I do still have my doubts if this last test really shows the standing wave resonance and not again the LC resonance.
I think that this Stalker was talking about the voltage potential flapping at the ungrounded end of the coil in
this video.
It is important to remember that the ground lead acts as a counterpoise for the antenna (the coil). It is an electric reflector that has non-mirror image currents flowing in it. The 1/4 wavelength antenna and the counterpoise together form a
1/2 wavelength dipole antenna. This is the same mechanism as with the
Vertical Monopole Antennas. This means that electrically, the counterpoise needs to be at least as long as the antenna (ideally: an infinite conducting surface).
For example, in order to have the best omnidirectional reception of a 100MHz radio station, the vertical whip mounted at the center of car's roof needs to be 1/4 of the wavelenth of 100MHz ...or 75cm. The non-mirror image currents flowing in the metal roof act as the other leg of a vertical
1/2 wavelength dipole antenna.
The E-field of the flapping coil end can be sensed (via capacitive coupling) with a neon bulb on a metal stick or with a naked scope probe (or SA probe) like shown in
this video. This of course presents danger to your test equipment if you get too close. Attaching a piece of aluminum foil to the probe's tip, extends its sensing distance.
Another way is to sense the H-field via induced current in a loop of wire positioned at the opposite end of the coil.
There is a third way to sense the standing wave, too. Namely, when the coil is ungrounded and driven at its midpoint (e.g. by a separate single loop) and treated as a 1/4 wavelength dipole, then its VSWR reaches maximum (ideally - infinity) and that means that the coil will STOP acting as a radio antenna and will not radiate any far-field radio waves (only near E & M fields, a.k.a. "reactive fields"). See
this video versus
this video.
This means, that a radio receiver positioned very far away (in the "far field") will STOP receiving the RF carrier wave, when the 1/4 wave antiresonance is achieved.
Conversely, when the coil is ungrounded and driven at its midpoint and treated as a
1/2 wavelength dipole, then its VSWR reaches minimum (ideally - unity) and this means that the coil becomes the best far-field radio wave transmitting antenna at this frequency.
The SA could conceivably be used as this radio receiver when a suitable
half-wave dipole antenna is attached to it. Just keep it very far away and galvanically isolated pwr. supply from the transmitter.
BONUS RANT:
Is is worth remembering that the quadrature magnetic near field strength falls off with the inverse-cube of the distance from the antenna (1⁄r ³), while the electric near field strength falls off with the inverse-square of that distance (1⁄r ²) and the amplitude of the classical radiated radio waves (far-field EM RF waves) falls off with the inverse of the distance (1⁄r ). This is why enginners and hams prefer to use the far field EM radio waves for long-range communications (although they are limited to the speed of light). There is a long-standing controversy around the claim, that the near fields transfer energy faster than light, though.
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Topic: Dally, Shark & Ruslan workbench (Read 309874 times)
