In the 1800s, taverns offered free
lunch to entice drinking customers.
The earliest extant references
to a notion that there is no free
lunch date to the 1930s (serendipitously, the
decade that witnessed the birth of W1XOJ,
the first FM station). Milton Friedman helped
popularize the idea in the 1970s.
Engineering an FM plant typically involves
tradeoffs, and that is our topic of discussion.
One of our goals is to get the most bang
for the buck. Sure, there is the public interest
component; but the harsh reality, whether we
want to admit it, is usually that the dollar is a
The FCC’s model tends to view all stations
with identical contour radii as equal in coverage,
but not all contours are the same. Stations
with high structure density in their service
areas may wish to opt for greater effective
radiated power and lower height. This allows
for greater effective field strength to punch
through building walls. Conversely, in a more
rural setting, the prudent choice may be to
consider less power and more height. In such
a situation, you would wind up with a larger
coverage radius for listeners in vehicles but still
be able to penetrate the frame construction typically utilized for building construction.
The tradeoff is one of front- versus back-end
economics. A situation involving lower height
and greater power may bring higher startup
costs for larger transmitter, transmission line
and antenna, and possibly lower initial costs
for the tower due to its height, though a larger
antenna may be required. Later, more power
means a larger utility bill. In the converse situation,
you may pay less for your box, antenna
and line, and perhaps enjoy a lower power bill;
but steel erection costs will spiral upwards.
Expect higher long-term maintenance costs for
a taller stick as well.
Nevertheless, even with a particular ERP
and height, there are additional parameter
changes that can be made, resulting in other
For example, a single-bay antenna with really
big transmitter will produce the same ERP as
a 12-bay antenna and smaller transmitter. At an
ERP of 50 kW, a single-bay circularly polarized
antenna would require input of approximately
108 kW. Although transmission line is available
to support such a set of parameters, you would
be pressed to find an antenna with that power
rating. At the other end of the spectrum, with a
12-bay full-wave spaced model, the input to the
antenna would be around 7.3 kW. So, with a
hunk of 1-5/8-inch, the plant is ready to roll.
More common scenarios involve six- or
eight-bay antennas, which give an input power
range of approximately 11 to 15 kW.
The 12-bay solution may seem kind of silly,
but in certain circumstances it may demonstrate
a brilliant tradeoff. In such a scenario,
antenna and tower costs will be greater, if
nothing else due to the difference in materials
utilized. However, long-term benefits of a lower
power bill may offset this, especially if the site
is located such that vertical radiation characteristics
are less important. In cases where a
site is between population centers, downward
radiation is less critical, given that radios tend
to be a rarity among field vermin (unless of
course your name happens to be Nicodemus or
perhaps Mrs. Frisby).
Other tradeoffs may be required due to the
mechanics of a particular site.
Spacing between bays of one wavelength is
the most common antenna design. Reducing spacing will lower gain, requiring a greater
input power to achieve the authorized effective
radiated power. This reduction may be necessary
due to structural concerns or to avoid
radiofrequency radiation exposure issues at a
site. This type of scenario tends to be common
on mountaintops where a short tower and high
ERP are utilized. While higher transmitter
power may be required and the antenna may
be more costly, this tradeoff may avoid lost revenue
for RFR coordination, eliminate the need
for a taller tower or reduce monthly rental fees.
THEORY VS. PRACTICE
Design and installation location also raise
questions of tradeoffs.
For instance, on paper, a “typical” FM antenna
and an antenna with a panel design will cover
the same area. In reality, that is not the case.
There really is no such thing as a non-directional
antenna because the environment always
influences the pattern. Face size, distance from
leg or face, and orientation relative to the face
or leg affect the radiation pattern. Therefore,
even though an antenna is considered non-directional,
it is not unusual to see an effective
boost in the pattern of a couple of dB along
some azimuths. However, once again, there is
no free lunch; other azimuths will have corresponding
reductions such that the RMS of the
pattern comes out close to the theoretical value
for the non-D antenna.
With panel-style antennas, however, this
“boost” disappears; the tower’s impact is less
because of the directionality of the individual
elements pointing away from the structure. I
observed this when a station in the eastern part
of the United States changed out a rototiller-style
antenna for a panel. Although the station received
numerous coverage complaints, field measurements
confirmed that the antenna was performing
as it was supposed to. The issue was that
the beneficial effects of the tower along certain
azimuths were not realized until the old antenna
was modeled. In this case, the station made a
tradeoff of “directional non-directional” coverage
for nearly non-D coverage. Unfortunately, the
population densities were sufficiently skewed
geographically to actually degrade the coverage.
This same phenomenon comes into play in
station upgrades, especially in the noncommercial
educational world, or where commercial
stations seek contour protected authorizations. If changing a facility from a non-directional to
directional configuration, you must take extra
care to ensure no significant audience loss. Obviously
if the station is starting up with a directional
antenna, there is no frame of reference. It is possible
that in converting from a non-directional to
a directional antenna while doubling the facility
ERP, you will experience net coverage loss.
One major antenna manufacturer has a nifty
utility that allows you to explore the impact
that a particular tower size will have on their
patterns. While it is not a substitute for actual
controlled measurements, it does show that
certain combinations of antennas and tower size
can result in a distorted relative field of 1.5 relative
to a non-directional RMS of one. The effects
of the tower therefore induce a relative power
of up to 2.25 along certain azimuths yielding an
ERP boost of more than three dB.
FCC rules are quirky in this regard, as they
ignore the contributions from the tower if the
antenna is non-directional but require them to
be considered if the antenna is licensed as directional.
Therefore, if your antenna is directional,
the maximum ERP on the license is the maximum
ERP radiated at any azimuth. If licensed
non-directional, it is likely the authorized value
plus some more.
Radomes present another tradeoff consideration.
For stations where winter rarely brings
anything other than liquid precipitation, radomes
probably are unnecessary. Further north,
they can make the difference between reliable
full-power operation (and by extension full coverage)
and substantial downtime. Radomes add
expense, installation and maintenance considerations
to an antenna purchase, but their impact
is greatest in structural loading.
Traditionally, vertical real estate is based on
a combination of lineal footage consumed and
installation elevation above ground; a radomed
antenna of a certain length results in the same
revenue or rental fee, depending on your side
of the equation, as its naked counterpart. Since
an antenna with radomes will soak up more
of the available capacity of the structure, they
should garner higher rental fees. If radomes are
proposed in a region of marginal utility, will
the potential loss of revenue from downtime
offset the added costs? On the flip side, if you
were the landlord, would the use of radomes by
a tenant unnecessarily preclude future revenue
The minutiae of antenna tradeoffs go beyond
what we have covered, but we have proved a
quintessential snippet of wisdom. Perhaps the
intersection of philosophy and technical stuff is
one of the main reasons that we dig playing with
radio so much. I’ll mull that one over lunch and
get back to you.
Ruck is the principal engineer of Jeremy
Ruck and Associates, Canton, Ill.