Adapted from March 2000 Radio Guide Article . by Chris Scott
Perhaps more than any other question, inquiries about new methods of increasing station coverage without an allocation upgrade come up most frequently. The fm broadcast antenna is perhaps the least understood of all the station hardware, but more than any other piece, has the greatest potential for producing coverage gain or loss. While not intended to be a treatise on antenna theory, this note will identify some of the considerations involved in the selection process, and hopefully address some junk science myths that have been propagating throughout the industry.
The most popular fm broadcast antennas use a transmission line "backbone" joined with EIA RS-225 flanged junctions and are end or center fed. Element spacing is usually one-wavelength (ten feet) but sometimes is one-half to eliminate the end-fire mode of phase addition, lessening downward radiation. (The one-half vavelength spacing reduces gain per element, so I don't recommend it unless RF radiation safety requirements, or directional antenna ("DA") needs require it) These are designed to be side-mounted on a tower or tower-top pole. Officially these antennas are omnidirectional, producing a circular coverage pattern. Because of the proximity to the metallic tower structure however, they rarely are.
Pattern optimization is offered by most manufacturers as an option, modifying as necessary the design to create the most circular pattern possible on your tower. This is done by precisely modeling your tower on the outdoor test range, sometimes at reduced scale, with the antenna side mounted. The manufacturer does exactly the same thing when creating a "directional" antenna, but the goal is to intentionally distort the circular pattern.
The pattern that results naturally when side-mounting an fm antenna on a steel tower is often far from circular. If we consider just the vertical component of the radiation, (which is received most favorably by vertical automobile antennas) there is quite often a boost in the signal in the direction of the antenna mounting. Typically, this is about two to three dB, or nearly a doubling of power. This gain is produced at the price of a loss in the opposite direction. When considering just the vertical radiation, the mounting method and the tower width (cross-section) modify this basic result, although usually not appreciably until the width nears five feet, which is about a half wavelength at fm frequencies.
Complicating matters further is the behavior of the horizontal component of the radiation, the portion received best by horizontal antennas. When the antenna is mounted on the side of narrow pole or small cross-section tower, this portion often resembles a circular pattern, but as the tower width increases, very strange patterns may result, sometimes with minimum radiation in the direction of antenna mounting.
Enterprising station engineers
learned that although the FCC classified their antenna as omnidirectional,
in truth it was not, and rather than rolling the dice with their station's
coverage, some have been known to tweak the antenna mounting while monitoring
signal levels in critical market locations, seeking the most favorable results.
Enter a gray legal area; some antenna manufacturers will not do
this because in the strictest sense, it represents directionalizing
the antenna, purposely orienting it in a way that may not produce the best
omnidirectional pattern. The line is clearly drawn when it comes to actually
modifying the structure by adding additional, or parasitic elements
near the antenna bays. There is a case on record where a station actually installed
screens behind each antenna bay that served as reflectors to direct the rf
energy toward the market, a clearly illegal modification. In another case,
a licensee replaced the metallic parasitic reflector elements in his licensed
DA with fiberglass rods - again a serious FCC violation in the making.
Some operators install a side-mounted antenna on a small pole at the tower top. This technique adds cost and may impose structural limits on the number of bays that can be used, decreasing antenna gain, but produces a more predictable pattern.
Operators that must mount below the top on a large cross-section tower do have a couple of options for ensuring omni-directional coverage. Reflector or cavity-backed panel antennas are available that mount evenly around (usually one on each face) the tower. These are usually very broadband, and are often the choice of stations that share one antenna between two or more transmitters. They aren't as popular as the side-mounted flavor due to their complexity, high cost, and high windload.
possibility is test-range optimization of a conventional side-mount antenna.
A very impressive system is the ERI Lambda (wavelength) mounting system,
which uses a customized tower section for the portion the antenna is mounted
on. The important feature with this system is that the horizontal bracing
lattice work repeats at precisely the same interval as the bay mounting,
ensuring that the range testing results will truly be repeated
in the field.
