Get a Tek 'scope with LCD display. "Universal Test Signal" right there.
--
"For a successful technology, reality must take precedence
over public relations, for nature cannot be fooled."
(Richard Feynman)
Except that there's quite a bit of current in the middle of the dipole, and if that part is aligned across the H field, you get magnetic induction.
E&M being what it is, this occurs at the same conditions the E field line up in. Samey samey. You can't say it's picking up one or the other because it's always doing both or neither.
Tim
-- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
yup, I was going to comment about that earlier,. It seems that you can't have one with out the other. It's like Evil and Good!
Jamie
Except E and M is always Good and Good :)
Tim
-- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
Except when you're doing compliance testing. ;-)
The real reason for using small "magnetic" loops on low frequencies is due to how they behave with some nearby interference sources.
In the far field from the interface source, both the E and H fields drop relative to 1/r and power density relative to 1/r² and the free space impedance is 377 ohms.
However, in the near field (less than 1 wavelength or less than 1/6 wavelengths depending of document) from the interference source, the situation is much more complex and the impedance varies greatly within this region. Going closer to the source, the magnetic field is proportional to 1/r² and very close to the source the electric field is proportional to /r³.
On VHF/UHF, the near field extends to centimeters or decimeters and are of interest e.g. for parasitic element design in an Yagi antenna.
However, on MF/LF/VLF etc. the near field from the interference source extends to tens or hundreds of meters, thus within the house and nearby streets.
When using an E-field antenna, the distant preferred signal is received with approximately constant strength. However, when moving the receiving antenna closer to the local interference source, the interface voltage increases relatively to 1/r³ and is summed to the wanted signal, quickly reducing the SNR to useless.
However, when using M-field antenna, the interference is increasing relative to 1/r², thus, the SNR is better _deep_ in the near filed than with an equivalent E-field antenna. For LF we are talking about interference sources within the house.
----
Compare this with a small LF/MF E-field antenna, e.g. a 1 m vertical whip connected to a high impedance (1 Mohm) built in preamplifier.
The extremely short whip is very reactive, thus the high impedance amplifier is needed to convert it to something like 50/75 ohms, in order to be used with coaxial cables and ordinary receiver inputs.
Within a room with electric wiring in the walls, there can be quite high capacitances from the wiring to objects within the room, at least
1-10 pF, possibly even more. Of course, coupling an oscilloscope probe with 1 Mohm input to some floating metallic object on the table, will produce a large 50/60 Hz hum display on the screen.These days with lot of high frequency noise in the mains wiring, the stray capacitance to the E-antenna whip would be in the same order of magnitude in the LF band as the amplifier input impedance, thus forming a 50:50 voltage divider and hence coupling a lot of electric noise to the E-antenna, thus making it more or less useless for indoor reception.
The shielded magnetic loop is not affected by this capacitively coupled noise.
Of course any _significant_ high frequency _current_ would connect to the magnetic loop through induction, but the high frequency noise current in domestic wiring is typically not very large. The situation is of course different near large VFDs driving big motors.
A metal structure will pick some of the local interference source E-field, unless it is perfectly balanced ("antenna effect"). The shield helps keeping out the E-field interference, when only the M-field is wanted.
All (V)LF and MF transmitter stations are vertically polarized. Since it would be impractical to erect 1/4 wave towers on LF, top hat capacitance loading is often used. In practice, several smaller towers are used, suspending an elevated network mesh of wires between them and the mesh is then connected with a vertical wire to the transmitter. The actual vertically polarization radiation occurs from this vertical line.
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.