Document text
CALCULA’ TiO ON
MAL FM CHANNEL ASSIGNMENTS
"i l
Y TY CHANNEL 6
By.
George V. Waldo
ond
1 James D. Visweil
Woshingion, D.C. 20554
July 14, 1967
CALCULATIONS’ FOR EDUCATIONAL FM CHANNEL ASSIGNMENTS
oe IN AREAS SERVED BY TV CHANNEL 6
By
George V. Waldo
and
~ James D. Wiswell
Report No. R-6702
Interference
caused to Channel
mitters operating
between the Grade
Channel 6 TV station.
sidered when Educa
July 14, 1967
SUMMARY
over a considerable. area will be
6 TW service if low-power trans-
on FM
À end Grade B contours of tne
Various factors must be con-
tional FM stetions are located
m
within the Grade A contour, or beyond the Grade B
contour,
determining these
report.
t.
of a Channel 6 TV station.
A design for
factors is described in this
Channeis 201-220 are located
INTRODUCTION.
In this report TV receiver performance data from various sources will
“be summarized and analyzed, and calculations of the extent of interference to.
Channel 6 TV service by. Noncommercial Edcuationel FM stations will be described.
The following analysis is treated in two aspects. First, the natural
limitations for Educational FM station operation beyond the Grade B contour
of a Channel 6 TV station are considered. The second viewpoint deals with
the limitations on such operation within the Grade A contour. In most
cases it would not be practical to operate these FM stations in areas between
the Grade A and Grade B contours unless there were no TV receivers within
“a considerable distance from the FM station. :
TV RECEIVER PERFORMANCE WA
Information concerning selectivity characteristics. of TV Receivers,
obtained from various sources and for various conditions, is. summarized in
the Appendix, The most significant information, from a statistical view-
point, was developed by the television receiver industry.and reported by
` RIMA (FCC Pocket No. 10315) in 1953, Although these measurements were made
some 15 years ago, this information dppears to be consistent with brief .-
information recently obtained, It is possible that recently developed
components. will improve the performance o£ TV receivers. The efficient.
utilization of the frequency spectrum will soon require considerable improve~
ment in the selectivity characteristics of TV receivers.. «
fe:
source in the band from 88,1 to 91.9 Mc/s which degrades the pte nur repro-
duction of a Channel 6 TY signal as received by average installations. |
This degradation may be much greater where two or more FM channels are. .
utilized in the same service area. ‘Information is needed on intermodulation.
effects in existing end future models of TV receivers. It.is the consensus
TV receiver experts that the upper adjacent channel interference char-
acteristics for both monochrome and color TV receivers are similar.
f لع ©
og Os
In some communities it has been shown to be practical to apply
traps or filters in TV receivers, effectively reduc the amount of
interference caused by nearby FM stations to Channel $ TV service,
INTERFERENCE BEYOND THE CHANNEL 6 GRADE D CONTOUR
Figure 1 is an overlay chart to ‘be superimposed over Figure 17 `
of FCC Report No. R-5502 ("Development of VHF and UNF Propagation Curves:
for TV and FM Broadcasting"). It shows the minimum distance separating
an Educational FM station with a given antenne he it. effective radiated
power (ERP), and FM channel number from the Grade B contour of a Channel 6
TV station, At this distance a picture of Grade 3 or better. quality will
be receives at 50% of the receiving locations during 90% or more of the
time. Under average conditions this service limitation occurs only near :
a point on the Grade B contour intersecting a line between the two stations.
Interference will be increased et most of the locations nearer the FM
station and no Channel 6 reception will be possible in its immediate
“vicinity without special antennas and filters on the TV receivers.
Ae $ tege ETE
; Figure l should be reproduced to scale on a transparent sheet for
- maximum usefulness. It is obvious that Figure 1 can also be used directly
side by side with the other chart.by projecting the desired coordinates to
the required parameters. If a transparency is used place it over the 1
F(50,50) graph (Figure 17, R-6602), with the coordinates projected to the
desired parameters, thus allowing direct reading of the chart. The two.
graphs, Figure 1 of this report and Figure 17 of R-6602 may be used for
the determination of the relative magnitudes of two remaining parameters
when any two of the following parameters are known or are specified:
(1) Distance .of the FM station beyond .the Grade B contour of a
Channel 6 TV station.
