Antenna :
Screened balanced loop, diameter about 1meter.
==> The screening of the loop is ONLY connected to the OUTSIDE of the metallic antenna box. <==
Nominal loop capacitance :  2x125pF (max. 2x 150pF) as good as possible capacitive balanced !!
Maximal loop circumference for the following coax types :
-  2,7mm RG316             : 2,8 m.
-  11mm RG213              : 3,15 m.
-   50mm LCF158-50JA   : 4 m.
A to low loop capacitance must be compensated by placing C5 and C10 onto the amplifier board, until a total input capacitance of 2x125 pF is accomplished. Both loop halves should have exactly equal capacitances !
But a to high total loop capacitance (circumference) will lead to sensitivity reduction above 21 MHz.

        The relation between loop inductance, amplifier noise floor and
       loop conductor thickness.
According to LZ1AQ, is a loop diameter of abt. 1m optimal. 
The loop amplifier has a balanced emitter input. Its impedance (2.8 ohms balanced) is constant over a wide frequency range (up to 25 MHz).

When the loop is loaded with such a very low impedance, it behaves like a current source.
The loop output current is constant over a wide frequency range (up to its 1/4 wave circumference frequency, 25 MHz), if in a constant electromagnetic field strength.
The loop source impedance (R + jXl) however, rises with frequency, because Xl rises with frequency.
Loop resistance R remains constant, and its value is low compared with Xl. 
The influence of XL becomes noticeable above 15 MHz.
As the loop current decreases with rising loop impedance, the high frequency noise floor of such an antenna system is mainly determined by the loop inductance. 
A lower loop loop inductance will therefore lower the amplifier noise floor above 15 MHz. 
A lower loop inductance can be achieved by using a thicker loop conductor. 
REM : A screened low inductance loop could be made using very thick 50mm Cellflex LCF158-50JA coax.

With this amplifier, which has a nearly constant amplification factor of 42dB up to 25 MHz, the loop antenna sensitivity is nearly constant -30dB up to 25 MHz.

As the inductance of a loop conductor only varies with the thickness of the conductor, only the thickness of the central coax conductor is at higher frequencies of importance to the amplifier noise floor. The kind of loop conductor metal (copper, steel, aluminum) is not of influence to the inductance of a loop, nor to the amplifier noise floor.

Below 15MHz, the by the loop received noise level is always higher than the antenna amplifier noise floor. 
REM : In practice, the "man made noise" levels above 15MHz, still remains higher than the system noise in most locations. 

Only in very quiet (non populated) locations, at frequencies higher than 15MHz, could the amplifier noise be stronger than the galactic noise received. Only in such locations, a low inductance loop could enhance the antenna sensitivity.