Open Communication
Figure 6. The dipole energy of the Yagi is concentrated and
directed in one direction, making the antenna directive.
Figure 5. The Yagi antenna uses a dipole as the antenna, but
parasitic elements are added to shape the radiation pattern.
ple dipole or a one-watt signal from a
Yagi. You can actually get much
higher gains by adding more directors. Yagis with up to 20 directors are
available to give super high gains.
The downside is that the antenna is
highly directive so you have to point
it correctly to get the signal.
Another common type of antenna is the parabolic dish (see Figure
7). It is used mainly at microwave frequencies above 1GHz. The dish, as it
is called, has a parabolic shape that
takes the energy from an antenna
like a dipole and concentrates all its
radiation into a highly focused radio
beam. The antenna signal reflects off
of the dish and, because of its unique
geometric shape, all of the waves are
concentrated into a very narrow
beam. This makes the dish very
highly directive, but the upside is the
super high gain. You can see why a
dish is used on
satellite TV. With
the satellite out
there in orbit
22,300 miles
away, the signal is
extremely small when it gets to
Earth. The dish has a very high gain,
so it, in effect, boosts the signal level
before it gets to the receiver.
Another type of antenna you don’t
see much of, but that is widely used, is
the patch antenna. It, too, is used
mainly at microwave frequencies. A
very common application is with wireless local area networks (WLANs) that
connect PCs and laptops. Popular
access points, called hot spots, allow
laptops to connect to the Internet at
many public places, like airports,
hotels, and coffee shops. The antenna
used in many of these 2.4-GHz radios
is a patch. It is essentially just an area
of copper on a printed circuit board
(PCB) about one-half wavelength in
size that may be square or circular
(see Figure 8). A copper ground plane
on the other side of the PCB acts like
a reflector to
focus the energy forward.
You can actually put multiple
patch antennas on a large PCB and
drive all of them simultaneously. This
gives lots of gain and directivity. You
can even control the phase of the
signals to or from the antenna, making it possible to shape the radiation
pattern or the gain on the fly. These
are called phased arrays and they
are widely used in military radar, but
they are becoming more popular in
cell phone systems and WLANs.
Anyway, I am out of space for
this issue. I will follow up in a later
column with more antenna types.
For now, pay attention to your antenna. It will make your seemingly failed
wireless project a true success. NV
For Your Info ...
The February Open Communications column
covered the Ramsey QRP transmitter kits. In
lieu of the schematic diagrams, the photos were
accidentally printed instead. Ramsey has made
the schematics available for those of you
who need access that information. Visit
http://ramseykits.com/qrp
Figure 7. Another type of antenna is the parabolic dish
similar to any satellite dish system.
Figure 8. The antenna used in many 2.4-GHz radios is just an area
of copper on a PCB about one-half wavelength in size.
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APRIL 2005