(2) Frequency or channel number. of the FM station..
(3) Effective radiated power of the FM station.
(4) Antenna height of the FM station.
The horizontal axis at the bottom of each graph must always coincide |
to form a common line. The transparency is then moved horizontally until
, the FM ERP coincides with the antenna height indicated on Figure 17.. The
vertical coordinates of these two parameters will now coincide, The point
"of this ‘intersection on thé line for op FM channel shown on the transparency
‘will be directly over the point indicating distance on Figure 17, This.
. shows the required distance beyond the Grade B contour. tu
in Figure 2 ah example is given of an FM station on Channel 213,
with an antenna height of 400 feet and an ERP of 10 dBk. Using the technique
outlined above, the ‘transparency is shifted until the antenna heignt and
ERP coincide; then the Channel 213 line on the overlay will intersect the
interpolated mileage of Figure 17. This reading of 5.3 miles is the mini- .
mum separation allovable between the TV Grade B contour and the FM station.
Similariy, for 2 Channel 201 FM station the allowable distance between the
Grade B contour and the station vill be 25 miles, Table I lists other
possible combinations of values that can be obtained vith the overlay in
the same position. ١ `
The graph in Figure 1 was designed from the equation,
Fy = Fp ~ A + Gp- Gyr Ap tay Py FR. . [69]
' where F, = Field strength, F(50,50), of the interfering FM station, in
dB above 1 uV/m for 1 kW.
Fp = Field strength of the Channel 6 TV station at the Grade B
- contour for rated power = 47 dB above,l uV/m. >
A = Fading- ratio = F(50,50) - F(50,90). ‘This may be obtained from
Figures 17 and 18 of R-6602. Assuming a normal distribution,
A = F(50,10) - F(50,50) = 8.5 dB, when the distance is approxi-
mately 70 miles. "s .
“Gp = Gain of TV receiver antenna on Channel 6, relative to a
A/2 dipole.
Gy = Effective gain of the same TV receiver antenna in receiving
the interfering FH signal, considering average difference in
directions of arrival of the desired and undesired signals.
It is assumed that Gp - Gy = 6 dB. ١ . 3
-20 log fp + 20 log fy, where fy end fy are the frequencies
of the desired and undesired signals.
op + Ay
Py = Effective: radiated power of the FM station, dBk.
R = Interference ratio for average TV receivers as shown in
fable II. At 47 dB desired signal field strength, as in
Grade B service, this ratio is defined. as the power differ-
ence (in dB) between the undesired and the desired signals
(U - D) at the receiver antenna terminals, for average `
picture reproduction of Grade 3 or better,
INTERFERENCE BY FM STATIONS LOCATED WITHIN THE CHANNEL 6 SERVICE AREA
In the primary TV service area greater protection than that afforded
beyond the Grade B contour is mandatory. We may assume that protection
is satisfactory if the average receiver delivers a Grade 1 or 2 picture |
(interference barely perceptible) at practically all locations. For pracy |.
tical purposes pictures. of "Grade 1-1/2" quality may always be considered
as Excellent, and this may be taken as a "Grade 1" for the present pur- .
pose. Improving the picture quality from Grade 3 to "Grade 1-1/2" requires
a reduction of the interfering signal by approximately 10 dB. For grade 2
about a 7 dB reduction is required. We may further assume that most TV
receivers located within an interference area will require special technical `
modifications, or will suffer objectionable interference, and that "free-
space" field strengths will, on the average, prevail at this range for tne
undesired signal. ` y :
To provide for the natural variation of field ch caused by
terrain and path environment, data from the New York City UHF-TV Project
(FCC Report No. R-6303) were examined. If 99% of the locations around the
periphery of the interference zone is considered to be "practicéily all"
of the locations to be protected, a factor equivalent to the dif£erence
between the median and 99% must be applied. The same procedure was followed
in the case where 90% of the locations are to be protected. In the 99%
` case the New' York measurements indicated a factor (L) of 30 dB for `
indoor antennas and 37 dB for rooftop antennas when receiving Channels 2 or
:7. For the 90% figure the factor was 15 dB for indoor antennas. The 30
and 15 dB values are believed to be most appropriate for the present appli-
cation since the pertinent areas of concern are those where field strengths
permit the use of indoor antennas. Ne o
At most of the locations of interest in this analysis, the distances
involved from the FM station are such that the average field strength will
approximate that for "free. space". The "free space" field strength of the
FM station at a distance d (in miles) will be
Fy = 102,8 - 20 log 4 + Py ROG | (2)
The field Strength; in dB, of ‘the TV station for interference-free service
at this location must be at least
Fe Fg Lo ^. JL Ln DUM NO!
