This part was inspired by Loewe Opta Dolly T37K,
where I have seen a "strange" winding in the middle of the ferrite antenna.
It seems to serve as MW, LW and SW electric length extension coil for the rod antenna especially, if this radio is used in a car.
This Loewe Opta Dolly T37K radio covers the FM/MW/SW (6-10MHz) frequency ranges.
Dolly T37 covers FM/MW/LW with an additional LW winding on FA.
The same is Quelle Universum Modern M66.
This sideways added winding in the middle of FA was new for me and I was curious about its properties.
Here the feritte antenna signal input part of T37K-FM/MW/SW and the same of T37 or M66-FM/MW/LW:
Left picture with Short Waves: L9 is the side-winding, L8 is MW and L10/L11 for SW are outside of FA.
Right picture with Long Waves: L9 is the side-winding, L8 is MW and L11 on FA is the LW winding.
Directly measured inductivity of L9 is about 4 mH. Resonance frequency of L9 is about 300kHz.
So the own capacitance of the winding should be about 70pF.
With parallel capacitance of 390pF the resonane frequency is about 120kHz.
Inductivity of L9 without ferrite rod is about 2 mH.
Therefore I executed some measurements with conventionally wounded
ferrite antenna rod and also with sideways winding.
I have used ferrite rod 100mm long with Ø 8mm and Ø 0.3mm wire.
Pic.1:
Winding: 20 turns.
Measured L(inductivity): 35µH.
There is a considerable difference between winding on pic.1 and pic.4 in the wire length:
Winding from pic.4 is 4 times longer as winding from pic.1.
(1 turn conventional: π*8mm, 1 turn sideways: π*32mm).
But we have to consider, that the inductivities (35 vs 42 µH) are not same,
therefore the quotient is less then 4 times.
Only inductivity was measured, other properties (Q, beam-directivity effect)
where not measured.
Both of them are surely different.
