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Why do most radio receivers detect the E field and not the B field - or do they? — Parallax Forums

Why do most radio receivers detect the E field and not the B field - or do they?

ElectricAyeElectricAye Posts: 4,561
edited 2012-12-03 19:24 in General Discussion
Do most radio receivers pick up the E field (instead of the B field)? And if so, why is that?

Comments

  • LeonLeon Posts: 7,620
    edited 2012-12-03 15:37
    It's because that is the way that most antennas are designed. Magnetic loop antennas are used by some radio amateurs, because of their small size.
  • Mark_TMark_T Posts: 1,981
    edited 2012-12-03 15:51
    Unfortunately that's a rather meaningless statement - the fields cannot exist without each other. Different kinds of antenna have different ratios of near-field E and B, but the far field is always
    going to have the same ratio (E/B = c).
  • ElectricAyeElectricAye Posts: 4,561
    edited 2012-12-03 16:31
    Leon wrote: »
    ...Magnetic loop antennas are used by some radio amateurs, because of their small size.

    Thanks. But now I have to ask: if the magnetic loop antennae are smaller, why aren't most antennae made that way?
    For example, why don't HAM radio operators use smaller loop antennae rather than the traditional large wire thingies?
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2012-12-03 16:43
    ElectricAye wrote:
    if the magnetic loop antennae are smaller, why aren't most antennae made that way?
    It's partly because they have extremely high Q, which means they have a very narrow bandwidth for a reasonable SWR. Consequently, they almost always have to have integral capacitive tuners. But, due to the high-Q, the voltage across the cap plates can easily exceed a kilovolt, even with a low-power transmitter. Also, for the best efficiency, the loop element needs to have a large diameter. Typically, 1" or larger copper pipe is used; wire just won't cut it.

    Short answer: Despite some significant performance advantages and good noise immunity, they're much more complicated to make and use than a dipole or vertical antenna.

    -Phil
  • Duane C. JohnsonDuane C. Johnson Posts: 955
    edited 2012-12-03 16:47
    Most B-field antennas are for the lower frequency bands where the E-field antennas would be unwieldy large.
    Conversely at higher frequencies the B-field antennas are physically to small to be practical.
    OK, this is a bit simplistic but generally correct.

    Duane J
  • Tracy AllenTracy Allen Posts: 6,667
    edited 2012-12-03 18:25
    Common in old AM radio receivers, several large loops of wire glued to the masonite back-plate, or for miniaturization (transistor radio!) a loopstick antenna consists of ferrite rod with 50 or so turns of wire.

    Loop antennas are found in radio direction finders on the lower bands or for things like WWVB receivers (60kHz).

    1) loopstick antenna on wwvb receiver. Ferrite permeability concentrates the B field.
    WWVB_rcvr.jpg


    2) DF shielded loop antenna from probing EMI fields. These are useful up to GHz.
    DF-loop-probe.jpg
    WWVB_rcvr.jpg
    576 x 333 - 55K
    DF-loop-probe.jpg
    432 x 271 - 52K
  • ElectricAyeElectricAye Posts: 4,561
    edited 2012-12-03 19:24
    Phil Pilgrim (PhiPi) wrote: »
    It's partly because they have extremely high Q, which means they have a very narrow bandwidth for a reasonable SWR. Consequently, they almost always have to have integral capacitive tuners....

    Thanks, Phil. I'm surprised I actually do understand what you're saying. :)

    Duane C. Johnson wrote: »
    ...OK, this is a bit simplistic but generally correct....

    Hey, simplistic works for me! Thanks, Duane.

    Tracy Allen wrote: »
    ...Ferrite permeability concentrates the B field...

    Interesting. I always wondered what those charcoal stick-looking things were doing. Thanks, Tracy.

    I've been reading the manual for the Ham radio Technician's license, and might even try taking the exam, but I just couldn't find any quick and dirty explanation for the loop vs. other types of antennae. Thanks, gentlemen, much appreciated.
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