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Carries TV Programs Over Great Salt Lake To Station KID-TV - by L. H. FOLLET, JR., Broadcast Microwave Equipment

From a mountaintop location near Salt Lake City, Utah, television programs are being beamed the length of Great Salt Lake, then over rugged terrain, a total distance of 225 miles, to serve Station KID-TV in Idaho Falls, Idaho. This is accomplished by a two-hop RCA microwave system, one hop of which is 136 miles, the other hop, 89 miles.

The 136 mile microwave hop from Salt Lake City to Albion, Idaho, is believed to be the longest point-to-point television microwave relay ever accomplished. High quality results being obtained on this hop are all the more impressive because the signal travels 75 miles over salt water - one of the worst enemies of microwave relay. Also color performance is being achieved throughout the system.

Completes Overall Program

According to the plan, microwave station facilities would be provided at strategic locations between Salt Lake City and Idaho Falls, in order to relay the programs directly from their source rather than utilize "off-air" signals which were previously used.

It was felt that a direct microwave relay interconnection was the best approach to securing reliable, high quality programs for transmission by KID-TV.

The Salt Lake microwave link was engineered by RCA specialists working in conjunction with Melvin B. Wright, General Manager, Skyline Advertising and Sales, Inc. and Carroll Secrist, Chief Engineer, Station KID-TV. Installation of the 136-mile hop completes the overall plan to provide clear network television signals from Salt Lake City to the KID-TV transmitter in Idaho Falls.

Future expansion is under consideration which will eventually extend high quality television programs elsewhere into Idaho as well as northward into Montana.

Microwave Brings Picture Improvement

Prior to the Salt Lake installation. Station KID-TV, late in 1959, installed an RCA Type TVM-1A microwave link from Albion Peak, 22 miles southeast of Burley, Idaho to the transmitter site on East Butte, 34 miles west of Idaho Falls, Idaho.

Once established, this 89 mile microwave path brought about an immediate improvement to both the picture quality and program reliability of KID-TV. It was to become a key link in the expansion program.

Two 10-foot diameter parabola antennas were installed along with the TVM-1A microwave relay equipment. Throughout the first few months of operation, it became evident that this path was to become highly successful. Considerable improvement was realized in the "off-air" signal strength at Albion.

The Albion microwave site, however, was still 136 miles (line-of-sight) from the network program facilities at Salt Lake City. To complete the plan, the remaining 136 miles between Salt Lake City and Albion were linked with one more vitally important microwave system.

Tough Problem to Tackle

Probability of success in the 136-mile hop depended on two considerations: first, path propogational reliability might be limited due to the extreme path distance (136 miles), and second, the beam would have to cross the entire length of the Great Salt Lake (75 miles).

Over-water microwave paths are normally critical in that strong reflections are likely to occur from the water surface. These reflections if not blocked may arrive at the receiver antenna and cause severe, cancelling effects to the main beam, since they normally undergo a phase reversal during the reflection.

Salt water reflections are usually stronger than those from most other earth surfaces and for this reason, alternate microwave routing around such surfaces is usually recommended wherever possible.

Path Analysis Undertaken

Extensive studies of this proposed path were made by RCA microwave specialists in order to ascertain is practicality.

"United States Geodetic Survey" (USGS) maps for the area were studied along with accurate altimetric measurements for all critical areas of the path.

A study of the prevailing area weather conditions was made, while countless recordings taken from other cross-water paths were referred to for indications of possible fade depth which might be expected, from highly reflective water surfaces.

The microwave beam emanatine from the transmitter atop Coon Peak would begin its cross-water route at the base of the mountain, six miles away. For the next 75 miles the microwave beam would cross the Great Salt Lake, cutting across the salt flats at the north end of the lake, and traveling through a 5400-foot pass in the Raft River Mountains bordering Utah and Idaho finally reaching its destination across the Raft River Vallev in Albion.

Favorable Path Conditions

Notwithstanding the path length of 136 miles, two important considerations seemed favorable for the success of the path:

1. The generally high elevation with constant low percentage of relative humidity and continuous prevailing wind conditions along the path, making the possibility of dielectric inversions of the atmosphere responsible for "inverse bend" types of fading extremely remote, and

2. The strong cancelling reflections associated with this path (estimated at a probable 40 db) occuring at the extreme north end of the Great Salt Lake would be blocked by a 5400-foot pass in the Raft River Mountains 35 miles south of the Albion receiving location.

New TVM-1B System Installed

When the path studies had been completed, RCA shipped one of the first models of the new Type TVM-1B Microwave Relay System to the respective sites for installation. Under the supervision of Carroll Secrist, Chief Engineer of Station KID-TV, two more huge 10-foot parabols antennas were installed, one at Coon Peak and the other at Albion.

Dual signal level recorders were installed at Albion, connected across the receiver's AGC line, and the equipment was turned on.

Excellent Test Results

After final antenna orientation had been completed, the non-faded signal level at the input to the receiver was measured at -70 dbw. This rf input level produced a video signal-to-noise ratio at the output of the TVM-1B microwave receiver of 64 db PP/RMS (measured over 4.5 mc bandwidth).

With signal level data being suppled constantly by the two recorders at Albion, see Fig. 5, it soon became evident that the 136 mile link was performing according to calculations and that, no deterrent reflections from the Great Salt Lake were being received. They were being masked by the mountain pass as predicted.

A noticeable reduction in the average depth of multipath fading was evidenced by the recording charts during the parabola alignment (see Fig. 6) when the polarity of the antenna feeds were rotated from the horizontal plane to the vertical plane. This reduction amounted to about 3 db from the average received signal level.

It was also noted from the graph recording that the amount of multipath signal variation (frequency of occurence) on the 136 mile path was substantially equal to that of the 89 mile path. The maximum fade depth experienced for both paths was about 30 db.

Greater-Than-Expected Gains

During certain periods, actual gains in signal strength above the free space (non-faded) calculations were observed. These gains ranged in amplitude from a few db to a maximum well above 10 db in some cases, (see Fig. 7).

At one time, for a period of about 3 minutes, the signal level rose to a level 18 db above its normal value. This condition can be accounted for by the phenomenon called "trapping" or "ducting".

Trapping is a condition whereby RF energy from a microwave transmitting parabola deviates from its normal free space dispersion pattern and follows a preformed dielectric contour through the atmosphere to the receiver.

These atmospheric ducts vary in regard to their effect on the strength of the microwave signal depending upon their particular structure and elevation.

Other late afternoon recordings taken from the Albion receiver showed that in some cases, the average received signal rose about 6 db over the normal level and lasted from about five minutes to several hours.

During these periods, the depth of multi-path fading was greatly reduced resulting in a marked increase in signal stability, see
Fig. 8.

Fully Operational 225-mile System

Late last year (1960), after several months of continuous recordings had been studied of the receiver signal levels, the long path was declared ready for use and the dual recorders were removed from the Albion receiver.

The resultant 136 mile path from Salt Lake to Albion in series with the 89 mile path from Albion to East Butte is now fully operational over a total of 225 line-of-sight miles, giving excellent results in terms of quality and reliability for both color and monochrome signals.


FIG. 1. Looking from Coon Peak northwest to Albion over the 136-mile microwave hop, 75 miles of which spans the Great Salt Lake. Also see map. upper right.

FIG. 2. Carroll Secrist. Chief Engineer, KID TV, makes last minute adjustments to TVM-1B microwave receiver at the Albion receiving location.

FIG. 3. Receiving location at Albion, Idaho. Parabola at the right faces Coon Peak. 136 miles distent. Parabola at the left faces East Butte. 89 miles away.

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