where L is the location distribution factor, and R' is the interference.
ratio for the grade required (Table III). Substituting for Fy from (2),
Fp 2102.8 20 log d+ Pg EEN ” Ou
-In equation (4) the distancé d, in miles, may be considered as the
radius from the FM station of the area where interference may affect
Channel 6 TV reception. This distance should be calculated from the center
of the antenna array, and the effective radiated power, Py, should be |
estimated by considering the vertical radiation pattern Qf the antenna. :.
For.example, .at.a.distance of. 0,20 mile..(1056 feet) from an antenna radia- `
tion center 500 feet above ground, the actual horizontal distance from the
base of the antenna tower. is 0.17 mile (946 feet), and the depression angle '
is 26.4 degrees.; These calculations apply to a receiver antenna 30 feet.
above ground. $
If an Educational FM station is to be located within the Grade B or-
Grade A contour of a Channel 6 TV station, information must be obtained on.
the density of population in the areas described above, on the utilization
of the service provided by Channel 6, and on.the practicality of modifica-
tions of TV receiver installations within these areas for the reduction of
interference which may be caused by the FM station, For average locations;
examples may be considered where ‘the limit of the interference ares is at
0.20 mile from the radiation center of the FM antenna.. At this 0.20 mile
distance, pictures having Gra 2 or better quality at no less than 90%
of the locations, may be considered tolerable. Destructive interference
will be caused at most of the locations at distances closer than 0.20 mile.
The curves in Figures 3, 4, 5 and 6 were calculated from equation (4), and
indicate the channel and maximum mileage separations required for the pro-
tection of locations beyond 0.20 mile, for various-effective radiated powers..
The limits over the FM educational band of the relation between
the separation required for the FM station from the Channel 6 TV station,
and the radius of the resulting interference area, are shown in Figures 7
(for. FM Channel 201) and 8 (for FM Channel 220). For 10 watts ERP, at
the Channel 6 Grade A contour, the radius. of interference for Grade 2 picture
quality, is 8.5 miles for FM Channel 20l and 0.75 mile for Channel 220. In
AWA
the example for a radius of interference of 0.20 mile, under any conditions,
the power. of the FM station must be considerably less than 1 watt. For
effective radiated powers greater than 1 kW, a Channel 220 FM station must
be located at a maxim distance of about 6 miles from a Channel 6 TV
station, This separation diminishes as the frequency is decreased in
approaching Channel 201, and for the first few channels: the FM station
antenna must be located on the same tower with the Channel 6 TV station's
antenna.
r higher effective radiated powers minimum interference would
be DE if the TV and FM station antennas both were, at a common location.
.For this condition, the effective radiated power of the FM station must be
less than
Py £ Pp -R'-K- E : (5)
where K is a factor for equalization of incidental differences in the.
directivity of.the two antennas. It is suggested that this should be
K = 10 dB for average conditions. Correlation of the two signals is
such that a location distribution factor will not be required... "m
1 In the above examples the interference ratios listed for Grade 2.
' pictures in Table III were applied.
CONCLUSIONS : . : 1 |
In average conditions, any transmitter with an ERP more than l watt
in the band from 88 to 92 Mc/s will cause serious interference in the area
between the Gzade A and Grade B contours of a Channel 6 TV station. Mounting.
FM and. Channel_6 TV antenna arrays o the same str
“various factors must be
located within the Grade A
14
fesults
considered wh
contour, ox beyond the Grade B contour, of a Channel 6 TV station. À design
for determining these factors is described in this e In
paremeters to any given receive z location where TV service
ranges of variation in recoiver "And ante performa in signal strengths,
and in cross-modulation effects with strong signals, must all be n into
consideration. The calculations shown: in this report will be useful in
estimating average interference effects for average conditions. At many of
the locations where average TV reception will be degraded by FM station inter-
ference, it will be possible to reduce this interference by installing traps or
filters, or by improving the receiver antenna system.
