How can you get the most out of your FM antenna system
Radio World asked three prominent broadcast consulting firms to
respond to a number of topics about FM antenna performance. Last
issue, we printed their answers to the first two questions. Their
replies continue below. More will follow in future issues.
The participants are Ben Dawson of Hatfield & Dawson; Bob
Culver of Lohnes & Culver; and Don Markley of D.L. Markley &
Associates.
Note that any consultant would need specific site and application
information to provide an accurate recommendation for a given situation.
Topic: Impact of antenna sidelobes on multipath performance
in the area served by the main lobe of the antenna:
Dawson: Reduction of sidelobes, or even shoulders of the
main lobe in the case of antennas with a small number of bays, can
substantially reduce foreground reflections, which create multipath,
particularly in some environments with high ground reflectivity.
Culver: Multipath reception problems are not caused by the
transmitter antenna. They are caused by reflections of the signal
anywhere along the propagation path, most likely at intermediate
points or closer to the receivers than to the transmission antennas.
But if there are unusual conditions near the transmitter antenna,
be aware of them and be prepared to minimize their impact.
For example, if you seek a transmitter site in the mountains and
will be below or "in front of" a ridge, you should seek
to not send a strong signal into that ridge. If you do, you will
build in a permanent and stable signal propagation delay.
It will create some pattern scalloping depending on the strength
and phase of the reflected signal. Beside, there will be very poor
service beyond the ridge and I hope there is no important population
there. If there is, you have the wrong transmitter site.
Similarly, if the transmitter antenna is on a short pole on top
of a ridge and a strong side lobe is directed toward the ground
nearby and reflected into the service area, the same problem arises.
Look for an antenna with reduced side lobes or a taller pole.
In summary, there is very little one can do to correct "multipath"
from the transmission end. But you can and should take a careful
look at the proposed transmitter site to make sure you are not building
a problem into the system.
I have had some anecdotal information that 1/2-wavelength antennas
perform quite well. But you must be ready to pay the extra antenna
cost, wind load, transmitter power and ERP trade-offs.
Markley: The impact of side lobes on multipath seems to
be the fad du jour. We really haven't seen any proof
that this makes a big difference.
Topic: Effects of tower face width, orientation and guys on
the pattern performance of a side-mount antenna.
Dawson: Side-mounted antennas should never be used without
manufacturer's measurements of the effects of the specific
tower and mounting.
Culver: Everything in or near the antenna aperture will
affect the resulting pattern, both the vertical and horizontal plane
(horizontal mostly) and both polarizations (vertical mostly).
I did a paper once regarding the "native" patterns or
non-directional antennas. That was at the time of the promulgation
of the 73.215 contour protection rules.
The element itself has a slight directionality (some, like panels,
are highly directional and an omni horizontal pattern is achieved
only by the array around the tower axis). Adding the feed line makes
it worse. Adding the mounting pole, worse yet. Mounting it on a
tower, at what distance and orientation and you get the idea. The
resulting pattern can get really bad.
I had dozen or so illustrations of this. The pattern null ranged
from about 2 dB to well over 15 dB, the maxima were from a small
fraction of 1 dB to over 3 dB.
If you just slap the antenna up, you will not know what you have.
You can try to calculate the resulting pattern, but it is tough.
You can measure the pattern much more easily, but only in the horizontal
plane where a one- or two-bay model is available.
The entire antenna and a longer section of the tower are needed
to investigate vertical pattern effects, but they should not be
too pronounced because the vertical tower is much larger than the
antenna and represents essentially an infinite length reflector
in the vertical plane.
Horizontal polarization is affected by the horizontal components
of the tower, which can be quite small or can be at a critical dimension
and have a serious impact. The vertical structure is always large
and affects the vertical polarization in a more uniform way.
Both polarizations are affected by the distance the antenna is
mounted from the tower.
Markley: If the most non-directional operation possible
is desired, a pole or narrow-face tower is needed, 18 or 24 inches
at most.
A tower with a face width of 42 to 48 inches causes terrible pattern
distortion. The cross bracing of the tower is approximately 1/2
wavelength long and seem to contribute to significant pattern distortion.
Larger towers, 10 or 12 feet in width, create strange patterns
with multiple lobes and nulls that may require some work to make
usable.
Richard Fry is a retired FM applications engineer with almost
35 years of service with major U.S. broadcast transmitter and antenna
manufacturers. Reach him via e-mail to rfry@adams.net.
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