AM Radio waves propagate by groundwave at all times, but at night the signals also propagate by skywaves. Medium wave signals bounce off the E layer of the ionosphere and return to earth at substantial distance from the transmitter. During the daytime the sun excites the D layer of the ionosphere. The D Layer is below the E layer of the ionosphere and absorbs medium wave radio signals so they cannot be reflected by the E layer. When the sun is not exciting the D-layer (at night) it dissipates quickly. The D layer is effected by seasonal variations because the sun can more thoroughly excite the D layer during the longer daylight hours of the summer. It is also effected by solar activity, with more excitation during times with large numbers of sunspots, and less during quiet solar periods.
During night hours the E layer causes AM signals to interfere with distant stations in ways that do not occur during the daytime. The transition from day to night is not immediate, but actually takes place over several hours at dawn and dusk.
Present FCC regulations and the US – Canadian Agreement on AM Broadcasting recognize that for Class A stations at least, daytime skywave interference can occur for the hours before sunset and after sunrise.
FCC regulations and the US-Canadian and the US -Mexico agreements on broadcasting also recognize that skywave signals don’t just “turn on” like a light at sunset. These transitional times are particularly important to the success of AM broadcasting because they occur during “Morning and Afternoon Drive” at some times of the year.
In light of the need to protect AM stations from interference it is proposed that stations protect each other during the two hours before sunset and the two hours after sunrise using a formula:
F =(900+ 4.5*P *D)/f
where F = maximum permissible radiated field in mV/m at 1 km toward a point on the protected contour of the effected domestic co-channel station,
P = protected contour value in mV/m of the effected station,
D = the Distance from the transmitter site to the protected contour of the effected station in kilometers, and
f = the frequency in MegaHertz.
Only high power stations on higher frequencies will be effected by this new requirement. The fact that there were substantial complaints between high powered class B stations during the last solar minimum indicates that this is a real and serious concern. Similar to the present regulations, if the licensed facility transmits more than this value, the licensed radiation value will be the maximum permitted.
The density of the Ionospheric D layer is dependent on solar activity, which changes over he eleven year sunspot cycle. During years of high solar activity the D layer is sufficient to attenuate medium wave signals during the critical hours. It is proposed that domestic critical hours limitation on radiation be required only for times when the solar activity as reported by NOAA is less than a specific value. It is proposed that when the monthly averaged Penticton 10.7 cm Flux for the preceding month is above 90, no critical hours protection is required. Since daily reports of solar flux are available on the internet in machine readable format, stations should also be permitted to access the daily value and remain at daytime facilities when the daily value is above the threshold. Stations should not be required to use day power during critical hours when the solar flux is high.
Proposal for Transitional Hours
FCC regulations and the US-Canadian and US-Mexican AM Broadcasting regulations provide for special operation during the two hours before sunrise and after sunset. The present regulations are overly complex and should be overhauled. A proposal to revise these regulations is as follows:
All stations may operate from 6 AM local time to local sunrise at its the licensed day or night power limited to 500 Watts, using its licensed day, night or auxiliary antenna provided:
- Full night protection is afforded to foreign stations excepting Canada, Bahamas or Mexico
- Full night protection is afforded to Canadian, Bahamian and Mexican Class A stations using diurnal factors as described in the appropriate agreements.
- Full co-channel night protection is afforded to domestic class A stations using diurnal factors
All stations may operate from 2 hours before local sunrise until local sunrise and from local sunset until two hours after local sunset at up to licensed day or night power using its licensed day, night or auxiliary antenna, provided:
- Full night protection is afforded to foreign stations excepting Canada Bahamas or Mexico
- Full night protection is afforded to Canadian Bahamian and Mexican stations using diurnal factors as described in the appropriate agreements.
- Full night c0-channel protection is afforded to domestic stations using diurnal factors
Authorizations would be secondary, and subject to modification, as with the present PSSA and PSRA authorizations.
Authorizations would be by minor change application to the FCC specifying the power, and authorized antenna is to be used for each 30 minute period of operation.
Rounded minimum power from 6AM local time to sunrise and from sunset until 6 pm will follow the rounding guide of 73.99(f)(6)(iv)
Existing PSRA and PSSA authorizations would continue as under current rules for those stations presently holding them.
Where it can be shown that foreign stations and allocations have been vacant or silent for at least one year, temporary authorizations may be issued for higher power or different antenna where that station(s) or allocation(s) is (are) not protected. An alternative specification for operation shall be made at the time of application showing the proposed conditions of operation when that foreign station(s) resumes operation. When the foreign station becomes operational, the transitional hours authorization will automatically revert to the alternate parameters which fully protect the foreign station.
This simplification and extension of the current PSRA and PSSA rules will permit expanded operation during the very important “drive time” programming hours providing a more uniform (to the listener) experience year round.