It should be noted that co-channel ‘and adjacent channel (IV Channel 5)
interference will be present at some locations and at some times within the
Grade B contour of a Channel 6 TV station. This report considers only the
interference which may be caused by transmitters in the band from 88 to
92 Mc/s, and other sources of interference are ignored.
AAN
Channel No. of FM Station
o c qp ee oT ceo en
Effective Radiated Power of FM Station in.dBk
| FIGURE I-
Effective Radiated Power of FM Transmitter
Ir)
+20 dak:
ao n3
FM Transmitter Antenna Height in Feet `
El SEE دس a
FIGURE 2.
FTE ETE EE CCST Tm IA سبي en a
-p omiy. £ وھ |
aptag «snipes au 2° Q Te Ika LOT V «Kispead aad t spe ‘Spa Sep Z* o To www dr ع amad فر
honey AL 9' ونان "y 0007 "it XAP 02 d3 "ele AL - C “Y GOOT FH ‘AEP OZ
S'I3NNVH2 td SNOIIVA UO! 5310/1510 KOlLvuvdas i iel grano wa Shania O4 S3ONVLSIO NOlivuvazs
DE “fouanbesy uojjeys WA
: sm "Duarte! uoneys WA .
d 26 2 16 63*
5 MA OO d
: MY 001
E *
E S
ES ; a
e 5
i MI E
E =:
2 9
Sen 3
2 2
RK 2
MIT ES 2 = 2
3 F^ E: uc
ES 2 &
E m S
m E S E
3 E a z
3 8 E 8
ES
2
3
E
2
DEA TA i mot SCH
* OLY 7 , 5 2649-3
9 aanbry
amgang 7 apen “Snipes SW 2*0 72 seu 4 Ot قمع
E “4 0001 1H “ABP OZ dua gjouro AL
STINNVHO MA SRORIVA 303 5331971510 NOJLVUVAZS
EE ‘Kouanbasy uerg wa
26 _16 os 68 m
uonels WA jo 3
H i IS WA yo +
ne
DE
"sop ‘SUONAIS ALI lavUeYD pue Wy Jo uoriezedas
€ enb-
“supero moja z apa) “sepas ET zo ye one AT
H Coot W “109 02 dua "give AL
Em na Snow 403 S3ONVASIO NOLLVYY42S
GE “fouonbasj MORES wi
2 16 . C6 68 53
UOHEES WA Jo 3
2069-1-4
SEW ‘SUONPIS ALO ونادناناة] pue WJ Jo 035
D [E
“Áyienb nas go se n p 89 gene
‘U 0001 "it ‘xap oz n *8 E k "y 005 H Tez punya wy
e Se x E *4 0007 "i ^72 02 Jua “orujo AL
SHOLIVAS Rd ONY AL NEZNI S3ONVISID NOMVIVASS "SA SAIVI SOUL ` © | SNOLIVIS WA ONY AL Leed SEDNVLSIO HOHVEYC2S "SA SOYA DEN
SIJIN ‘sug EIS uoowog LojjeJedas : TET. É Sol ‘SOS uoat5g uonz4ecog
4 eunt
Shin v» f
< 5 _Oleocos » ¢ z Oleszos o « z LM
T : d
ES
=. i
o à B 4
og "a 1
E
= 2 ;
a 7 3 5
3 El
2. g
=
E E
Ei
9 =
ot tg
1019-1-204
“FCC Docket No. 10315 (Feb. 1953) contains the most extensive data” `
in: the: Commission! S: record. on JV receiver iE in rejecting:
: Adjustments of ‘data werê: necessary £o form à common basis of
analysis: A Grede 1 picture was. considered. 'excellent",: but for Che:
"purpose of: ‘this: report: a practical estimate of Grade 1 would be Grade 1-1/2.
