It should be borne in mind the best form of radiator is just an
isolated straight length of wire. But curved or bent wires do almost
as good.

Loop antennas are nearly always single turn conductors and provide a
means of getting as much wire into space as possible with the largest
possible linear dimension being dependent on the size of your back
yard. Or your attic. Loops are space savers.

Subject to the space available, for a given perimeter, the larger the
area enclosed by a loop the better are its radiating properties and
its power efficiency. However, when an antenna is already 80 or 90
percent efficient, there is seldom any point in attempting to improve
it. 100 percent is impossible. And to improve it to 95 percent
results only in an entirely imperceptible 0.2dB or 1/20th of an
S-unit.

Again considering space requirements, the physically smallest loops
are descibed as Magloops. Magloops, not difficult to design, are the
most efficient of all small antennas of any type.

When the perimeter of a Magloop is only 1/3rd of a wavelength at the
operating frequency, efficiency can approach that of a full size
1/2-wave dipole. The disadvantage of a small magloop lies in its
restricted range of operating frequencies. Three adjacent HF bands at
most. The cost of a vaccuum variable capacitor cannot be neglected.

But even at frequencies as low as 1.9 and 3.6 MHz, magloops with
perimeters as small as 1/15 wavelengths, diameters as small as 1/50
wavelengths, are quite usable in small backyards and attics. Expect
signal strengths 2 or 2.5 S-units worse than a full size half-wave
dipole. There are reports of improved signal to noise ratio on receive
although I have not experienced this myself.

For design of magloops at any HF frequency and any reasonable size of
loop, download program MAGLOOP4 from website below.

For performance of much larger horizontal loops, for use in your
backyard, download program RJELOOP4
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Reg, G4FGQ.