balun is used to maintain, as far as possible, the balance of the antenna and hence reduce
local common mode noise being coupled to the receiver. It works! As with most
housing developments in the UK, the garden is quite small. So noise levels from all
sorts of electrical appliances can easily reach S7 during the evenings on a marconi-fed
antenna. Local noise levels on the loop are, typically, about 10 dB lower relative
to my marconi-fed antenna.
Note that, from a receiving point of view, this ATU provides
very little immunity to out-of-band signals. In particular, HF signals may be
coupled to the receiver front end with very little attenuation. Hence use of this
ATU still requires a good pre-selector or band-pass filter in the 136 kHz receiver.