Hi Freddie,
Yes, its quite an interesting effect, its when the wire runs in the direction of the TX
But its also a nuisance, if your doing AM measurements of field strength
The tests that STx did near Orford should be near, as although survey road has phone lines
they run along part of the road, but Not in the TX direction, so effect is minimal

Other interesting effects with AM....In the town I live, there are Two railway bridges
these are parallel, right next door to one-another, with a gap of only a few feet between them

With AM if you drive under the first bridge, signal goes off, as expected, but the weird bit is
It comes back on, in the small gap between the bridges, then off again under the second bridge

Other Bits I have looked into, is Wave Drag effect, If measured at the top end of Medium wave
this is quite a pronounced effect, and is useful for getting better Nulls on Frame/loop/ferrites

Rod Watts