years ago, in the mid-1980s, I was charged with the task of improving
the signal and coverage of our Chicago Class B FM station. At the
time, that station was operating at 30 kW and 400 feet above average
terrain. My task was to get it up to full Class B facilities, which
was not an easy task because of grandfathered short spacings; but
that’s a story for another time.
plot showing a station’s standard (red), modeled undisturbed
(green) and modeled disturbed (blue) patterns. The disturbance is
from a nearby communications tower.
(Click to Enlarge)
much engineering as well as months and months of wrestling with the
FAA, I did get approval for a 525-foot tower and a CP from the FCC
for a full 50 kW with the height increase. What a thrill it was when
that document came in the mail! But then I saw an ominous special
condition on the last page of the CP. There was an AM directional
array less than two miles distant, and the FCC was requiring me to
make pre- and post-construction proof measurements on the station.
it turned out, that station was in bad shape. I don’t remember all
the particulars, but when I contacted their Washington consulting
engineer for the documentation from the last full proof, something of
a panic started with the licensee. From what I could gather, the
pattern was out of tolerance, monitor points were probably high and
the sampling system was damaged.
licensee certainly did not want me making measurements and submitting
them to the FCC showing those issues, so I was provided with a letter
from the licensee, which the FCC accepted, taking all responsibility
for any pattern disturbance that might result from the construction
of our new tower. With that in hand, I was able to go ahead and
build, and the rest is history.
COULD HAVE BEEN WORSE
did recognize, however, that I had, to some degree, dodged a bullet.
Running a partial proof on that six-tower directional array twice
would have taken a lot of time and effort (there were 16 radials, if
I recall correctly).
then, I have been on both sides of the AM directional array issue
many times. Starting in the early 1990s, it seems that there was
always a cell tower or monopole going up close to one of our arrays
somewhere. With limited tools to evaluate the potential impact, we
often resorted to pre- and post-construction monitor point
measurements, which provided ambiguous information at best.
then I ran into the loophole in the FCC’s rules for protection of
AM antenna systems from nearby tower construction. Other than
broadcast, wireless and public mobile services, there were no rules
in place to require other FCC permittees to protect such antennas.
Part 90 (Private Land Mobile) licensees in particular did not have to
make notification, perform measurements or provide detuning. The
county sheriff could build his dispatch tower right across the road
from an AM antenna, totally wreck the pattern and have no liability
at all, and it happened a lot, all over the country.
PROTECTION AT LAST
number of years ago, the AM Directional Antenna Performance
Verification Coalition, the group that was the impetus behind the
moment-method model proof rules that went into effect a few years
ago, began pushing for some omnibus AM antenna protection rules. Last
August, the FCC finally enacted those rules, and by the time you read
this, the rules should be in effect.
rules were placed in Part 1 of Title 47 of the Code of Federal
Regulations, “Practice and Procedure” (§1.30001 – §1.30004).
That placement makes them applicable to all
services regulated by the FCC. While the rules have many provisions
and requirements, a good understanding can be had by hitting the high
far-field plot of a null radial. The inverse-distance line represents
the standard pattern IDF for the radial. This plot shows that the
nearby tower does indeed cause the stations IDF to exceed the
standard pattern value for this radial.
(Click to Enlarge)
the FCC set some screens for distance and height that will eliminate
a lot of structures from consideration. Distances and heights were
based on the AM station’s frequency (rather than employing a
straight distance/height screen).
structures near non-directional AM antennas, only towers within one
wavelength that are taller than 60 electrical degrees at the AM
frequency must be considered, and detuning is required only if a
pattern disturbance of more than 2 dB results.
structures near directional AM antennas, only those within the lesser
of 10 wavelengths or 3 km and taller than 36 electrical degrees at
the AM frequency must be considered. Detuning is required if
radiation on any azimuth exceeds the AM station’s licensed standard
mounted on buildings no longer have to be considered unless the tower
itself (not the tower and the building combined) meets the above
seems that wireless licensees are always making changes to their
antenna systems and towers, and the FCC set some screens in the new
rules for modification of existing structures to help weed out
these screens are a height change of more than five electrical
degrees at the AM frequency for non-detuned, non-insulated towers; or
the addition or replacement of one or more antennas or transmission
lines on towers that are detuned or base insulated. Those screens
will take care of the vast majority of such modifications while
protecting the AM pattern where a tower has already been detuned.
that an antenna structure that falls outside the screens is built and
the AM licensee notices a corresponding change in his antenna
pattern. What then?
new rules provide for this. The AM licensee can, within two years,
make and submit a showing, using either moment-method modeling or
measurements, and if the FCC agrees that the pattern was indeed
disturbed, it can order detuning. A moment-method showing can be made
before construction if an AM licensee suspects that a proposed tower
outside the screen distance and/or height would have an impact on his
(MoM) modeling has long been used by consulting engineers as a tool
to evaluate the effect that a nearby antenna structure may have on
the radiation pattern of an AM antenna, and in the new rules, the FCC
made use of MoM modeling a requirement for such evaluations.
Alternatively, for stations licensed pursuant to a conventional
proof, before- and after-construction monitor point measurements can
be used to show no change in the pattern
I was concerned about as we proposed these new rules was existing
“exempt” structures. Other coalition members and I have arrays
that have been impacted by towers for Part 90 and other formerly
exempt services. What about those? The FCC provided a one-year window
after the new rules go into effect for affected AM licensees to
submit showings demonstrating pattern distortion as a result of such
nearby antenna structures. If the FCC, after examining a showing,
agrees that the structure is causing impermissible pattern
distortion, it can order detuning.
rules also contain criteria for mounting various antennas on AM
towers. These rules mostly replace those from Part 73 that were
deleted and moved to Part 1. In a nutshell:
installing antennas on a non-directional AM tower, if the base
resistance changes by more than 2 percent, a Form 302-AM must be
installing on a conventionally-proofed AM directional array tower, a
partial proof must be run and filed with a Form 302-AM.
installing on a MoM-proofed AM directional array tower, if the base
impedance of the tower changes by more than ±2 ohms and ±4 percent
resistance and reactance, a new model must be run and filed.
new rules provide a win for just about everyone. They are a win for
AM licensees because they provide good protection for their antenna
patterns. They are a win for wireless licensees because they
eliminate many of the costly and labor-intensive measurements they
have long had to do when constructing or modifying their facilities.
And they are a win for the FCC as they can eventually do away with
many of the “permanent STAs” under which AM stations with pattern
distortion from exempt structures operate. The only folks that may
not be too happy are the licensees of existing pattern-disturbing
structures that may have to add detuning.
I were building that Chicago FM tower today, it would be a simple
matter of running a moment-method model on the AM array, then adding
a “wire” representing the FM tower and running the model again. A
comparison of the predicted pattern and the electric fields on the
null and lobe radials would show that the FM tower had no effect on
the AM pattern. I would then file this as a showing with my Form
302-FM license application.
would certainly be a lot less work than beating the bushes for a
couple of weeks running before and after proofs on the station. And
the AM licensee could rest assured that his pattern would be
undisturbed. I’d call that a win all the way around.
Alexander is the director of engineering at Crawford Broadcasting and
a past recipient of SBE’s Broadcast Engineer of the Year award.