You can find most of the information by doing a search for
"magnetic loop antenna", though "magnetic loop" is not
really a good description of how the antenna works.
There are several links to the subject on the late AC6V's
antenna links page here:http://ac6v.com/antprojects.htm
Presuming that you want to transmit on it, the most
common implementation is to use a fat, high-conductivity
single-turn loop that is tuned to resonance with a variable
capacitor of some sort opposite the feedpoint (though there
are some variants with the capacitor at other locations.)
Radiation resistance is very low, so currents and voltages
are very high even at modest power levels. One of the
early military loops was made from 8 sections of aluminium
tubing, and they found they had to gold plate the joints to
keep the losses low for good efficiency. Many loops are
built from copper water pipe or tubing, sometimes 5cm or
more in diameter to reduce losses.
You can build a single loop and use a variable capacitor to
tune it to multiple bands, but the required minimum /maximum
capacitance ratio required to do so (as well as minimum vs.
maximum usable lengths in wavelengths) usually make it
impractical to tune a frequency much greater than 2 or 2.5 : 1.
(You might squeeze 30 to 10m into a single loop, but not down
to 40m without a serious loss of efficiency.)
SWR bandwidth is very narrow - perhaps 5 to 10 kHz on 80m
without retuning. That is why many designs use motorized
capacitors for remote tuning, which adds an extra level of
complexity to construction.
You can see some photos of Russian military mobile loop antennas
at the Antentop Radio Magazine site here:http://www.antentop.org/004/starec.htm