ARLA/CLUSTER: Antenas Frias.

[António Matias]

Como tudo começou, leia o artigo do Lubijca  Popa  YU7EF
Muito interessante !

*EF0210LT*


*ANTENNA WITH EXTREMELY SUPPRESED SIDELOBES *

*TO MINIMIZE MAN MADE NOISE PICKUP*

*( PUBLISHED IN DUBUS 4/2006 )*



Ever since I started working on 144MHz, back in 1985, my major problem was
fighting with a city noise (man made noise) since I live in a medium sized
city but with a lot of industrial facilities. Sometimes noise was just
unbearable in some particular directions and it was almost impossible to
listen for a weak signals. At that time I was using very popular and well
known DL6WU 11 element yagi – very good performance antenna but obviously
still not good enough – now I see and know why. Most of my activity those
days was high speed CW MS and all of you that have done that kind of
communication know that very often reflections were weak, particularly when
we operated out of major meteor showers, trying to use sporadic meteors.

Later I have built a new 15 element DL6WU antenna that gave me better
results noise vise and more gain but it was not that efficient in collecting
signals reflected from meteors due to narrow beam width.

As soon I got my first PC in 1991 (286/12MHz with math coprocessor and 5MB
RAM) I came across “Mininec 1” software for antenna simulations and started
“playing” with it. It was very useful to visualize antenna patterns and
excitation parameters. However, due to very slow computer it was painful to
do it - just for one calculation I had to wait for 15 to 20 minutes to end.
Very quickly I got W7EL program “Elnec 1.0” that was much more user friendly
so I was spending more and more time analyzing various antennas and trying
to do some modifications to existing ones. As a result, I came to somehow
modified design of 4 WL DL6WU antenna and have built it immediately to see
if that really works. To make it short, it was noticeably better than
original antenna and that gave me first ideas of where design has to go to
reduce city noise pickup. With those two antennas stacked vertically I even
worked quite some number of EME stations in early 90’s, only in CW, using
only my TR-9000 and about 500w power amplifier.

Later, with AO software that was even better, having optimization feature
available, actually I started designing my own antennas, rather than just
doing modifications to published designs. Once I got NEC2 program things
were much better and results were more reliable and I am using it ever
since. There are many different programs at the market today but most of
them are giving similar results and the difference is just how the authors
have developed user interface and couple of different options. For several
years now I am using 4nec2 software from Arie Voors, in my opinion currently
the best software available.

Being intrigued with antenna performance I have spent many hours analyzing
any antenna I came across in various magazines and publications, just to
verify declared specifications and see if they are really that good as they
say. Well, I must say that probably > 70% of those antennas were not even
close to declared specifications and were all but good and just a few
actually confirmed declared specifications. Doing those analyses I also
realized how many mistakes were made and some statements were false, no
wonder good antennas were rare those days. Having all that in mind, ever
since then, I never used any other antennas but my own designs.

Naturally, it took me many years and thousands of hours on simulations and
optimizations until I came to designs that are compatible to some of most
famous designs. For a long time I was actually walking in the dark, not
being able to make a brake through how to get rid of those side lobes since
I did realize they are to blame for increased local, man made noise pickup.

Thanks to Peter Sundberg program Yagi Analysis 3.54 I finally had an
opportunity to actually “see” where the problem really is, since it is
obviously showing what part of radiation pattern is to blame for good or
poor noise specification of the antenna. Only with that program I could see
that total antenna temperature is mostly dependant on suppression of side
lobes and clear pattern. At the same time, it was obvious that F/B ratio is
not so important since difference from say 20dB to a 25dB would reduce total
temperature only few °K which is not worth too much sacrifice. Many designs
might have been much better if only people doing them were not so much
focused on F/B ratio that, on the other hand, led to poor side lobe
suppression.

As every optimization is a “game” with various trade off parameters, I was
focused on getting design with pure 50 Ohm impedance, reasonably wide
operating range, gain as close to FOM as per DL6WU formula and suppressed
side lobes to acceptable level in order to get as good G/T ratio as
possible.

Most of those designs were tested by myself or my friends and results were
very good. However, for a very long time I did not publish my designs for
various reasons. Finally, after several friends told me I should do it, I
have sent files to Lionel VE7BQH for his review and opinion. After they were
published in his chart, many people started asking questions and information
and I decided to publish them on the WEB as I was not able to answer
numerous mails I was receiving on daily basis.

However, I am confident that each and every of these antennas can be
improved even more, so I will continue working on them and hopefully will
come up with even better results so there is definitely more to follow.

*****************************************************************************

When I started this article I was thinking what antenna should I present and
it didn’t look reasonable to pickup one of the antennas already published on
the WEB. Instead, I have decided to be an antenna that was especially
developed for those living in extremely noisy areas such as big cities or
huge industrial facilities who are suffering the most from man made noise.

