General > General Technical Chat
About fields difference between AM and FM *receiving* antennas... (EDIT: if any)
EPAIII:
I have been at both and AM and FM transmitting sites are not terribly different in size. A small shed, about the size of a lawn shed at a private home, is all the building either needs. Of course, some may have larger facilities, but that generally is to have space to rent out to other services that need tower space and again, it can be much the same.
Both need towers of approximately the same size and those towers will usually require about the same amount of land for the guy wires. As I said above, the AM antenna IS the tower and the FM antenna is at the top of a tower. Why?
AM has a much longer wavelength and an effective/efficient antenna is around 300 feet or 100 meters long. And, being vertical, it benefits from a ground plane which effectively doubles it's length. An AM antenna wants to be on the ground. The longer AM wavelengths will follow the curve of the earth better so they naturally travel further.
FM has a shorter wavelength, around 3 feet or 1 meter so the antennae are much shorter. And those frequencies are much more "line of sight". While AM can penetrate things like trees and buildings fairly well, FM is stopped by them much more easily. The only way to overcome this disadvantage of FM is to have the antenna high up. That way the signal passes OVER the obstacles near the transmitting site and can get down between trees and buildings near the receiving site better. Here in the US the FM band was in the middle of the VHF TV band and this is why you saw so many TV antennae on 30 foot/10 meter masts or shorter ones on roof tops but at that same height. From what I have seen in photos and movies, it must have been much the same around the world.
Yes, you could have FM broadcast antennae on shorter towers, but that would severely limit the propagation of the signal far more than curvature of the earth calculations would predict. The signal would simply need to penetrate more things like trees and buildings before reaching the receiver. If you haven't chased around the countryside measuring signal strength, as I have, you can't really appreciate how bad this can be. The only practical application of lower powered, FM, repeater stations with shorter antennae is to fill in natural dead spots caused by hills and valleys. The repeater is on a hill top and serves the dead spot behind that hill. Or for a distant community where the terrain may be flat but a taller receiving antenna is needed to pick up the signal of the main station due to curvature of the earth.
AM's lower frequency and longer wavelength does follow the curve of the earth and that is why it is better for covering long distances. This is most effective at night due to atmospheric conditions. As I alluded to above, some AM stations here in the US are given what is called "clear channels" at night. Other AM stations with the same frequency are required to shut down at night and the "clear channel" stations increase their power to cover large percentages of the country and even beyond into Canada, Mexico, and perhaps some other countries. People in northern states, like Michigan, can listen to a station on the Gulf Coast people in the West can listen to an East Coast station and other similar situations.
I doubt that the AM radio band will go away any time soon. There's not enough bandwidth there for the communication and network people to fight over. And the AM stations do make money as well as serving national defense purposes. I suspect that stations with "clear channels" at night have been spread around the country with this in mind.
The proximity of cities in Europe may be a factor, but doing away with AM radio there may be a mistake. The internet, with it's reliance on satellite and microwave transmission between cities, may be rather fragile in a disaster or war.
--- Quote from: Andy Chee on March 12, 2024, 05:49:35 am ---
--- Quote from: NiHaoMike on March 12, 2024, 01:29:59 am ---I think that's just propping up a dying technology.
--- End quote ---
In Australia, the AM broadcast band 530kHz - 1600kHz is a necessity in order to cover the vast distance of our land.
In Europe, the cities are closer, hence why AM band is becoming obsolete over there.
Australia could replace large AM sites with multiple small FM transmitter sites, but I haven't done the back of the envelope calculations whether it's worthwhile doing so.
So if AM band is a dying technology, Australia will probably be the last to get rid of it.
--- End quote ---
VinzC:
--- Quote from: vk6zgo on March 12, 2024, 12:27:06 am ---From the original posting, I had no idea of what you knew or didn't know.
The characterisation of antennas as AM or FM, made it look like you were a "Noob", so it was surely logical to try to give you an attempt at an overview of the how & why of antennas.
--- End quote ---
I totally get that. However that causes the response to fall completely off-topic and not to address to the actual question, which was:
* are loop antennas more sensitive to the magnetic component of an RF field?
* are whip (wire) antennas mode sensitive to the electric component of an RF field?
--- Quote from: vk6zgo on March 12, 2024, 12:27:06 am ---The point I was attempting to make was that antennas for receiving MF broadcasting do not have to be "coils" & can be lengths of wire.
"Frame coils" that were their predecessor were not anywhere as effective as loopsticks, although they were much larger.
The "coil" in a loopstick does not do a lot of magnetic pickup by itself, as can be shown if you remove the ferrite rod.
Working normally, it certainly does work with the magnetic field component, but the ferrite rod concentrates that field in the vicinity of the coil.
I expected to Google & find a plethora of diagrams showing the operation of a loopstick, but they are quite rare & this is the best I can find:-
https://loop-antennas.ceyhunsezenoglu.com/en/latest/magnetic-core-loops/introduction.html
Transmitting antennas for all frequencies use the same basic principles, & don't care whether you intercept the electric or magnetic field with your receiving antenna.
MF Broadcasting antennas are vertical, as the basic requirement is that transmission from them should be omnidirectional.
The main takeaway is that receiver antennas can be very far from optimum with very few consequences, due to the high sensitivity of receivers & the high field strength available in the service area of transmitters.
Interestingly, when VHF TV was close to arriving in Australia in the mid 1950s, your "As for FM, the frequencies are high enough so that a mere wire can be used." was reversed, with dire warnings that we couldn't "just use a random length of wire", as we had become used to with MF AM radio.
By the time we got FM broadcasting, radios for that service were so sensitive that you could use the random wire.
--- End quote ---
That again is key and unquestionably important. Yet still doesn't answer my questions :) .
I understand people who are active here are tempted to infer what the OP knows or not (although I'd rather expect them to actually **ask**) but it is also a trap that often leads to actually drifting away from the initial question — as you can see, the staggering majority of answers are about "why coil/why wires" instead of actually addressing the initial question. It's something I often experience on forums and I don't know why. I can be as precise and accurate as I want or not, directive or not, there's always a step where the flow of posts goes off-topic and completely misses the point. And it's generally sooner than later.
As a result, I'm not sure someone browsing *this* topic will easily find that my question was actually answered as it appears completely lost in the flow¹...
Thanks for taking the time anyway. I wish you a great day :-+
¹ Out of 14 responses thus far, only **one** hit the nail on the head. The rest is just off-topic nerd talk, I'd say...
NiHaoMike:
--- Quote from: Andy Chee on March 12, 2024, 05:49:35 am ---In Australia, the AM broadcast band 530kHz - 1600kHz is a necessity in order to cover the vast distance of our land.
In Europe, the cities are closer, hence why AM band is becoming obsolete over there.
Australia could replace large AM sites with multiple small FM transmitter sites, but I haven't done the back of the envelope calculations whether it's worthwhile doing so.
So if AM band is a dying technology, Australia will probably be the last to get rid of it.
--- End quote ---
So then why not reuse the band with a modulation scheme that's much more noise resistant?
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