LOOK UP IN THE SKY!
IT'S A BIRD! IT'S A PLANE! IT'S A YAGI!!
We
were discussing yagi and log periodic antennas and their related brethren, and
the fact that those antennas we refer to as "yagis," often are
something else, entirely.
Both
these types are frequently stacked vertically and/or horizontally, to make up
dual and quad arrays. An array may be
used to increase the gain of the antenna even further, or to further tailor the
directional pattern of the basic antenna.
When antennas are stacked, the physical space between the units is
critical, as is the length of the harness cables used between the antennas and
RF splitters/combiners.
The
quad array pretty much represents the maximum practical gain available from a
given antenna. Theoretically, in order
to realize a 3 dB gain over a quad array, we would need to go to eight antennas. In the real world, the additional cable
harness, connector and combiner losses eat pretty badly into even that gain
figure. So we've reached the point of
diminishing returns.
It's
also common when using these antennas for transmitting purposes to use arrays
of "skewed yagis" (antennas pointing in different directions) to
produce many weird and wonderful directional patterns. In this case, the power dividers used can
also be arranged to produce unequal power divisions, to even further enhance
the number of choices available.
Most
of the time, these antennas are oriented for horizontal polarization, although
they may be used for vertical polarization as well. An additional consideration when mounting
them for vertical polarization is that the antenna should then be located
several wavelengths above ground in order to function as specified. Otherwise ground effects can affect the
impedance as well as the directional pattern and front to back ratio of a
vertical yagi or log periodic antenna.
Yagi's
can be used in creative ways to eliminate co-channel interference. One technique is to mount two yagis, both
oriented in the same direction, but staggered such that the incident desired
signal arrives at the first antenna one-quarter wavelength before it reaches
the second antenna. A special harness is
constructed such that an additional quarter-wavelength delay is encountered by
the feed from the first antenna before it's combined with the output of the
second antenna. Net result: incident
signals on the main lobe of the antennas are delayed equal amounts, and sum
normally. Signals coming in from the back
of the antennas end up opposite in phase and cancel out at the summing point. The front to back ratio of the antennas is
increased significantly (at one frequency of interest).
A
more general technique to reduce co-channel interference from a specific known
direction involves using trigonometry and the known velocity of wave travel to
calculate the phase delay between two antenna positions as seen from the source
of interference. This distance is
adjusted until the undesired signal arrives at the two antennas 0.5, 1.5 or 2.5
wavelengths apart. Summing the antenna
outputs causes phase cancellation of the undesired signal, in effect placing a
deep asymmetrical null in the array's directional pattern at that frequency --
in the direction of the interference. Reception of the desired signal is relatively
unaffected.
It
is even possible to make up a circularly polarized array by coupling a
horizontal and vertical antenna through a phase delay harness. The real benefits of circularly polarized FM
transmissions have never been fully realized: a CP FM receive
antenna has a powerful mechanism to reject multipath reflections (in addition
to directivity, that is), since reflected circularly polarized signals
"spin" in the opposite direction as the incident signal. This would be an advantage for receiving
fringe CP FM transmissions at a rebroadcast site or cablevision headend, for
instance.
The
sheer physical size of such a contraption would pretty much prevent its
acceptance on FM frequencies by consumers, however.
Coming
up next, we'll stir the pot a bit in a discussion of circular and horizontal
polarization for FM broadcast stations.