As I said before, I was always fighting with heavy city noise and was always
trying to figure out way to reduce it as much as possible. In order to get
antenna with maximum suppression of side lobes it is necessary to give up
much more gain than usual. Idea is based on my consideration like this:

If my antenna has to pick up noise from all directions, and it does, let me
than reduce pick up from all other directions but from main lobe. I will
give up some more gain than usual for superior side lobe suppression but
incoming signals will be heard with less birdies and other junk.

As a result, antenna that will be presented here is something that can not
be considered as a top class antenna by looking to gain or G/T ratio. As a
matter of fact, if someone will compare gain of this antenna with some other
designs of the same boom length, it is about 0.5-0.6dB below the DL6WU
F.O.M. (figure of merit) for that boom length. I am considering it worth
doing and acceptable if I can reduce city noise pick up.

This boom length is chosen as, from my experience, such antenna has optimal
-3dB beam width for MS and terrestrial communications. Two of those
vertically stacked antennas will give us more than sufficient gain for
successful work on daily basis and H stack of four antennas should be
sufficient even for EME .

Since there are so many different ways of antenna construction and element
mounting I am always giving free space dimensions for elements – letting
everyone to execute building preferred way. Naturally, depending on the
chosen construction technique, appropriate boom correction to the element
lengths has to be applied in order to maintain original specifications.

Table with dimensions:

*EF0210LT ANTENNA FREE SPACE DIMENSIONS*

* *

REF

DE (10mm)

D1

D2

D3

D4

D5

D6

D7

D8

SPACINGS

0

280

387

744

1318

1984

2727

3536

4390

5190

5 mm

1028

1004

970

957

936

916

896

876

840

79

Driven element length shown in table is for an open dipole but, as always, I
highly recommend use of folded dipole whenever possible for many reasons. If
nothing else, folded dipole will collect far less static noise from the
atmosphere than open dipole. Since impedance is optimized to pure 50 Ohm,
folded dipole can be easy fed using traditional l/2 * k  4:1 balun.
Since I learned from the correspondence with many hams that determining
correct dimensions of the folded dipole seems to be a problem for many
people, for your convenience here is drawing with complete dimensions for
this particular antenna with 5.0mm elements



Single antenna free space gain



F/B ratio



SWR Diagram



Single antenna horizontal plane radiation pattern:



Single antenna Vertical plane radiation pattern:



As it can be seen from radiation patterns, -3dB beam width is very good and
therefore antennas can be stacked much closer which is very convenient. As
per DL6WU stacking formula

                                                 Sopt  =  l / sin j/2

optimum stacking distance for this antenna would be:

Horizontal      =  3.0m
Vertical         =  2.8m

With antennas stacked to this spacing, 4 yagi bay specifications will be:

Gain                             19.78 dBi

Ta                                224.5 K

G/T                              -3.73 dB
and first side lobes would be down 11 – 12 dB - radiation patterns would
look as shown:



Optimally spaced 4 yagi bay Horizontal plane pattern



Optimally spaced 4 yagi bay Vertical plane pattern



However, for those who still may think first side lobes will still pick up
too much unwanted junk from the neighborhood, antennas can be stacked even
closer to

Horizontal      = 2.5 m

Vertical         = 2.4 m

This way, with sacrifice of some additional 0.6dB of gain we will get almost
ideal radiation pattern


Horizontal plane pattern



Vertical plane pattern



Close spaced 4 yagi bay 3D pattern



In this case, specifications of the 4 yagi bay are:

Gain                             19.16 dBi

Ta                                211.4 °K

G/T                              -4.09 dB

and first side lobes would be down to as much as 17 – 18 dB.


Closer spaced 4 yagi bay gain diagram:



Closer spaced 4 yagi bay F/B diagram:



Closer spaced 4 yagi bay SWR diagram



What is the difference of this antenna to other published designs?

Well, take a look to the following patterns and you will se how this antenna
looks compared to popular and known designs. As an useful example I have
chosen to compare it to a DL6WU 11 element yagi and 9 element DK7ZB antenna
that are of about same boom length.


Horizontal plane pattern comparison:



Vertical plane pattern comparison:



By looking at this comparison, I consider it worth giving up 0.5 – 0.6 dB of
gain for such a clear pattern that will significantly reduce man made noise
and junk pickup.

All remarks and notes I have given on the WEB about construction of
published antennas apply to this antenna entirely and they are the same so
probably there is no need to repeat them.

Unfortunately, I didn’t have time to actually build this antenna and have it
tested but definitely it will be done in some time. If anyone of you would
considerate worth trying it, I would appreciate honest, critical reports of
performance as well as pictures of built antennas.
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