Despite the hiatus in nighttime operation for AM IBOC, antenna
research is continuing in an effort to comply with the FCC's existing
requirements of two antennas for FM IBOC, and develop a system using
one antenna that will satisfy the FCC's requirements.
The AM situation is still fluid while daytime operation is being
practiced by a number of stations with varying reports of its success.
Nighttime AM IBOC operation and the effect of skywave are still being
examined. It appears that the most difficult problem to solve is the
adjacent-channel situation, which is reported to cause considerable
interference and signal degradation from daytime IBOC. Some engineers
have reported that even strong, desired signals are being affected by
adjacent channel hiss with subsequent listener loss.
In the FM field, an FCC decision determining the number and form of
antennas allowed is still pending. The ultimate transmitting antenna
configuration has a great deal to do with the type of transmitter
installation and transmitter design, and has considerable impact on
transmitter and installation cost. At first glance, it appears that a
radiator for analog and digital signals would be the best because it
should be the most cost-effective approach. However, the various
combinations of combiner, isolator and antenna can add considerably to
the cost of an installation.
Figure 1. A simple coaxial RF signal combiner.
The type of antenna selected is governed greatly by budgetary
considerations as well as the number of stations feeding signals into
the antenna. The panel-type antenna is possibly the most easily adapted
antenna, but the decision to use it is governed by the number of
stations involved. Interleaved standard radiators offer a
cost-efficient alternative, provided that sufficient isolation between
the analog and digital antennas can be achieved.
The commission is expected eventually to allow the use of separate
antennas for analog and digital signals. This will probably make it
convenient to use a station's auxiliary antenna for the digital signal,
provided that it is no higher than the main antenna, within a specified
distance and sufficient isolation can be achieved. There is one caveat
that must be remembered and followed — control of non-ionizing
When the original installation was designed, the environmental
radiation values were calculated using the main antenna field to check
for clearance, and the operation of both antennas at the same time was
not envisioned. The addition of “X” kilowatts from the
close-by auxiliary antenna may cause the RF field to exceed the safe
limits for the various EPA RF levels.
As usual, the level of RF power has a tremendous impact on the cost
of equipment. A circulator is essential when using separate antennas
because there is usually an isolation of about 20dB. An isolator for a
500W-or-less digital transmitter costs around $4,000. However, an
isolator for a 1kW transmitter would be about $13,000. Isolation is
critical when using two antennas and anything less than a 20dB
rejection can allow too much signal to feed back into the system with
considerable effect on the isolator/combiner design and costs in
addition to wasting signal power.
Variations on a theme
Figure 1 illustrates a coaxial RF signal combiner that consists of a
cavity illuminated by two signals with different polarizations. One is
the analog signal and the other is the digital signal. Mounted at the
far end of the cavity is a circularly polarized receiving device that
intercepts both signals and combines them into a single output.
Figure 2 shows a single antenna with combiner circuitry. Figure 3
shows an interleaved antenna. The separate transmission lines are shown
together with the isolator. This is an acceptable way of converting to
FM IBOC. Maybe the Commission's original objections to separate
antennas was the consideration that one signal might enjoy better
propagation conditions than the other and result in improper IBOC
Figure 2. A typical single antenna with combiner circuitry.
Interleaving standard FM antenna bays is a simple operation, and by
adjusting the radiator location on the tower, identical centers of
antenna radiation above average terrain can be achieved, thus producing
effectively equal signal in most locations.
Put into practice
At the heart of Entercom's operation is the 200 ft. tower located
3,000 ft. above sea level on West Tiger Mountain near Seattle. Ten FM
stations originate from this site. Their frequencies cover the entire
FM band through a combination of Shively model 6810 antennas, ERI
cavity-backed panel antennas and a pair of rototiller antennas.
For its IBOC tests, Clay Freinwald of Entercom uses the four-bay ERI
antenna for the analog transmitter, and the two-bay rototiller
auxiliary antenna for the digital signal. The vertical spacing between
the antennas is between 15 ft. and 20 ft. There aren't problems from
excessive power feedback through the isolator, although as power is
increased it is possible that power feedback problems may be
encountered. The ERI panel antennas have three input connectors. In
preparation for further work on converting to IBOC, ERI added a fourth
input connection to allow an additional digital input.
Figure 3. A typical interleaved FM antenna.
So far engineering evidence supports the use of a single antenna or
a combination of two radiators for the transmission of IBOC signals. By
the addition of more bays and rearranging the existing antenna so that
interleaving is satisfactory, the antenna system costs can be kept to a
reasonable level no matter whether one or two antennas are used.
E-mail Battison at email@example.com.