Forum: Author Hangout

I've heard that in the U.S., there's a limit of 50,000 watts on any AM radio station. I also remember, from decades ago, that there was a radio station in Del Rio, Texas, that had 75,000 watts, and could do that because its transmitter was in Mexico. I don't remember the call letters of that station, but I do remember that it started with a X.
So what does that mean in practical terms. I received the Del Rio station at night in the Southern Appalachian mountains. Does having more wattage mean the radio signal travels farther.
FCC restrictions aside, what would happen if an AM station transmitting from the top of the Blue Ridge Mountains with 250,000 watts? Could they hear that in Mexico? Canada? U.S. West Coast?
What if it were an FM station. Are there similar limits on FM signals and if so, would a 250,000 watt transmitter travel farther (or shorter) than an AM signal?

Take this with a pinch of salt as it's relying on my ability to recall aspects of physics I've only had passing need to retrieve from my memory banks.

Here are the bullet points in no particular order:

for a given wattage with no other considerations AM will be receivable at a longest distance from the transmitter. AM is at a lower frequency than FM an because of this and less signal degradation because of the way the signal is encoded and it does not lose fidelity as soon as FM.

FM is more sensitive to interference but 'bounces' more readily off upper layers of the atmosphere allowing reception in 'patches' of clear and poor signals.

FM also used to partially overlap with uhf/vhf TV signals so you could get signal crossover.

The last major factor is the antenna. here civilians are capped at 5w but with the right choice of antenna that signal can be received over 100Mi away and that is allowing for a signal at a much higher frequency and should have worse attenuation.

Things I'm maybe 50% sure of:
FM is more efficient at converting AC to RF.
FM works better in areas of physical interference.
FM was originally the poor cousin with few stations and less commercially viable as a result.

The 50KW limit was all about politics.

https://en.wikipedia.org/wiki/Wheeler_resolution

Resolved, That it is the sense of the Senate of the United States of America that the operation of radio broadcast stations in the standard broadcast band (550 to 1600 kilocycles) with power in excess of 50 kilowatts is definitely against the public interest, in that such operation would tend to concentrate political, social, and economic power and influence in the hands of a very small group, and is against the public interest for the further reason that the operation of broadcast stations with power in excess of 50 kilowatts has been demonstrated to have adverse and injurious economic effects on other stations operating with less power, in depriving such stations of revenue and in limiting the ability of such stations to adequately or efficiently serve the social, religious, educational, civic, and other like organizations and institutions in the communities in which such stations are located and which must and do depend on such stations for the carrying on of community welfare work generally.

Stations on the AM band have a limited number of frequencies they can use. A high power transmitter can be heard over a greater distance, so they are limited and the same frequency can be used outside the distance covered.
On the FM band again power can cover a greater distance, though an obstruction like hills can create shadows to coverage. Most FM stations tend to serve smaller community areas.

Transmitter power output (wattage), along with the transmitting antenna height and location, determine how far radio signals travel.

(Subject to atmospheric conditions -- for the inevitable pickers of nits)

The reason for a limit to power is so one powerful station doesn't prevent local broadcasters from using that same frequency. A powerful enough transmitter located in the middle of the USA could make dozens of local stations unusable due to interference, even if you couldn't actually get good reception from that powerful station. And local broadcasts were where the money was to be made, especially when radio first started out. Local news was a lot more important to local listeners than what might be happening in Council Bluffs.

AM radio has some drawbacks, like static, fading, and electrical noise from local sources like electric motors, etc. And broadcasting "high-fidelity" music was not possible.

That's why FM. Shorter range due the higher-frequency broadcast band, but nearly immune to interference and much better fidelity.

For example, I can receive WLS AM from Chicago (at night) which uses 50,000 watts. I cannot receive WBCT FM in Grand Rapids - about the same distance - even though they transmit 320,000 watts.

I do not remember the frequencies used (and lazy to look it up), but suspect it was AM band that was used.

Several radio stations transmitted from abroad into the Soviet Union, including in Latvian, such as "Amerikas Bals" (Voice of America) and "Brīvā Eiropa" (Free Europe). Those were in turn intentionally suppressed by Soviet authorities by transmitting their own signal (a very screamy beep) in the same channel.

However, using a sensitive enough receiver and skilled operator it was possible to listen to those suppressed stations, but you had to constantly adjust. The suppression signal was VERY laud, but it was narrow, on the edges of it the useful, much quieter signal could be heard. Suppression knew it, and their signal wiggled over the band bank and forth, but not entirely randomly. So one could catch the edge and surf it rythm, then sharply cross and continue on the other side, and so on. Resulting in a speaker voice that faded and became louder in waves, intersected by a loud beep occasionally.

Why the Soviet authorities were unable to cover the whole width of the "enemy" signal I don't know. They used warships to transmit the suppression signal at first, but eventually purpose built antennas (4 in territory of Latvia), and yet the effort was unsuccessful.