Differences between makes.
This is the popular
question, which in the author's humble opinion, is the easiest to answer.
If we analyze differences involving quality of construction, ease of installation
and maintenance, immunity to icing, and reliability, there exists significant
variance between brands and models.
Most people ask what a different brand will do for their coverage. I'll quote a candid remark from a principal in a major fm antenna manufacturing firm referring to coverage: "It's not the antenna that counts, but what is between the antennas." This was a very honest statement coming from a source who risked losing sales to unknowing stations wishing to experiment with new brands. The context of the remark translated to meaning that the antenna height and the intervening terrain in a given fm antenna - market environment were far more important factors than the very minor differences between brands.
Sometimes when a station changes to a new antenna, coverage changes for better or worse are noted. Actual difference (as opposed to mentally invented difference) is due primarily to the previously described directional effects produced by the tower-antenna mounting; one brand, because of slightly different mechanical geometry, happens to work out better or worse in that particular installation on that particular tower, for the particular azimuth. The same antenna may produce entirely opposite results on a different tower.
Originally the fm service was slated to be horizontally polarized. This
worked best with horizontal antennas. Station power level is still considered to
be that which is radiated in the horizontal plane. Later the commission
authorized stations to use an equal amount of power radiated in the vertical
plane. By controlling the phase relationship of the two radiated planes, a
rotating or circular polarization is created. This is compatible with both
horizontal and vertical receive antennas. The quality of the circular
polarization is the horizontal to vertical energy balance, or axial ratio, and
should ideally be one to one. This is rarely achieved with side-mounted fm
antennas in all directions. Does perfect axial ratio make much difference? I
suggest that it does not.
Antenna pattern measurements
Often an existing, installed antenna pattern is questioned. The coverage and antenna pattern of am stations can be accurately characterized by taking a (large) series of field-strength measurements in radial locations around the antenna. Things are not so easy with radio frequencies near 100 MHz because reflections and multipath wreak havoc with the consistency of the readings. Even under ideal conditions, these readings are significantly less accurate than their am counterparts. The requisite degree of accuracy is approached only when large numbers of locations are sampled with the utmost care. Casual techniques usually produce contradictory and inconsistent data. Experienced engineers view data gathered casually with great skepticism, having watched the field strength meter's indication change by ten dB while walking ten feet.
The author has researched the airborne method of fm antenna pattern measurement. With recent advances in satellite navigation, automated field strength information can be correlated to precise positional data, recorded at or near the elevation of the antenna. Repeated circles are flown around the tower at various distances. Repeatability is better in most cases than intensive ground-based measurements, although multipath is not entirely absent. The biggest challenge is getting consistent pattern control from the aircraft-mounted monitoring antenna.
If it is not essential
to measure the antenna pattern in situ, it can
be carefully modeled and measured on an outdoor antenna range prior to installation.
This is the most accurate method, particularly when done in full scale with
a repeatable tower section behind it, but can be somewhat costly.
It was previous noted that
constructional and maintenance related differences exist between various
makes of antennas, but generally not much differences in performance. Knowing
this, one can be tempted to purchase a least expensive variety,
which may be fine, for a while, but in most cases one gets what one pays
for. Some low-power antennas use flexible transmission line in the feed system.
These same antennas are usually unpressurized, which places them in the same
category as land-mobile antennas, at risk. While these systems are capable
of reasonably long term reliability if first rate waterproofing techniques
are used, they are not failsafe. Pressurization of the antenna, interconnecting
feeds, and main transmission line is essential for long term reliability.
Many catastrophic failures have been caused by moist air corroding internal
connections after the line or antenna was left unpressurized for a length
The propagation of the
signal from the transmitter antenna to the receive antenna is ideally via
line-of-site path, without buildings, towers, hills or water tanks adding
reflected images to the mix. The advertised specifications of transmission
systems such as signal-to-noise and crosstalk between channels are based
on this theoretical fantasy. In the real world, propagation via multiple
paths represents the true nemesis of fm stereo. The most familiar result
is gross distortion or even station dropout when a mobile listener waits
at a stoplight. Sometimes creeping forward just a few feet clears the reception.
