About fields difference between AM and FM *receiving* antennas... (EDIT: if any) - Page 1
Forum section: electronics / general

author: VinzC
author_rank: Frequent Contributor
timestamp: March 10, 2024, 11:28:11 am
content: Hi all.It always was a curiosity to me as to why AM and FM (receiving) antennas were different¹ but I was obviously not curious enough as to dig deeper... until I've had to build my own FM radio circuit. I'm not here for a deep technical explanation though, rather than verifying if my "intuitive" understanding is correct (or not).The fact that AM *receiving* antennas are loops (i.e. a dipole, a coil, basically) and FM *receiving* antennas can be a simple wire, tells me the former are more (or completely?) sensitive to the *magnetic*, while the latter to the electric field. The reasoning is that a coil, used as a *receiving* antenna, by definition, "catches" variations in magnetic fields. Since a wire acts like a capacitor, coupled with the ground as the second electrode, the principle of a *receiving* FM antenna is based upon electric fields.Is that reasoning correct(-ish) ?Is that also why one is said to be polarized vertically (AM, right?) and the other horizontally (FM) ? Since electric and magnetic fields are 90° apart (geometrically, not electrically), that would make sense to me.EDIT: Here'sa nice, short excerptfrom a book (I assume is) about antenna theory that guided my deduction.Thanks in advance for shedding some light on this.¹ Read "One is a coil, the other a simple wire".

author: Ian.M
author_rank: Super Contributor
timestamp: March 10, 2024, 12:15:58 pm
content: There is no difference! The only thing that matters is the frequency band, which at lower frequencies favours coil and loop antennae because they can be far more compact. You are probably confused because AM broadcast stations were (and the remaining ones still are) HF (shortwave) bands downwards and FM broadcast on lower frequency bands is generally impractical because the required bandwidth 'footprint' for the sidebands not to interfere with adjacent AM stations is excessive with respect to the frequency range of the band.

author: Andy Chee
author_rank: Super Contributor
timestamp: March 10, 2024, 12:35:10 pm
content: Quote from: VinzC on March 10, 2024, 11:28:11 amNope!The correct explanation is that AM broadcast radio operates at around 1MHz, whereas FM broadcast radio operates around 100MHz.For a fairer comparison, have a look at VHF AM airband antennas, which operates between 108MHz and 137MHz.

author: ejeffrey
author_rank: Super Contributor
timestamp: March 10, 2024, 04:31:32 pm
content: Parlly. First off it all comes down to frequency, AM being around 1 MHz and FM at 100 MHz . If you use amplitude modulation or frequency modulation on different bands or different purposes than terrestrial broadcasting everything changes.You typical AM receiver coil antenna is indeed mostly sensitive to magnetic field. Because it's mich smaller than the wavelength, you can measure them separately.Due to FMs 100x higher frequency, we can more easily use a quarter wave dipole, which is detecting both the electric and magnetic fields. But a short whip antenna mostly senses the electric field.As for polarization: EM wave polarization is defined in terms of the electric field direction. The difference doesn't have anything to do with the angle between E and M fields. AM radio always uses vertical polarization. This is necessary due to its low frequency and ground wave propagation. Horizontal polarization would cause the ground wave to destructively interfere and make terrestrial reception poor. FM due to its higher frequency is basically line of sight and the polarization doesn't matter as much. I think now it's often circularly polarized so that both horizontal and vertical antennas will work.

author: IanB
author_rank: Super Contributor
timestamp: March 10, 2024, 04:48:15 pm
content: Quote from: VinzC on March 10, 2024, 11:28:11 amAn AM receiving antenna can very easily be a long wire hung in the air, possibly paired with a ground connection. In fact, almost anything can act as an AM antenna. When I was a child, I was surprised to discover that merely attaching an earpiece to a large metal structure allowed me to hear a local radio station at quite high volume. This was without needing any traditional elements of a receiver such as a tuning circuit or a diode. Both of those elements were presumably parasitically present in the system.Portable radios use a coil as an antenna as it is obviously not practical to use a large external antenna for this purpose, but this is not the only possible antenna design.

