Richard Clark wrote in
:

Shielding does nothing but describe a balance. You can as easily
remove the shield and obtain identical performance IF you guarantee
balance. This was done for decades before coaxial cable was common.

Hi Richard,

For Dave's benefit, I might explain that the risk attendent in using a
small loop on a long transmission line is that the outside of the
transmission line becomes a significant radiator. In the limit, the loop
becomes just a means of exciting the outside of the transmission line as
the main element of the antenna system.

That is often undesirable because it spoils the pattern and / or results
in pickup of undesirable signals, especially from sources close to the
transmission line that has become the antenna.

There are other methods of trying to isolate the transmission line (as
Richard noted), the shielded loop construction is not the only way. For
example, a BALUN is a device that is designed to permit transition from
an balance device (the loop) to an unbalanced device (a coaxial
transmission line).

The shielded loop is widely used for instrumentation purposes, where the
Antenna Factor (related to gain) is calibrated and needs to be
independent of feedline length and routing (within reason).

Owen