NEC-LIST: What if the CFA works differenyly?

Hi,

It is claimed that the D plate of the CFA as a capacitor above ground
is by virtue of the displacement current dD/dt creating a surrounding
H field, and due to this field being prependicular to the E field
created by the E plate (cylinder) a Poynting vector is created.

If we calculate the H field thus created by taking the line integral
in a circle midway between the D plate and ground there are two
currents that have to be accounted for, the displacement current dD/dt
and the charge current in the wire feeding the D plate. They are equal
and opposite, so no H field is created.

What if the D plate instead acts as an active elevated "ground" in
parallel with the surrounding passive ground and the E plate is just
the top load of a vertical antenna. A circuit equivalent is a
generator G1 feeding a short antenna with the equivalent circuit of a
very small radiation plus loss resistances R in series with the much
larger reactance of the E plate capacitance C1+C2. In this case, the
top capacitance to ground is split in two. C1 to passive ground and C2
to active ground, that is to a generator G2 that goes to ground.

For the generator G1 to see a purely resistive load R ("matching to
free space"), the voltage and phase of G2 may be adjusted so as to
eliminate current in C1. For that we get that the voltage ratio of the
generators G2 to G1 needs to be U2/U1=j*Xc2/R, that is U2 feeding the
D plate needs to be much larger and 90° out of phase. This is not
to say that this is the optimum adjustment, as overall bandwidth and
efficiency depends on the ratio C2/C1 and the particulars of the
losses and tuning in the matching networks. The two sources G1 and G2
would see widely different loads, losses and bandwidths, making a
tricky feed from a single source.

Greetings,
Kristjan Kristjan Benediktsson,
TF3KB
Received on Mon Dec 13 1999 - 05:05:01 EST