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antenna array - RF Cafe Forums
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Hai4u_2000
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Post subject: antenna array
Posted: Wed Jan 28, 2004 1:33 pm
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Hi all, I have 2 10-db helix antennas and want to
put them together. what is the distance should be
if I want to get maximum gain and not having high
sidelobe level. Does anyone have any ideas about
it. YOur advise will be very appreciated.
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JGP |
Post subject: antenna array
Posted: Wed Jan 28, 2004 2:10 pm
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Welcome to forum,
First off, I need to know
the following: Frequency band (it would be nice
if you could provide center frequency (Fc), RF power
level, applications (specific application) and transmission
and receiving radius. Is that an Yagi type antenna?
Is that for Ham or commercial radio applications?
Well, let me give you some basic example
to illustrate the (as yet unwritten) points above.
And helix that shows excellent overall performance:
L=48", N=30, Gain=18.8dBic. First sidelobe level
-22.4dB.This is much better than the typical uniformly
wound long helix, which typically shows 8-10dB sidelobes.
With a clean pattern like this, the array design
process is selecting the element spacing to balance
between maximum gain (maximum spacing) and minimum
sidelobes (minimum spacing). The two goals trade
off directly (high gain means higher sidelobes,
low sidelobes means less gain).
Here is
a low-sidelobe approach:
The array has 20.6dBic
gain (only 1.8dB more than a single element!) but
the sidelobes are <-30dB all the way out past
45 degrees (this is better sidelobe performance
than the individual element!). This happens because
the array factor null is moved out to hit the element
pattern just past the -20dB point, thereby suppressing
the overall radiation pattern well below 20dB. The
array factor takes care of business in close to
the main lobe; the cleanness of the element does
the rest past 45 degrees.
Here is the maximum-gain
approach:
This array has 24.1dBic gain (!),
a 5.2dB improvement over a single element. In exchange,
you get a -15dB sidelobe right next to the main
lobe. By increasing the spacing, you move the array
factor null in closer to the main beam (thus reducing
the overall half-power beamwidth, and therefore
the gain), but the closer you move the null to the
beam peak, the more of the main lobe "leaks" out
behind the null, producing a sidelobe in the overall
pattern. One nice thing about this pattern is if
it is aimed in space much above about 25 degrees
in elevation, that sidelobe points into cold space
(rather than at the warm earth) and G/T is greatly
improved.
How to decide? The goal here is
to maximize G/T. We see that in the selection of
array spacing, we can change the G term by 3.5dB;
the question is to determine how much the T term
changes between the two approaches.
Or...who
says the array has to be square? One compromise
approach is to pick the maximum gain spacing in
the horizontal direction (azimuthal plane), and
the minimum sidelobe spacing in the vertical direction
(elevation plane).
I thought this might
help you. Good luck!
Sincerely,
John
Pereira
Note: You may want to check these
books and authors below:
The "definitive"
measurement campaign for helical elements, oft-cited,
is: King and Wong, "Characteristics of 1 to 8 wavelength
Uniform Helical Antennas", IEEE Transactions on
Antennas and Propagation, vol, AP-27, January 1979,
pp. 72-78.
Jasik and Johnson, Antenna Engineering
Handbook, 2nd ed., Chapter 13. An excellent NEC-2
modelling program was reported by Emerson, "The
Gain of an Axial-Mode Helix Antenna", ARRL Antenna
Compendium Vol. 4, pp. 64-68.
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JGP |
Post subject: antenna array
Posted: Wed Jan 28, 2004 2:33 pm
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Keep in mind that this type of antenna can work
for both, directional and omnidirectional pattern
and well into Microwave region. I have prototypes
for both in 2.4 GHz frequency range and work very
well.
Sincerely,
John Pereira
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hai4u_2000 |
Post subject: antenna array
Posted: Thu Jan 29, 2004 12:37 pm
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thanks a lot John. Let forget the sidelobe for
a moment. the gain of the antenna is maximum at
certain distance between the two elements. and if
the distance too far, the gain begins to drop. the
question is: what should the distance be?? thanks
in advanced H. Le
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JGP |
Post subject: antenna array
Posted: Thu Jan 29, 2004 6:26 pm
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For my own prototype in 2.4 GHz (ISM band), I have
used as following and it is working very well. Now,
I don’t know your applications, frequency band,
gain, sidelobes desired…etc. It would be nice if
you could provide more information. I think
it is wise to go futher with explanation, thus you
will get the results you are asking for: For frequencies
in the range 2 - 7 GHz this design is very easy
to do, practical and not difficult at all. This
contribution describes how to produce a helix antenna
for frequencies around 2.4 GHz which can be used
for an example; high speed packet radio. Developments
in WLAN equipment result in easy possibilities for
high speed wireless internet access using the 802.11b
(as know as WiFi) standard. The helix antenna
can be considered as a spring with N turns with
a reflector. The circumference (C) of a turn is
approximately one wavelength (l), and, the distance
(d) between the turns is approx. 0.25C. The size
of the reflector (R) is equal to C or l, and can
be a circle or a square. The design yields circular
polarization (CP), which can be either 'right hand'
or 'left hand' (RHCP or LHCP respectively), depending
upon how the helix is wound. To have maximum transfer
of energy, both ends of the link must use the same
polarization, unless you use a (passive) reflector
in the radio path. The gain (G) of the antenna,
relative to an isotropic (dBi), can be estimated
by: G = 11.8 + 10 * log {(C/l)^2 * N * d} dBi
(a) The characteristic impedance (Z) of
the resulting 'transmission line' empirically seems
to be: Z = 140 * (C/l) (b) Ohm l
= (0.3/2.43) = 0.1234567 m
(12.34
cm) (c) The diameter (D) of one turn = (l/pi)
= 39.3 mm (d) Standard PVC sewer pipe with
an outer diameter of 40 mm is perfect for the job
and can be obtained easily from a (do it yourself)
shop or a plumber. PVC isolation and a 1.5 mm thick
copper core. Winding it around the PVC pipe will
result in D = ca. 42 mm, due to the thickness of
the isolation. With D = 42 mm, C = 42*pi
= 132 mm (which is 1.07 l) (e) Now d = 0.25C
= 0.25*132 = 33 mm (f) For distances ranging
from 100 m - 2.5 km with Line Of Sight (LOS), 12
turns (N = 12) are sufficient. The length of the
PVC pipe therefore will be 40 cm (3.24 l).
I hope this will help! John Pereira
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hai4u_2000 |
Post subject: antenna array
Posted: Thu Jan 29, 2004 9:23 pm
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John, I think you misunderstand my question. The
antennas are made, and what I want to do now is
to use these 2 antennas as a 2-element array(uniformly
excited). So, how far apart should I place them;
in terms of wavelength (lamda). BTW, thanks
for your information. that helps a lot...
H. Le
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JGP |
Post subject: antenna array
Posted: Mon Feb 02, 2004 1:33 pm
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In order to give you an exact answer, you must provide
the following:
Frequency band [it would be
nice if you could provide center frequency (Fc)],
RF power level, Vertical or Horizontal pattern,
radio applications (specific application) and transmission
and receiving radius, VSWR desired, Line of Sight
details, type of transmission line used, Antenna
mount (mounting top or roof top)…etc.
Sincerely,
John Pereira
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guest |
Post subject:
Posted: Sat May 08, 2004 12:44 am
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do you want more Hor or Az directivity ? I like
vertical mount. This way, the hor. beam is not too
sharp. Best distance you can find out with spectrum
analyzer. The match is not straight forward. gl
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Posted 11/12/2012
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