7

for protection of the site. The present level of radio noise at the site at the 50 MHz frequency is within about 4 db of average galactic noise, 14 db below suburban man-made noise levels and 30 db below urban man-made noise levels estimated by the Joint Technical Advisory Committee. Expressed in terms of ratios of noise powers, these figures mean that at the 50 MHz frequency, the present Table Mountain noise level is 2.5 times greater than the galactic noise; and that suburban and urban man-made noise are 25 times and 1000 times, respectively, greater than the present noise level at Table Mountain.

6. The research activities in which ESSA is engaged, involve unique and nationally important radio research programs. The following are some of those activities. The Ionospheric Telecommunications Laboratory renders service to other government, commercial and scientific agencies in a consultative and advisory capacity, and is engaged, at Table Mountain, in acquiring technical information on radio propagation factors affecting the design and use of radio systems, and the information obtained is directed toward guidance of engineering practices, allocation and use of radio frequencies, and evaluation of system capabilities and limitations. A 25-element VHF electronic scanned array is used in propagation studies, such as propagation of signals by the sporadic-E layer, and by the ionospheric scatter mode. A back scatter radar receiver facility can measure simultaneously high frequency ground back scatter signals, direction of arrival information, signal phase variation, fading characteristics, and signal intensities. The Table Mountain site is also utilized for a number of experimental programs for the Tropospheric Telecommunications Laboratory which conducts research on the propagation of electromagnetic waves as affected by the troposphere and its boundaries. Two general-purpose parabolic antennas with 60-foot diameters, steerable in azimuth and elevation, have been used at frequencies ranging from 400 MHz to 11 GHz to study tropospheric scatter on paths up to 770 km. in length, diffraction measurements over a mountain ridge, measurement of bistatic scattering cross-sections of thunderstorms, and evaluation of typical characteristics of large antennas. The Radio Astronomy Laboratory has a large radio-telescope operating on 10 MHz at Table Mountain which yields information on the nature of the extraterrestrial radio sources in the HF region, such as the sporadic radio emissions of Jupiter, the nature and extent of regions of ionized gas in the plane of our galaxy, the structure and extent of our galaxy, etc. This information is used by astrophysicists in formulating cosmological theories, in understanding the physical nature of our galaxy, and in determining the physical processes at play in the "radio stars". The High Altitude Nuclear Detection Studies project at Table Mountain utilizes radio, optical, acoustic and magnetic sensors in research and development of sensors which might detect effects from high altitude nuclear detonations. Studies are also conducted on the physical, chemical and electrical properties in the upper atmosphere. This includes direct measurements from rockets and satellites. See also paragraph 36 of the initial decision.

This act prohibits the establishment or construction of industrial or other facilities within 2 miles of Table Mountain if such facilities would emit electrical noise radiation in excess of certain prescribed levels which could cause harmful interference to the Table Mountain installation.

7. ESSA proposes an interference criterion of 10 mv./m. at Table Mountain for the FM band (see initial decision, par. 38) which would guarantee that new FM signals would not add significantly to interference presently received. The 10 mv./m. criterion is 3.16 times, or 10 db, greater than the Commission's requirement for minimum field strength over the principal community to be served, and is 10 times, or 20 db, greater than the 1 mv./m. signal required for service to other areas. This criterion was selected, in part, so as to avoid denying new services to the Boulder area and, at the same time, to avoid significant or dominant contributions to interference potential at Table Mountain. 8. The examiner, in his initial decision, has made findings on the various tests, measurements, and analyses which have been made and submitted by the parties. Of the three showings made in those findings concerning the expected field intensities at Table Mountain, the Board believes that the one based upon field intensity measurements made on the simulated operations from each of the proposed sites (see pars. 42– 43 of the initial decision) would most likely represent the expected fields on Table Mountain from the proposed operations. The other two showings, one based upon free space propagation theory and the other upon the FCC curves, have common factors mitigating against their use here. Both involve prediction methods, which are based, in part, upon certain assumptions which have not been established as being valid for the particular circumstances and purposes here.10 Accordingly. the Board relies upon the following measured field intensities at a receiver antenna height of 30 feet, made principally at location A, in evaluating the impact of the proposed signals on the Table Mountain facilities."

* The record herein, for the most part, reflects ratios between powers and between voltages expressed in decibels (db), a logarithmic relationship of those quantities, calculated by a well known and recognized formula. The relationship between db and voltage, and power ratios are found tabulated in numerous handbooks. For example, see "Reference Data For Radio Engineers" 5th Edition, International Telephone and Telegraph Corp., 3-13. When voltage ratios and power ratios are expressed herein as numerical ratios instead of db, there is no difference in meaning; rather it is a different way of expressing the same relationship.

