Omnidirectional Antennas

Omnidirectional antennas can be end fed and end mounted, or they can be center fed.
The example shown in Figure 13.5 is two quarter-wave halves fed in the center, cabled
down inside one of the dipole arms. Two end-fed dipoles stacked on top of each other
give 3 dB of gain (our squashed doughnut). The beamwidth is narrowed in the vertical
plane, but the omnidirectional pattern is maintained in the horizontal plane, and the
radius is increased.
Four dipoles in a stack will give 6 dB of gain. In Figure 13.6, the inclusion of VSWR
(Voltage Standing Wave Ratio) is a figure of merit. This gives an indication of how well
the antennas will match to the transmitter—that is, how much power will be transmitted
into free space and how much will be reflected back to the transmitter. An antenna
with a VSWR of 1.5:1 will dissipate 95 percent of the power applied to it. In very wideband
devices, VSWR can be 2:1 or worse. This example shows how matching deteriorates
as you move away from center frequency.
If you change the phase matching between antenna elements, you can uptilt or
downtilt the antenna. This is the basis for the adaptive downtilt antennas used on some
Node B base stations.

If you bay two omnidirectional antennas, you can create any number of different
footprints by changing the physical spacing between the two antennas. You can make
this adaptive by electrically changing the phase (rather than physically moving the
antennas). This is the basis for adaptive antenna design (changing the E-plane and Hplane
characteristics of an antenna array).
If you take an omnidirectional colinear and give it a substantial amount of gain, the
cell radius will increase, but a hole will appear in the coverage close to the base station.
This can be reduced by using electrical downtilt (though it is generally better to have
designed the antenna installation correctly in the first place). This is known as the
“polo mint” effect.
When you mount antennas on a metal structure (that is, a mast), the antennas
should always be at least two wavelengths away from the structure—not particularly
a problem at 2 GHz. More of a problem is having multiple antennas close to each other,
which then intermodulate with each other. Generally, an isolation of at least 40 to 45 dB
is needed between a transmit and receive antenna, and 20 to 25 dB between two transmit
antennas.