I've used a number of horizontal loops over the years, and they
can work well, even when the shape isn't a nice tidy regular
polygon. On some bands and in some directions they are
stronger than a dipole, in other directions they might not be.
Here is a
collection of radiation patterns for different shapes,
bands, and feedpoint locations.
Just like a dipole, the vertical angle of radiation is strongly
affected by the height above ground.
More loop articles.
The
Field Day Loop article shows an antenna that I used
for several years, that worked quite well with QRP. Note
that it also had the advantage of ground slope near the
top of a hill. I specifically chose that shape to align the
major lobes in useful directions.
The
Bow-Tie Loop shows one option for installing a loop
on a single support: I used a ~28' center mast and tied
off the 4 corners to posts about 12' high. It would have
been better to get the ends higher, but that was what I
could easily manage at the time.
So if you are short on supports, you might consider one
of the delta varieties, or elevating two opposite corners
of a square loop.
Now, let's consider some trade-offs.
Height makes a big difference for working DX with
horizontally polarized antennas. If you have to invest
in supports, consider whether it makes more sense to
use just one taller mast (either in the center of a loop,
or to support an inverted vee) rather than several
shorter ones (for the corners of a loop).
For example, I have 100' of the military 4' mast sections.
I can put each corner of a square loop up 24', or I can
put up a single mast about 40' high (limited by what I can
manage by myself, rather than the amount of mast).
My guess is that, except possibly in some specific directions,
the 40' inverted vee is going to work better than the lower
loop for DX.
That's not to say that you won't work DX with the lower loop:
you can, especially as conditions pick up compared to the last
couple of solar cycles. But the higher antenna would likely be
a little bit better in many cases.
When you are limited to particularly low heights, then vertical
polarization may work better than horizontal. You can hang
a vertically polarized delta loop off a single support, although
they tend to work best over a more limited range of bands.
Other options include the heavy duty
fiberglass masts from
MGS, which go up to 50', or the telescoping
SpiderPoles,
which are available up to 85' (although the top section isn't
useful for much besides supporting a very light vertical wire).
You likely will need guy ropes with the taller masts.
(MFJ sell masts similar to the MGS ones, for about the same
price, and you may be able to save on shipping costs if you
can order it through a local Ham Radio distributor. Check with
the store and see.)
Now, another thing to consider is the effect of a sloping wire.
With an 1/2 wave inverted vee (or dipole), maximum radiation
is from the high point of the antenna. That makes good use of
your available height. As the wire is made longer, particularly
for multi-band operation of the same antenna, you get radiation
from lower points along the wire, which lowers the
average height
of the antenna. That's not as good. And maximum radiation
tends to be more towards the end of the wire rather than broadside
to it. That can be useful, but when the wire is sloping at too
steep of an angle, you lose gain because the major lobes are
aimed too low. That can be the case with an OCFD or EFHW
installed as an inverted vee, especially on the higher bands
like 15m and 10m.
My practical suggestion: first, get on the air.
Remember, no antenna is permanent. (Although some of my
"temporary" antennas have stayed up longer than my
"permanent" ones.) You can always change your antenna
if it doesn't seem to be working for you, or to try something
new. Make sure your support has a halyard (like a flag pole)
so you can raise and lower the antenna without having to
take down the support.
Start with something simple. A horizontal loop can be
great if you have 100' Douglas Fir trees to hang it from.
If you have just one support, cut dipoles for 20m and
40m and hang them as inverted vees from a common
feedpoint. Add other bands as desired. The wires can
hang below each other, or be tied off in different
directions (possibly acting as guys for the mast).
Don't get too bogged down at this point in making the
"perfect" antenna. Instead, put up a "usable" antenna
and get a feel of the bands. Day-to-day propagation
differences will be much greater than the differences
among many of the antennas that have been discussed
on this thread: all of them can work on a good day and
hear nothing at all when the band is dead.
From that you can see where you want to make
improvements. Maybe you use one antenna for 20m
through 10m, a small beam for 6m, and a vertical
for 40m DX. Or you might have a couple different
antennas that you can string up for specific
occasions: with a halyard on the mast and a center
insulator on your coax that makes it easy to change
the wires around, you can experiment with different
antennas and see which ones work best for your
specific interests. (I often hang two halyards on any
mast that I install, so I can test one antenna while
using the other.)