Based upon FCC curves, the field on the nearest point of Table Mountain to the sites proposed would be 88 mv./m. for the IEDC proposal and 9.9 mv./m. for Shaffer's proposal, and on the basis of free space calculations, the fields would be 68 mv./m. for Shaffer and 119 mv./m. for IEDC.

This is not to hold that the FCC curves should not be used for the prediction of contours or that measurements take precedence over the curves except in the particular circumstances such as here where several valid measurements have been made of signals of stations over a limited area and where the impact of signals upon a specialized receiver installation is involved.

ESSA is principally concerned with field intensities, in excess of the criterion of 10 mv./m., which would prevail over a significant portion of the site. Measurement point A

9. The record reflects that, in highly sensitive receivers of good design, unwanted responses are produced in the presence of interfering high-level input signals (a) by the mixing of the interfering signal with the harmonic signal generated by the first oscillator and fed into the intermediate frequency band pass amplifiers, (b) by intermodulation products formed by two or more input signals, resulting from non-linearities in the receiver and/or nearby structures, and (c) by harmonic products of strong input signals generated in the receiver due to nonlinearities of the receiver circuits. All receivers have nonlinearity characteristics. The record further establishes that intermodulation products (see appendix I) are, in fact, generated all the time in receivers, and that the actual level of the products is variable. The level of such products is dependent upon the intermodulation parameter constant (see paragraph 16, infra and Tr. 610, line 25). Thus, as demonstrated in appendix I and found in paragraph 12, infra, the level of the intermodulation products increases with an increase in level of an interfering signal, and, hence, the noise level is increased.

10. ESSA expects a principal source of interference to reception to be from signals generated by intermodulation products. The intermodulation products problem, as noted, is illustrated by data in the following table and the attached appendix I. These tables are based upon assumed field strength of 10 mv./m. and 40 mv./m. for the proposed operations on 94.7 MHz and utilizing signals of the following existing FM and TV stations whose measured field intensities at Table Mountain are greater than 10 mv./m.

The attached appendix I entitled "Intermodulation Products" shows a partial list of the third order 12 of intermodulation products and their levels derived from the intermixture of the proposed signal with existing signals.13 The bandwidths, in MHz, in the table are the sums of the bandwidths of the contributing signals.

11. ESSA also made a graphical showing of the effect of the addition of the proposed signals at various levels of field strengths upon the generation of intermodulation products. These graphs depict the

is located near the center of Table Mountain, point B is approximately 800 feet from the south-central edge of the site, point C about 1900 feet southwest of point A and approximately 1100 feet from the westerly edge of the site, and point D some 1100 feet from the northeast edge of the site. ESSA's evidence indicates that the measurements made at the boundary of the site reflect strong folds which are not really significant to the operations at its site. Accordingly, the field measurements farthest removed from the edge of the site are the most significant ones.

12 These are the most significant products in that they are the new frequencies generated by the intermixture of three fundamental input frequencies, and, in some instances, the intermixture of a second harmonic frequency with another fundamental frequency. 13 IEDC stated that it had no question concerning the equation (equation 1, ESSA exhibit D) which was used to derive the intermodulation product frequencies.

intermodulation products for existing signals exceeding 10 mv./m. at Table Mountain, and for new products created by addition of a signal at 94.7 MHz at three levels: 10 mv./m., 40 mv./m., and 80 mv./m. These signal strengths correspond, respectively, to available power levels. of -63, -51, and -45 dbw received by a 2 square meter antenna aperture, which has less than 2 db gain over a half wave dipole; this same basis was used for the table in appendix I. From these graphs, ESSA shows that addition of a 10 mv./m. signal (-63 dbw) at 94.7 MHz can introduce new products throughout the spectrum (portrayed up to 300 MHz), but at levels mostly 10 db or more below the already existing products; that the addition of a 40 mv./m. (-51 dbw) signal can create 40 new products (an 80 percent increase over the existing 50 products) at levels of the same order of, or exceeding, existing products; that these additional products associated with a proposed 94.7 MHz signal at the 40 mv./m. level could be dominant over a bandwidth of about 72 MHz of the spectrum below the 300 MHz frequency, or 24 percent of the region; and that, at the 80 mv./m. level, the interference products dominate in more than 40 percent of the spectrum below the 300 MHz frequency, and that the levels of the new products are in most cases 5 to 14 db greater than existing ones."

