598 Federal Commnnications Commismon Reports
Antenna, Directional, see also Directional Antenna
Engineering Standards
Commission letter relaxes policy on assignment of monitoring
point limits to AM directional stations on experimental basis for
one year. "Direct ratio" method adopted and lowering of limits
based on partial proofs of performance ceased if not in excess of
200% above measured values. "Seasonal proofs" not permitted.
BEFORE THE
FEDERAL COMMUNICATIONS COMMISSION
WASHINGTON, D.C. 20554
In the Matter of
Donald G. Everist, Association Of Federal
Communications Consulting Engineers
Monitoring point limits to AM directional
broadcast stations
December 6, 1979
IN REPLY REFER TO:
8800-DW
,
Mr. Donald G. Everist, Chairman
FCC Processing and Procedure Committee
Association of Federal Communications
Consulting Engineers
1015 - 15th Street, N.W., Suite 703
'Washington, D. C. 20005
Dear Mr. Everist:
I have your letter of Octoher 22nd, written on behalf of your
committee, requesting modification of certain Commission engineering
practices used in assigning monitoring point limits to AM directional
broadcast stations. Your letter formalizes suggestions developed in a
t series of meetings, begun well over a year ago, between your
committee and members of the Broadcast Facilities Division's engi 
neering ,staff concerning the policies and procedures governing the
preparation and processing of various types of applications. The
interest shown throughout this period by your committee in helping
improve "our processing procedures has been helpful and is greatly
appreciated.
74 F.e.C. 2d
Monitoring Point Limits
599
Specifically, your committee feels that, under the present policy,
monitoring point limits are often assigned which are unnecessarily
restrictive and urges the adoption of a policy whereby the assignment
of these limits is based on the "direct ratio" method. The committee
also urges the establishment of a policy whereby stationss~bjectto
seasonal conductivity changes can achieve relaxed lill)its upon submis·
sion of "seasonal proofs." Additionally, the committee requests that
the Commission refrain from altering monitoring point limits based on ,
partial proofs of performance if "substantial conformance" of the
radiation patterns is demonstrated and the antenna parameters are
either essentially unchanged or, if changed, adequately justified.
In response to your first suggestion, I am pleased to announce that
we have, on an experimental basis, adopted the policy of assigning
monitoring point limits using the direct ratio method. Under the direct
ratio method, monitoring point limits are obtained by multiplying the
measured field strength at a monitoring point by the ratio of the,
authorized maximum radiation divided by the unattenuated radiation
established in tbe proof of performance. This method simply restricts
unattenuated radiation to within its maximum authorized value
whereas the traditional method, in many cases, restricted radiation
much more severely. Theoretically, objectionable interference is not
caused if antenna radiation is maintained lJelow its maximum autho 
rized value. 'Assuming, therefore, that changes in monitoring point
field strength correspond directly to changes in antenna radiation,
monitoring point limits determined by the direct ratio method should
be adequate to avoid interference. However, since the assumption of a
linear relationship between monitor point readings and antenna
radiation becomes somewhat questionable with excessive changes, we
do not intend to assign limits higher than 200% above proof values. In
addition, because operation with monitoring point field strength in
excess of the direct ratio limit could result in objectionable interfer 
ence, we will continue to deny requests to exceed those limits.
Your second suggestion addresses a problem encountered in many
areas of the country where complete proofs of performance are done
during the summer months when ground conductivity is significantly
lower than during the winter months. Often monitoring point limits
resulting from such summertime proofs are not sufficient to accommo 
date higher readings encountered during winter. In such a case
increased limits are obtained by collecting supplemental wintertime
data in the form of a partial proof of performance consisting of at least
10 measurements on each radial estahlished in the complete proof (see
Section 73.154(a) of the Rules). You suggest that the Commission
accept "seasonal proofs" for this purpose in lieu of partial proofs. A
seasonal proof wouW consist of "at least 20 field strength measure 
ments, both nondirectional and directional, on each of the radials
specified in the construction permit and at least one radial in the major
lobe."
