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".
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.