12. ESSA's exhibit E has sample calculations showing how the intermodulation product frequencies are computed and also how the power level of these products are determined. One such calculation illustrates the computations for the 165.65 MHz product and the resultant power levels which involve 94.7 MHz signals having intensities of 10 mv./m. and 40 mv./m.; the power levels are - 164 and -152 dbw, respectively. Further, by simple substitution into the equation (p. 10 of ESSA's exhibit E) of a received power level of -45 dbw for an 80 mv./m. signal (see page 8 of the same exhibit), the power level of -146 dbw is obtained for the same 165.65 MHz frequency product. Thus, it may be observed that as the level of signal intensity at 94.7 MHz is increased in steps, first from 10 mv./m. to 40 mv./m., and then from 40 mv./m. to 80 mv./m., the power level of the 165.65 MHz product is increased by 12 db and 6 db, respectively, a total of 18 db, or over 63 times greater for this particular intermodulation product. Using the same method of calculations, it can be further determined that for a signal intensity of 100 mv./m., the power level of the intermodulation product would be 144 dbw, an increase of 20 db over the 10 mv./m. level, or a 100 times increase in power.

14 IEDC has raised questions about this showing and also the showing made in the attached appendix I concerning the level and bandwidth of the intermodulation products. First, it points out that it is well known that power when related to a television channel, refers to peak power, while when related to an FM channel, power refers to RMS power, and that the same references apply to field strengths; and that ESSA has not shown whether it has used the necessary factors to properly relate both types of service to the same reference before entering the equation which was used to compute the intermodulation power levels. And, second, it objects to the implication that the intermodulation product levels shown in the appendix I table would be present throughout the bandwidth shown in that table, stating that since power is not evenly distributed throughout the allocated channel for either television or FM, a simple addition of the bandwidths of each is not a true indication of the bandwidth for a given power level in any case. As to the latter point, the Board will not, in its conclusions, give any substantial weight concerning the bandwidth. However, as to the first contention, the objection is not of great significance in that inspection of the equation (equation 2, ESSA exhibit D) from which the level of the intermodulation products have been calculated shows that the use of the necessary factors Would not materially affect the significance of results obtained, i.e., that with an increase in field intensity of an interfering signal, the level of the intermodulation products will also increase.

Thus, the increase of one high level signal could bring to an observable level a number of unwanted responses which were previously below the level of observability. ESSA also demonstrated that with the addition of each new signal, the number of intermodulation products rapidly increases.

13. From the above, the following conclusions may be drawn. With the addition of a new signal to those already present at Table Mountain, the number of intermodulation products increases substantially; that, as the strength of the new signal increases, the power level of many of the new intermodulation products also increases; and, at high signal intensities, they become the dominant intermodulation products at substantially greater power levels than the existing products and present serious problems to the reception of signals untainted by interference.

14. IEDC, in rebuttal, showed that fourteen of the 40 intermodulation product frequencies shown in the table of appendix I can be formed from the five existing signals and do not depend upon the addition of a new 94.7 MHz signal. However, IEDC did not rebut ESSA's showings that, with the addition of each new signal, the number of intermodulation products rapidly increases; that, as the signal intensity of unwanted signals is increased in value, the level of the intermodulation products also increases; and that their level, thus increased, could be at a level of observability.

15. The record further shows that, as intermodulation products increase in number, case by case corrections of a specific receiver installation, subject to such interference, becomes increasingly difficult because of the large number of such products. Moreover, in some instances, the elimination of one specific interference situation will often result in encountering another, and also in degrading the performance of the receiver. In this connection, IEDC showed that by use of a simple filter, an unwanted signal could be reduced by a factor of 80, allegedly without seriously introducing loss in the receiver. However, increased selectivity ahead of the first stage or active device of the receiver, as obtained by a filter, introduces losses which reduce sensitivity. Thus, such a technique creates serious problems for very sensitive receivers. Moreover, sometimes ESSA's antenna systems must respond over a wide frequency band with uniform impedance characteristics at several wanted frequencies. Filters adapted to remove specific frequencies do not have such characteristics. In addition, there is an attendant degradation of receiver noise figure with the introduction of filter losses.

16. In computing the power levels of the intermodulation products, ESSA's witnesses employed an equation 15 which involves the summa

15 IEDC has objected to the use of this equation on the ground that it is an arbitrary and unverified formula; that its source, its validity and its usefulness have not been shown; and that its authors have not been made available for cross-examination. Thus, it asserts that it cannot be relied upon either in or out of context. As the record shows, the equation is obtained from "Interference Notebook" RADC-TR-66-1, Publisher: Braceland Brothers, Inc., 1966. The source of the equation was not initially identified by ESSA, but when it had been identified, and the notebook was still apparently unavailable to IEDC, an IEDC witness testified that he would like to examine the reference to determine under what conditions the equation could be applied, and what the method of derivation was so that he could apply it to see if the results were reasonable. Subsequently, on July 26. 1967, the notebook was made available to IEDC, and the hearing was continued until Sept. 8, 1967. Thus, IEDC had sufficient opportunity to examine the notebook for the