ITM Prediction Accuracy for TV Signal Loss Before and After 1960

[Steven]

I tried using the Irregular Terrain Model (ITM) to predict the signal loss of TV stations, utilizing data from multiple yearbooks. Interestingly, I found that after 1960, the predictions align well with the increase in TV households in a given area. However, before 1960, the model's predictions do not match as accurately.

I'm wondering if there were technical improvements in TV station broadcasting, changes in transmission power, or modifications in antenna technology that could explain this discrepancy? Were there policy changes, shifts in frequency allocation, or advancements in signal propagation techniques around this time that might have impacted TV signal reception?

Would appreciate any insights from those familiar with historical broadcasting infrastructure and signal propagation modeling. Thanks!

 

[PTBoardOp93]

Nothing fundamentally changed about television transmission, at least on VHF channels, circa 1960.

The standard VHF power levels were established in 1952 at 100kW for channels 2-6 and 316 kW for channels 7-13, both atop a 1000ft tower. These rules were in use until the end of analog TV broadcasting in 2010.

The antennas weren't changing much. The RCA Super Turnstile ("Batwing") antenna existed from the beginning of commercial television in the late 1940s. The design remained popular, especially for VHF channels 2-6, throughout the analog TV era.

However, throughout the 1950s and 1960s, there was a lot of investment in television. Many stations sought to centralize their towers, increase power to the limits and raise them to the highest heights.

In 1960, UHF was still a very young medium. I do not have at my fingertips the date of the final FCC rulemaking on UHF television power levels.

 

[Dondd]

The standard VHF power levels were established in 1952 at 100kW for channels 2-6 and 316 kW for channels 7-13, both atop a 1000ft tower. These rules were in use until the end of analog TV broadcasting in 2010.

Curious, why 100kw was sufficient for the lower frequencies, but 316kw for the higher VHFs.
FM, of course, is different but 100kw is high power for that medium.

 

[PTBoardOp93]

The signal loss through the air increases as frequency increases. If you go into some physics/electromagnetics equations, 314kW on channel 13 will produce the same field strength as 100kW on channel 4. I don't know if that's the precise calculation the Commission used when setting the rules, but it gives you an idea.

The power limits for the FM broadcast band were set more than a decade later by a different set of FCC commissioners, so there wasn't necessarily consistent logic.

 

[Mark Roberts]

But here's a related matter. Looking back through FCC history cards of TV stations, I see that audio power was usually 50% of visual power. Then, in the 1960s, something changed, and that ratio was reduced to 10-20%. Stations had to file to reduce audio power, and most did. I haven't found any historical material on why that change occurred. Anyone know? This is a matter that was just slightly ahead of my time.

 

[PTBoardOp93]

The aural power was changed to a maximum of 20% of visual (47 CFR § 73.1560). I couldn't find right away when that rulemaking was passed through the FCC, but it must have been in the mid 1960s, long after the 1952 Television Report and Order.

The best timing I can find comes from the FCC's 1965 annual report, p. 164:

FCC engineers participated in studies of the feasibility of channel
splitting (fitting two channels into frequency space previously as-
signed to one) to provide more assignable frequencies in the 450-
470 Mc band, and worked with industry engineers exploring ways
and means of accommodating more and more stations in the land mobile
radio services. Similar cooperative efforts resulted in the adoption
of a new, lower power level for TV aural transmitters.