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 10, 2024, 09:10:36 pm
content: Quote from: Ian.M on March 10, 2024, 12:15:58 pmWhy couldn't they use NBFM which only requires a few kHz of bandwidth?

author: MT
author_rank: Super Contributor
timestamp: March 10, 2024, 09:26:45 pm
content: Old tube guitar amps with inbuilt spring reverbs where the springs had a tendency to act as antennas picking up lorry dudes AM com radios!

author: Ian.M
author_rank: Super Contributor
timestamp: March 10, 2024, 09:31:07 pm
content: Back in the day cheap FM broadcast receivers used slope detectors, which only required a single winding IF transformer and a single detector diode (which before the Germanium semiconductor industry developed, were expensive), and making the slope steep enough for NBFM was problematic.Once there was an installed base of FM broadcast receivers designed for 100KHz or 200Khz channel spacing, changing modulation schemes was very difficult. Look at the length of time it took for DAB to become popular.

author: vk6zgo
author_rank: Super Contributor
timestamp: March 11, 2024, 01:44:10 am
content: Quote from: VinzC on March 10, 2024, 11:28:11 amYour intuitive" understanding is incorrect.The "Medium Frequency" (MF) AM Broadcast band is in most countries in the region of 0.5 MHz to 1.65MHz, compared to which,FM Broadcasting is in the band 88MHz to 108MHz in the VHF Band.Leaving aside modulation types, I'm not sure if you are familiar with the frequency to wavelength conversion, but wavelength in metres = 300/frequency in MHz.Grabbing convenient values, this yields for 1000kHz (1.0 MHz) a wavelength of 300m, & for 100MHz, a wavelength of 3m.The large vertical radiators for MF normally consist of a vertically guyed mast, which is electrically insulated from ground, & from 0.25 to 0.75 of a wavelength long.As the wavelength is so large, various workarounds are often used, but the main idea is to obtain maximum transmitted signal over the service area, so this entails a large, efficient, antenna.Ideally, the listener would have an equally large antenna, but this is obviously impossible.Luckily, radio receivers are very sensitive so do not need very efficient antennas.In earlier times, it all varied on how far you were from the station, with many home listeners having relatively long horizontal wire antennas in their backyards, whereas others could make do with just a short length of wire out the back of their radios, or even enough pickup via their power cord.Portable radios were the main impetus for "coil type" antennas, many having a "frame" type antenna mounted inside the cabinet, which also did duty as the radios aerial coil.After the advent of ferrite "loopstick antennas", they became popular in home receivers as well.When FM broadcasting commenced at 88-108MHz, receivers for VHF were mainly specialist designs for military or communications use, but consumer level equipment needed to be achievable at reasonable prices.The early FM receivers were fairly insensitive, so were normally used with roof mounted antennas similar to those used for TV.To obtain the maximum signal, these were "Gain" antennas such as Yagis, etc, which are feasible at the frequencies used.Time & progress happen, so that FM portable radios with much better sensitivity became available which could use far from optimum short whip antennas.In strong signal areas, things like my clock radio, which has a short length of wire hanging out of it became useable.In the AM MF band, loopsticks have reigned supreme.In both bands, receiver antennas are far from optimum, & a "real" antenna would do much better, but "good is the enemy of perfect", & "good" does a very adequate job.

author: vk6zgo
author_rank: Super Contributor
timestamp: March 11, 2024, 02:09:43 am
content: Quote from: NiHaoMike on March 10, 2024, 09:10:36 pmNBFM is "sort of" OK for comms use, but as you reduce your deviation, the noise immunity of FM becomes degraded.The other problem is that NBFM is basically "comms bandwidth", which wouldn't work in those countries which have a standard AM broadcast programme bandwidth of 9-10KHz. (an RF bandwidth of 18-20kHz)AM, certainly in the MF band, very seldom has any major noise problems in the primary service area of the station.There are other methods of modulation used by some HF broadcasters such as "Digital Radio Mondiale" (DRM), which offer better quality than ordinary AM, but it hasn't been taken up widely.

