APPLICABILITY OF PUBLISHED MAN-MADE NOISE STATISTICS TO CONTEMPORARY ENVIRONMENTS

05 * CD

Percent of time Fa is exceeded

The man-made noise statistics presented are largely based on measurements that were made more than 20 years ago in North America by Spaulding and Disney (22). More recently, Spaulding has warned that the CCIR data may now be inac­curate due to technological advances (23). This is largely based on the fact that emissions from newer automobile igni­tion systems, a major contributor to man-made noise in urban

Frequency

(MHz)

ol

Business

Ol

Residential

ol

Rural

0.25

6.1

3.5

3.9

0.50

8.2

4.3

4.4

1.00

2.3

2.5

7.1

2.50

9.1

8.1

8.0

5.00

6.1

5.5

7.7

10.00

4.2

2.9

4.0

20.00

4.9

4.7

4.5

48.00

7.1

4.0

3.2

102.00

.8

8.

2.7

3.8

250.00

3.8

2.9

2.3

Q.

E

CD

Table 2. Location Variability in Terms of the Standard Deviation for Various Environments

40

^ 30

-a; 30

a>

>

о

JD

Я

“ 20

Ж 10

areas, have decreased dramatically over the years. After re­viewing more recent measurements and trend analyses, Spaulding concluded (23) that in the business environment ‘‘at 100 MHz in the 1970’s time-frame, Fam was on the order of 20 dB but now is probably approximately 20 dB less.’’ This conclusion, however, is not based on a comprehensive set of noise measurements as would be necessary to update the pre­vious survey described in Ref. 23.

While the improvements in automobile ignition systems have likely affected the noise levels in business and residen­tial environments, emissions from gap discharge and corona in power transmission and distribution lines have probably not decreased with time. Figure 10 (22) shows Fam under, and one-quarter mile from, a 115 kV line in rural Wyoming. It is interesting to note that the noise measured one-quarter mile from the power line is about the same as that predicted for a rural environment. A possible conclusion is that if power and distribution lines are the primary noise source in rural envi­ronments, rural man-made noise is not likely to have de­creased. However, one would not expect noise in an urban environment to be less (than rural), as would be the case with the estimated 20 dB reduction in Fam.

Another factor that could significantly affect the level and character of man-made radio noise is the proliferation of elec­tronic devices (e. g., computers, electronic switching devices, microwave ovens, etc.) that are unintentional RF emitters. Such devices have become ubiquitous in business, residential, and rural environments and could affect both the magnitude of the noise power as well as its frequency dependence.

The man-made noise data presented in the previous sec­tions are applicable to North America; the validity of exten­sion to other parts of the world cannot be determined pre­cisely. CCIR Report 258 describes very high frequency (VHF) measurements made in business and residential areas of the United Kingdom where the noise power was found to be some 10 dB below that shown in Fig. 3 (16). This is attributed to differences in patterns of utilization of electric and mechani­cal appliances and regulation of interference. The report also states that due to such differences, the noise statistics should be used with caution. It should be noted, however, that if an overall 10 dB reduction in urban noise can be justified, the

іУ A-

0 3 6 9 12 15 18 21 24

Hour (MST)

Figure 11. Median, mean, and peak noise power near an office park (24).

man-made noise environments near 100 MHz would be bounded by what are now classified as rural (worst) and quiet rural (best) environments, as shown in Fig. 3.

Relatively recent noise measurements at 137 MHz (24) show that the statistics of man-made noise are significantly different from what is predicted by CCIR Report 258. For ex­ample, Fig. 11 shows the median, mean, and peak (exceeded

0. 01% of the time) values of Fa measured over a 24-h period in a business environment. Diurnal variations corresponding to human activity are clearly evident. The relatively steady within-the-hour values of the mean power (Fa) are not consis­tent with the predicted within-the-hour distribution of Fa for a business environment (see Fig. 9). Figure 12 shows the dis­tribution of Fa measured at six urban sites plotted on normal probability paper. The distribution at a particular site was obtained by collecting statistics measured within two-minute intervals spaced about an hour apart from hours of continu­ous measurements made at that particular location. Hence, the results should correspond to the hour-to-hour time vari­ability, which, for the most part, is relatively low at most of

40

m

TJ

CD

>

О

m

TJ

Figure 10. Power line noise measurements near a 115 kV line in rural Wyoming (22).

30

20

10

0.1

Frequency (MHz)

Figure 12. Power averages from measurements at six urban sites

(24).

Office park, edge Office park, middle Office park, edge Downtown Denver Downtown Boulder Denver at I-25

5 10 20 30 50 70 80 90 95

Percent exceeding ordinate

99 99.9

the locations. Location variations however are quite large, ex­ceeding 12 dB in some cases. More importantly, these mea­surements show that there are business environments (down­town urban areas) where Fm is still nearly 20 dB.

In summary, the 137 MHz measurements demonstrate that important changes have occurred in both the level and character of man-made noise since the comprehensive noise survey described by Spaulding and Disney (22). While these measurements can only be considered as a ‘‘spot check,’’ they do show that standard methods used to predict man-made ra­dio noise are probably outdated. It is concluded that addi­tional comprehensive man-made noise measurements at RF frequencies into the ultrahigh frequency (UHF) band will be necessary to provide radio system designers and engineers with the required tools to effectively design modern radio systems.

Updated: 29.05.2014 — 08:49