Jim,
Another choice for the plastic box mounted antenna is a vertical dipole
inside the box. This can be built onto the RF module PCB. This also can be
modeled accurately using NEC. The feedline from the RF module to the dipole
can consist of two parallel lines. Decoupling can be achieved by several
means but a simple and no-cost is a sleeve balun type of arrangement
consisting of PCB traces. If you would like circular polarization, to allow
for random polarization at the other end of the data link, this can be
accomplished by adding a horizontal dipole on the PCB and feeding the two
dipoles 90 degrees apart. The radiation pattern is not truly isotropic but
the 3 dB reduction in vertically polarized energy is probably worth it to
achieve horizontal polarization over most of the azimuth.
Dave Cuthbert
From: David Cuthbert [mailto:[email protected]]
Sent: Wednesday, October 18, 2006 11:49 AM
To: 'Jim Eichner'; '[email protected]'
Subject: RE: Wi-Fi antenna location
Jim,
Let's say the large metal box has a monopole mounted on top. The radiation
pattern is omni-directional and is vertically polarized. The coverage is
uniform and the polarization is the same for each installation.
Now let's say the plastic radio box also has a monopole antenna worked
against a ground plane directly under it. The ground plane can consist of a
metal disk with a diameter of 3 cm or two 3 cm wires (in opposite)
directions can be used. The monopole is mounted vertically with respect to
the floor. This senario will also be omni-directional and vertically
polarized. But if the box is installed such that the monopole is horizontal
then the pattern is no longer omni-directional (referenced to the floor) and
the polarization is horizontal. There will be nulls in the radiation pattern
along the axis of the monopole.
Another senario is if the monopole is used without a ground plane. The
outside of the coaxial cable will now carry antenna current and the antenna
is a long wire of sorts. The performance will vary from installation to
installation.
I can run you some NEC antenna simulations (for free!) of some installations
that you describe and we'll have some numbers and 3-D patterns. This will
allow you to make your decision based on numbers and not quesses.
Dave Cuthbert
Linear Technology
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From: [email protected] [mailto:[email protected]] On Behalf Of Jim Eichner
Sent: Wednesday, October 18, 2006 11:25 AM
To: [email protected]
Subject: Wi-Fi antenna location
Question for the RF types in the crowd:
Consider 2 scenarios involving a product that uses an 802.11 radio for
remote monitoring. The product is a large metal box, and the radio is a
separate plastic box containing the 802.11 module and its antenna
connector.
Scenario 1: The antenna is mounted on the plastic radio box and the
radio connects to the host via cable and is mounted remotely (several
feet away or more). For the sake of discussion, assume the radio box is
mounted on a non-conductive surface (drywall typically).
Scenario 2: The radio is located inside the host product, and the
antenna is mounted on the metal skin of the host and connects to the
radio via an antenna cable.
Questions:
1. Should one of these scenarios theoretically have better range than
the other?
2. Should one of these scenarios theoretically have less directionality?
Thanks in advance for your help,
Jim Eichner, P.Eng.
Compliance Engineering Manager
Xantrex Technology Inc.
e-mail: [email protected]
web: www.xantrex.com
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