Experience gained by. TASO and by the Commission provided estimates of about
3 dB. difference: between Grade 1-1/2 and Grade 2, and about 7: dB difference
"bétween.Grà de 2and Grade 3 pictures.” Grade 3 pictures. ate considered to
be, passable, where interference degradation: is. not objectionable. The; »
‘to a Grade 3 level of interference by adding: 10 dB: Table LA shows:
interference ratios for various levels of picture degradation... Foi
signal. conditions in the Channel 6 primary service aréa, up to 20 mi e:
"from the IV station, “the interference ratio was decreased by from 15, to
20.dB, ;depending.on.the.frequency of ..the undesired signal,...to provide for.
decreases in receiver selectivity in accordance with information. ‘con:
‘tained’ in the Docket No. 10315 record. It was assumed that these conditions
should. apply for the Gredes 1 and 2 ratios, shown in. Table, 14::
` Figure ‘LA contains he saules of an analysis er the data on the
"ability of TV receivers to reject adjacent-channel interference: contained
: in Docket No. 14185. In order to make a comparison of all the rejection.
ratios, it was-necessary to make adjustments depending on the conditions:
“under which these ratios were ; Measured.
; The shaded area in, Figure 1A represeñts- data obtained by: the. Gane
“Department of Transport -(Intertm Report; December 8, 1966) in investig
of FM interference to TV Channel 6. The values shown were considered to
. be Grade 3 for "weak" signals, and therefore were used as reported without
adjustment. The upper and lower limits of the shaded arca are for the: 50%
of receivers closest tó the median value, among: 15 typical monochrome
receivers tested, : These interference ratios agree reasonably well with
those reported in Docket No. 10315, as represented by the dashed curves
in Figure 1A, p Dn dg dë CS
Data submitted by the. Engineering staff of Storer Broadcasting
“Company were. FM.station interference levels -from field ‘tests. conducted:
“around WITI-TV Channel 6 in Milwaukee yi. Wisconsin. The tests were conducted.
“near. 3 FM stations in the service area of WITI-TV and the severity of ..
interference was noted until no interference ‘was observed. Four levels of
interference were listed; severe, moderate, light,. and none. ` The ratias:;
;of the FM field strength to the TV field strengths. were calculated and
"adjusted to approximate Grade 3 picture levels as closely as possible.
"Severe" interference was taken to be Grade 5 or 4 and.14 dB was. sul
“tracted from the rejection ratio for Grade 5; and 7 dB was. subtracted `
bi be 3. and e
The. “Association: of Maximum : Service Telecasters, Ince, reported. ¢ on
aken on several: Channel. 6. stations. which have
urves from FCC Report No. -R=6602 were u ed to estimaté FM. field
strengths for the: distances. “involved.” Using the reported TV x cU
strength, the ratios” of FM to TV field: strength ‘were: “calculated...
plies ratios since they probably: EE
“average. conditions
` Comments ‘filed “by. the Engineering Department of.Triangle Publi-
ations,’ Ines: (Redio. and Televisión Division) : | Kea cand, Kennedy,
Consulting Engineers, contained ‘laboratory measurements of 2 monochrome
nd::l: ‘color. TV receiver. for ,adjacent-channel interference’ rejection.
both. strong and weak desired signatis. The ‘interfering signal was adjus
in small steps: through an. equivalent frequency spectrum. of 80.to.95
“Mc/s and the signal level noted where "just perceptible" ‘int e
occurred, It: was noted that’ as much as 20 dB of variation: occurred
‘the color receiver rejection ratio for small changes in frequency..."
was the result of the generation of cross-modulation, products and the
maximum and minimum occurred near intervals o£. 15 kefai These eff ects
of small frequency changes were not included in the. values: shown on
“comparison graph and so the values can be subject to a variation of
from. the actuel values. > To change these "just perceptible". levels ty
Grade 3, 10 dB was added to the rejection ratio. Kear and Kennedy made
spot field tests of "just perceptible’ interference to WFIL-TV ^
; Philadelphia, Channél 6 TV from FM stations. The values obtained differed
from the laboratory. tests by -18 to +14 dB for the color set and -13,5-to: :
“+9 dB for the monochrome set. These field tests are not shown on the
comparison graph.
. “The comparison of the rejection: ra atios from 211 the data appea
to be quite scattered but in general agreement to those used in .this.
. xeport. Much of the scattering, comes from the assumptions made as to
the. esting conditions and to the relatively few numbers of receivers:
“used,
at. the Grad 2B
: : C Contour = ="
Frequency: of (R =U ~.D)
Undesired: Signal “Grade 3 Pictures -
3.6 dB:
90 Me 5 ASA dB
91 Miz ` ATT eB
92 MHz yo. 89.0 dB: