Is that reasoning correct(-ish) ?
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.
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.
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's a nice, short excerpt from 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".
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.Why couldn't they use NBFM which only requires a few kHz of bandwidth?
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.
Your 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.
I'm not sure if you are familiar with the frequency to wavelength conversion
As the wavelength is so large, [...]
NBFM 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.
I think that's just propping up a dying technology.
I think that's just propping up a dying technology.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.
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" & 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.
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.