Michael Coslo wrote:
Jim Lux wrote:

The big problem is this: a small loop stores a lot of energy in the
fields around the loop (if the loop has a Q of, say, 100), and you're
radiating 100 Watts, that implies that there is 10kW circulating in
the loop between the loop itself and the tuning capacitor. The
energy moves between the magnetic field of the loop and the E field of
the capacitor every 1/4 cycle.



Jim, are you really saying that there is 10KW in the loop? Who needs
zero point energy if that is so? Or did you mean 10KV?

technically 10kVA.. it's reactive power circulating between the L of the
loop and the C that tunes it.

The very definition of Q is the ratio of stored energy to that lost per
cycle. In the case of the antenna, assuming it's lossless, the lost
energy is that radiated away, and presumably replaced by the transmitter
(assuming a steady state sort of system). If you have X Joules
radiating away each cycle, there has to be Q*X Joules stored in the
system, and X Joules added to the system.

In a lossy antenna (which these loops will inevitably be, barring
superconductors, etc.), some of the energy is lost to heat, but, again,
if you measure the Q, that's rolled in. (The Q of a lossless resonant
loop 1 meter in diameter at 7MHz would be spectacularly high, since the
radiation resistance is tiny compared to the reactance of the loop)

This is why small loops need HV capacitors and low resistance loops.

For what it's worth, the same sort of problems with near fields crop up
in superdirective arrays, because there's a lot of reactive power stored
in the near field that circulates among the elements. Fortunately from
the RF exposure standpoint, most amateur superdirective arrays (i.e.
Yagis) are mounted several array sizes away from people, and in these
arrays, the high energy density is almost entirely within the volume of
the array. Take a look at the cover of one of the Antenna Compendiums
(#3?) for a picture of this.

jim, W6RMK