The factors in loop efficiency is:
Radiation resistance, usually calculated as a Wheeler measurement would require an arena sized room.
Typical values are in the range of hundred of milliohms to very low ohms. For less than 1/10 wave
less than .4 ohms. The only way to increase the Rr (radiation resistance) is increase the loop diameter.
at above .1 wavelength the loop will not exhibit constant or near constant current and most of the
small loop equations look better than it will be.
AC resistance of conductors. Typical values are in 10s to 100s of milliohms.
More surface, skin effect is the know adversary here. With greater surface there
are edge crowding effects and phase of inside (id of the loop) vs OD of the loop.
this shows up when loops use very fat conductors causing enough of a differential
between ID and OD and adds a new loss factor, cancellation of currents.
ESR of the capacitor. 10s of milliohmms. This adds to the total AC resistance
of the loop.
Coupling and matching system and its losses. varies with coupling and type.
Loop coupling to loops generally has a K of less than 1, this adds loss.
Direct connections suffer from the very large transformation, usually
capacitor based is better than inductor or transformer is preferred for
lower losses, high Q caps are costly (vacuum if high voltages).
Generally loops, specifically small loops are high loss compared to radiation resistance.
That does not include environmental losses however small they may be. Also height
above ground usually 1 to 2 loop diameters is a minimum.
Multiturn loops are not more efficient, usually less so due to cancellation of fields.
In some case the offset is required for less tuning capacitance.
The greatest trade is bandwidth, the higher the effective Q the lower the bandwidth.
At 160M the bandwidth is often so narrow for high Q [low loss] loops sideband clipping
and high IMD from the transmitter as the swr across 2khz changes enough to load pull
the amplifier. The positive side is offending signals on the RX out of the tuned bandwidth
of the loop are attenuated. Loop Q tends to decrease with increasing frequency due to
Skin resistance and combined factors with a small offset due to increased size relative