I've used a number of different types of loops - each
have their uses.
My favorite is the horizontal loop, but that is because
I usually work 40m and often 80m and they work so well
for NVIS. They haven't been wonderful for DX on the
higher bands because most have been around 20' high
or so: getting them up to 40' would make a big difference.
Vertical loops don't work as well over a wide range of
frequencies, but a full wave loop can be pressed into
service on an adjacent band. Use a diamond at the top
of a mast, or a delta (point down) shape if it is
hanging from a rope between two trees.
Bi-square: another vertical loop, but this one is a
half wavelength on a side, and is open at the top (not
an actual loop.) Generally built in a diamond shape
and fed at the bottom with open wire line, it gives
some gain over a standard full wave loop.
Actually you can modify a full wave loop for 20m to be
a bi-square on 10m by opening the top and adding an
open wire stub of the proper length. It should be a
quarter wave on 20m, with the far end open (so it acts
like a short circuit.) On 10m it will be a half wave
and will act like an open circuit.
Although a full wave loop has a bit of gain over a
dipole, I think you'll find the antenna height makes
more difference in performance than the actual antenna
shape.
For a larger beam, you can hang several wire loops
from a catenary rope and make a multi-element quad. I've
often done this for 40m on Field day, but it works on
the higher bands as well.
One of the best antennas I ever used was an NVIS design
for 40/80/160m that could be hung up vertically for
the higher bands. Take two wavelengths of wire and
arrange them into a figure-8 or bow-tie arrangement:
this should look like two delta loops with the points
touching, and the wires cross-connected at the points.
Open one of the wires where they cross and feed with
twinlead, or coax and a 4 : 1 balun. On the design
frequency this gives two phased loops. On half the
frequency it looks like a folded dipole. Above the
design frequency the pattern breaks up somewhat, but
it's still usable.
Then if you have good supports and plenty of space you
can consider a Sterba or Bruce curtain - both are based
on loops. A Sterba is basically a half wave on a side,
while a Bruce is a quarter wave on a side. For
horizontal polarization, a Sterba is long, while a
Bruce is tall. I built a Bruce antenna for Field Day
one year using salvaged telephone wire and hung it off
of a fire tower in Pennsylvania, but it was absolutely
useless: all the Stateside signals were down in the mud,
and the strongest signal on the band was from Kuwait!
I've also had good luck with a bobtail curtain on 15m
with the bottoms of the tails up about 8 to 10' off the
ground. If you have a tall tree or tower and a lot of
open space, consider a sloping Vee beam - you can
rotate it by moving the tie-off points of the wires
Beyond that your choice of antenna will depend on what
you have available for supports.
Good luck! - Dale WB6BYU