General > General Technical Chat
About fields difference between AM and FM *receiving* antennas... (EDIT: if any)
VinzC:
--- Quote from: ejeffrey on March 10, 2024, 04:31:32 pm ---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.
--- End quote ---
Thank 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 am ---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.
--- End quote ---
Well, I'm sorry but after reading your answer, I no longer understand my question... ???
--- Quote from: vk6zgo on March 11, 2024, 01:44:10 am ---I'm not sure if you are familiar with the frequency to wavelength conversion
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I am indeed. Fact is I'm not a beginner anymore (see above).
--- Quote from: vk6zgo on March 11, 2024, 01:44:10 am ---As the wavelength is so large, [...]
--- End quote ---
I 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, a coil, is an inductor, right? Intuitively, I understand a coil is not sensitive to *electric* fields more than *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 field component".
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.
vk6zgo:
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.
NiHaoMike:
--- Quote from: vk6zgo on March 11, 2024, 02:09:43 am ---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.
--- End quote ---
NBFM 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.)
EPAIII:
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.
Andy Chee:
--- 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.
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