A few additional comments in the discussion of RX antennas:
The broadband feedpoint Z, termination and pattern of K6SE's optimized pennant
and flag designs is quite elegant. I'll repeat the usual caution regarding all
ground-independent RX antennas -- be sure there is good isolation between the
feedline and the antenna using a transformer designed for the task -- separate
primary and secondary windings using a core material and wire size that
minimizes the capacitance between the windings while proving a good impedance
Regarding the K9AY loop and "happy medium" design -- most of the terminated loop
designs are "lowpass" in their characteristics. That is, they are quite
consistent in feedpoint Z, termination and pattern below a cutoff frequency. For
the K9AY loop, the published dimensions in Sep. '97 QST have that cutoff around
160M, which means that it behaves similarly in the AM broadcast band, or lower.
A different termination is needed for optimum 80M performance, which is just
above the "cutoff" frequency. The tradeoff is in the received signal levels --
Smaller antennas, though more broadband, need more preamp gain. Larger antennas
capture more signal. The voltage across the feedpoint is proportional to the
area enclosed by the loop.
Finally, I want to point out something that not everyone appreciates:
Most of the published designs are based on optimum dimensions, but almost ANY
loop size and shape, grounded or ground-independent, can be made to work. First,
the feedpoint and termination must have significant separation (e.g. between the
two bottom corners of a triangle or square; or separated by the ground
connection, as in the K9AY Loop). Then, it needs the right termination to
establish the current distribution and phase along the antenna conductors to
create the directional pattern. The termination may be resistive, or it may be
complex, requiring R+L or R+C to obtain the desired pattern. The optimum
termination is likely to be narrowband in this "random" situation.
Many SWL users of these antennas have variable resistive terminations, typically
using the "Vactrol" devices. A few have designed more complex R +/-jX
terminations that let them put a deep null at any vertical angle off the back of
their antennas, at the frequency they want.