74 F.C.C. 2d
600 Fedeml Communications Commission Reports
In responding to this suggestion, it is helpful to understand the
approach used by Commission engineers in analyzing complete proofs
of performance. These generally consist of 20 or 30 measurements per
radial (see Section 73.186(a)(1» and serve as the reference for all
subsequent partial proofs. As you know, the fundamental problem is
distinguishing between the effects of conductivity and antenna
radiation. In making this distinction, we consider it imperative to
establish, as conclusively as possible, the size and shape of the
nondirectional radiation pattern. The nondirectional radiating system
is simpler (fewer variables) than the directional system and its RMS
(size) can be more accurately determined since each measured radial is
of more or less equal significance, particularly if the radials are evenly
spaced. With a directional pattern, many of the minor-lobe and null
radials do not contribute significantly toward defining the RMS,
leaving the remaining main lobe radials with a disproportionate
influence on the determination of the pattern size. For these same
reasons, the Commission relies entirely on nondirectional measurement
data in determining the extent of seasonal changes in conductivity.
Because of the crucial role played by the nondirectional pattern
resulting from a complete proof of performance, extreme care is used
in analyzing the measurement data. Experienced engineers who have
been carefully trained are used in this work All known external
factors'such as terrain features, reradiating structures, pipe lines, etc.,
are taken into account. Each radial is repeatedly weighed against the
others with constant attention to the resulting pattern shape and RMS
and the analysis is not considered complete until the importance of
each element of data is understood from the perspective of the whole.
Of course, the more extensive and "well behaved" the measurement
data, the more precise and confident the engineer can be with his/her
analysis. Once the nondirectional pattern is established, analysis of the
directional data can usually be done mathematically, rather than
,graphically, using either arithmetic or logarithmic averages. Any
subsequent nondirectional partial proofs which are submitted to the
Commission for the purpose of documenting suspected conductivity
changes are mathematically analyzed, point for point along each
radial, against the complete proof nondirectional data (see Section
73.186(a)(5». If the possibilities of distortion and changed RMS can be
eliminated from the partial proof nondirectional pattern, then the
extent of conductivity change along each radial can be determined and
applied to the directional partial proof data revealing whether, in fact,
oobserved changes in directional field strengths resulted from changes
in the radiation Pi'ttern or simply from conductivity changes.
The n?tion of a seasonal proof, to the extent that some of the proof
radials~ouldbe eliminated, strikes at the very heart of our approach
which is I an accurate determination of the nondirectionaI radiation
pattern. ,Although, under the committee's suggestion, the minimum
number of measure111ents on some radials would be raised from 10 to
74 F.G.C. 2d
Monitoring Point Limits
601
20, we do not feel the value gained from additional data on these
radials would be sufficient to offset the complete loss of data on the
remaining radials. This is also the case for directional patterns"where
changes in radiation in some directions can affect radiation Ill}- lother
directions and assumptions of pattern symmetry are generally unreli 
able. The Commission encourages supplementalmeasurem~nts'in
addition to the minimum of 10 per radial required by the Rules; this
should not be accomplished, however, at the expense of fewer
measurements on other radials.
Your last suggestion concerns the Commission's assignment of
monitoring point limits in response to partial proofs of performance
conducted following antenna repairs, refurbishment, construction or
readjustment. Often such proofs result in a reduction in limits below
those previously assigned because measurements were taken during
periods of' low conductivity or because antenna radiation in some
directions was reduced. The committee suggests we not lower limits in
such cases if the pattern remains in substantial conformance and the
antenna parameters (phases and current ratios) are either essentially
unchanged or, if changed, adequately justified. We believe this
suggestion has merit and have, also on ane~perimentalbasis, ceased
the practice of lowering limits based on partial proofs except when
such limits would exceed measured values by more than 200%.
We feel that the current mandatory use of type-approved antenna
monitors by directional stations and the widespread use of approved
sample systems permit these changes in policy at this time without
endangering in any way the technical integrity of our AM broadcast 
ing system. Nonetheless, because of the significance of these changes,
we intend to proceed on an experimental basis for at least a year,
gaining the benefit of practical experience, before permanently
adopting them. In addition, cases clearly falling beyond the scope of
these policies will continue to be handled on a case-by-case basis.
We are hopeful that the changes we have initiated in response to
your suggestions will provide many stations with operating tolerances
sufficient to accommodate variations which, under our old policy,
would have required a proof of performance and the filing of an
application with the Commission. Again, I would like to express my
sincere appreciation for the work done by your committee in bringing
forth these suggestions.
SINCERELY,
RICHARD J. SHIBEN, Chief, Broadcast Bureau.
74 F.C.C. 2d