Better FM Coverage Can Be Yours, Part 2
by Richard J. Fry
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.