author: VinzC
author_rank: Frequent Contributor
timestamp: March 11, 2024, 04:55:31 pm
content: Quote from: ejeffrey on March 10, 2024, 04:31:32 pmThank you **very much** for your explanation. I am no longer a beginner in electronics (although I've been mostly practising as a hobby for about 40 years or so) and I mostly wanted to make sense of the differences between loop and whip antennas. I understand my perception is only partial and I also know there's much to dig about that particular topic. But one needs to understand the basics before going further, right¹? ;-) *That* was the reason for me asking.I have to stress that your response is the only one (IMHO) that stays on focus and addresses the exact topics I introduced, along with short, layman explanations that I can make sense of, so thanks a lot again.Quote from: vk6zgo on March 11, 2024, 01:44:10 amWell, I'm sorry but after reading your answer, I no longer understand my question...Quote from: vk6zgo on March 11, 2024, 01:44:10 amI am indeed. Fact is I'm not a beginner anymore (see above).Quote from: vk6zgo on March 11, 2024, 01:44:10 amI understand the reasons to use a coil, i.e. given the wavelength of AM signals, an antenna in the form of a wire would have to be so huge it is impractical and a coil is used to artificially "increase" the length of the antenna. As for FM, the frequencies are high enough so that a mere wire can be used. I (think I) know that. That was just not the reason I asked.If my understanding is correct (after 40 years practising electronics, it better be, right?) a loop antenna, that is, acoil, is an inductor, right? Intuitively, I understand a coil is not sensitive to *electric* fieldsmorethan *magnetic fields*, right²? So the latter effect is predominant, still correct?And yes, I understand RF radiation is an *electromagnetic* wave, not just an electric field or a magnetic field — maybe I should have mentioned "the electric/magnetic fieldcomponent".Anyway.From ejeffrey's response above, I understood my perception, though partial, was not (completely) incorrect. Now you say it is...From that contradiction, I'm no longer certain where I stand now. That is unfortunate.¹ At least to me.² Given that variations in an electric field are what creates the magnetic field.

author: vk6zgo
author_rank: Super Contributor
timestamp: March 12, 2024, 12:27:06 am
content: 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.The point I was attempting to make was that antennas for receiving MF broadcasting do not have to be "coils" &canbe 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.htmlTransmitting 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.

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 12, 2024, 01:29:59 am
content: Quote from: vk6zgo on March 11, 2024, 02:09:43 amNBFM is still going to be more noise resistant than AM or SSB, otherwise nobody would use it.Some automakers are phasing out AM radios in cars, citing EMI problems. There's a proposal for a law to require automakers to have AM radios claiming it's important for emergencies, I think that's just propping up a dying technology. More reasonable would be to require some other receiver for emergencies if there's no AM radio, for example weather band and/or some amateur bands. (Both weather band and the 2 meter band are not that far from the broadcast FM band, so implementation should be relatively easy.)

author: EPAIII
author_rank: Super Contributor
timestamp: March 12, 2024, 03:38:15 am
content: Put another way, for FM the tower SUPPORTS the antenna. For AM the tower IS THE ANTENNA.An AM tower will have insulators on the guy wires, usually two or more on each wire, to prevent them from being an uncontrolled element in the tower antenna. That's an easy way to spot one from a distance. And not all AM towers are insulated from the ground. There are ways of connecting the feed from the transmitter at higher places where the ground acts as a null point and the signal is fed to the tower without any shorts.Another invisible feature of an AM transmitting site is a ground field. A network of conductors will radiate out from the tower's base and probably be cross connected with circumferential conductors at a distance from the tower. This ground plane is buried a snort distance below the ground and it serves as a reflector which effectively doubles the length of the tower/antenna. So a 1/4 wave tower becomes a 1/2 wave antenna. One AM station here in the states has an almost ideal location in the middle of a salt water swamp. The ground plane is excellent and their signal travels nation-wide at night when they can use full power.And some towers serve both AM and FM; the AM signal being attached to the tower itself and the FM signal running up the tower to it's antenna at the top. And then there can be TV and 2-way communications, microwave dishes, and whatever else. I once had responsibility for a tower with a TV antenna on top (my station) and so many smaller antennae all up and down it that I lost count. Everything but AM.I too built crystal radios in my childhood. I once had an antenna strung the length and width of our side yard. It worked great for local stations. It would have worked better if it had been vertical but that was not possible on my small allowance of the time. I even built one crystal radio with an amplifier but no batteries or AC power supply. It used the energy of a strong local AM station to power the amplifier.

author: Andy Chee
author_rank: Super Contributor
timestamp: March 12, 2024, 05:49:35 am
content: Quote from: NiHaoMike on March 12, 2024, 01:29:59 amIn 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.

author: EPAIII
author_rank: Super Contributor
timestamp: March 12, 2024, 07:10:46 am
content: 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 antennawantsto 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

author: VinzC
author_rank: Frequent Contributor
timestamp: March 12, 2024, 08:33:58 am
content: Quote from: vk6zgo on March 12, 2024, 12:27:06 amI 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 amThat 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 day¹ 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...

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 13, 2024, 01:40:16 pm
content: Quote from: Andy Chee on March 12, 2024, 05:49:35 amSo then why not reuse the band with a modulation scheme that's much more noise resistant?