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About fields difference between AM and FM *receiving* antennas... (EDIT: if any)

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VinzC:
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".

Ian.M:
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.

Andy Chee:

--- Quote from: VinzC on March 10, 2024, 11:28:11 am ---Is that reasoning correct(-ish) ?

--- End quote ---
Nope!

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.

ejeffrey:
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.

IanB:

--- Quote from: VinzC on March 10, 2024, 11:28:11 am ---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.
--- End quote ---

An 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.

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