Re - sparse wound toroids and the drop off of measured L, there is no black magic here -
Re T50-6 sparse wound coils at 220 degrees for an L of approx 2.9uH as per diagram quoted on VK Ham.
Toroid data is from tables provided by the Toroid King aka Diz of Florida USA.
Vkham article is at https://www.vkham.com/Info/ferro/tut_5a.html
Od is 12.7, Id is 7.7
Mean D which is the nominal D for flux run purposes is 10.2mm
The amount of run len which is avail for wiring at 220 deg worth of cover is D * pi * 220/360
which is 19.6mm. NB ten turns so pitch per turn is 19.6 mm.
( TEF notes sorry , minor cock up on the Catering Front here ..the use angle in the VK example is 200 degrees not 220. This has
but a minor effect so I have not redone the calculations below) ;
The HGT of the core is 4.3mm.
The nominal pitch angle of the wiring is therefore Atan (2/ 4.3) mm or 24.9 degrees. That's the nominal pitch
down angle of the B vector from the ctr line of the mag racetrack when the core is viewed al Horizonto...
The cosine of that is 0.91 which is the fraction of flux aligned with the racetrack. However that wire spiral is
three dimensional so therefore the correct figure for the aligned flux is Cos squared or 0.82.
0.4 uH original L for a close wound coil (as per the diag provided 0.39u) and as per The Toroid king calc.
(which can be desk checked) ,
this becomes therefore 0.4 * 0.82 = 0.32 approx uH. Which is rather close to the data given for the sparse wound
coil. And of course this makes no allowance for end to end capacitance which seems to be a bit of a black art
So there is no black magic there but the coil winding technique has a big influence on the final L and therefore
the detail filter performance. BTW the sparse coil will have higher Q (Xl at frequency / R ) . There is hard data on this in the Amidon
technical section on their website and ditto in the RF engineering publications you can find on the the
Unfort. there is no real corresp between the Micrometals core materials and the Amidon materials but the MicroM data
is better presented than the Amidon. Amidon materials are used in most amateur projects because they are easier
to get hold of in small qty - eg The Toroid king or in Australia, MiniKits of Adelaide.
So what you get with one hand is taken away with another. Ain't RF physics a bitch !
If you buy a Tcvr made by say ICOM to more or less military standards you have one level of engineering and test.
If you buy a LPF board on a dirt cheap tcvr out of Shenzen well you might get something else in terms of filter
performance and spurious emission.
BTW the other non aligned flux does not disappear - Faraday says it cannot - it becomes flux that loops back
around the creating wire and of course it becomes leakage flux too - more serious if the part is against
ferromagnetic casings and such like. Therefore toroids do leak - but the wiring configuration defines how much
leakage. Even 50 Hz toroid mains transformers with near radial wiring technique do have a small external field.