This phenomena can be observed near the base of the fm transmitter tower
where the signal strength is extreme, but the reception is terrible. The
problem is caused by the mix of the direct and reflected signals present
at the receiver, and is a function of local terrain and presence of reflective
structures, not defects
in the antenna or transmission system. A lesser-known antenna manufacturer
has recently made claims of measurable results in reducing multipath through "innovative" design
improvements. User reports are sometimes cited supporting these claims. It
is the author's view that this is nothing more than overzealous marketing
preying on people's natural wonder about the seemingly mysterious nature
of antenna performance.
Number of bays versus "penetration".
UHF television stations often use as many as thirty-two elements (bays) to achieve the gain (vertical beam compression toward the horizon) necessary for the effective radiated power (erp) authorized by the commission. For the intended purpose, this extreme number of bays works well. In the case of fm antennas, there seems to have been an odd trend toward using fewer bays in conjunction with unnecessarily large transmitters. Some consultants, known for otherwise brilliant allocations work, have prescribed these combinations for reasons that simply cannot be defended by rational engineering. The antenna manufacturers, having a financial interest in churning the installed antenna base, have not advised against these sales enhancing retrofits.
High-gain antennas combined with low-power transmitters producing licensed erp have many economic advantages. Their superiority in relatively flat terrain applications cannot be disputed. In the case of unusual antenna height over average terrain, a closer look is warranted.
The argument for using one or two bays hinges on the premise that multi-bay antennas produce such a thin elevation "beam" that close-in "under the horizon" listeners are neglected; the "beam" overshoots them. If the antenna is at the top of a very tall building in New York City, or Chicago, this is a bonafide concern. A one or two bay antenna might be best due to the unusual population density underneath the antenna.
Let us examine an extreme case of the antenna height / market distance relationship, deserving caution in selecting a multi-bay antenna. Many fm transmitters are located on Mt. Wilson, covering the Los Angeles area. The antennas there are more than one mile above most of the listeners. The nearest city, Pasadena, is 13 degrees below the horizon. A ten bay antenna would serve the fringe well, but would produce less signal into Pasadena than a one-bay antenna with a high power transmitter. This situation is unusual in that it truly represents an environment where a ten bay antenna is a poor choice, and a six bay antenna might be marginal. Several four and five bay antennas presently are being used there with excellent results.
In the eighth edition of the NAB Engineering Handbook, Peter Onnigan and Eric Dye present a revealing analysis of results obtained with typical antenna gain / transmitter power combinations. It is well accepted that signal strengths in excess of four times the city grade standard produce no advantage and can worsen blanketing interference problems. This signal level is 13 millivolts per meter.
The following comparison
shows a typical installation with antenna at 500 feet above average terrain,
and 50 killowatts erp. Surplus signal
strengths in excess of four times city grade are shown in red.
|Miles from Antenna||Downward vertical angle||2 bay antenna and 55 kw transmitter||10 bay with 2% null fill and 10 kw transmitter|
To summarize, it rarely
is the case where using fewer than four bays will improve market coverage.
Unless the antenna height over market is very unusual, a six bay antenna
will produce every signal advantage available, and will result in significant
savings in capital and operating costs. However, one antenna manufacturer
has pointed out an unusual case where fewer bays are truly justified to ensure
uniform radiation toward the horizon. In
the case of a tapered tower section typical of large self-supporting towers,
the effect of the sloping tower adjacent to the plumb antenna may cause some
unpredicted beam tilt, skewing the vertical pattern so that some of the energy
is no longer directed toward the horizon. In this case, without detailed
modelling, a four-bay antenna will provide the taller elevation pattern
to ensure below horizon coverage.
GoTo cs&a HOMEPAGE