.ca/mailman/listinfo.cgi/nec-list
-- 
The NEC-List mailing list <nec-list_at_gweep.ca>
http://www.gweep.ca/mailman/listinfo.cgi/nec-list