Compressing or expanding turns on a toroid.
DK
Group,
Being a retired chemistry professor ( which immediately disqualified me from discussing anything concerning electronics, :(), I will inject my two cents into the discussion of compressing or expanding turns on a toroid to change the toroid Inductance.
My understanding is that all one changes by compressing and expanding the turns is that one changes the distributed capacitance and not the inductance. Looking at the formula for the inductance of a toroid I see inductance being dependent on
the number of turns squared, the cross sectional area and inversely proportional to the radius.
Thoughts?
D.K. KD6TK
On Sep 12, 2020, at 10:44 AM, Alan de G1FXB <g1fxb.groups@...> wrote:


Manuel; DL2MAN
Hi,
You can actually measure, that inductance gets higher when compressing the windings. Either by LC meter or nano vna. The Inductance for a toroid is always calculated /given with the assumption of e.g. equally spaced windings over 3/4 of core. If you change the spacing, you change the Inductance. The more dispersed the windings are the lower the inductance. The more dense the windings are the higher the inductance. 73 Manuel; DL2MAN


Alan G4ZFQ
My understanding is that all one changes by compressing and expanding the turns is that one changes the distributed capacitance and not the inductance. Looking at the formula for the inductance of a toroid I see inductance being dependent on the number of turns squared, the crossAs a retired motor technician maybe I'm not qualified to wonder if it is possible to measure inductance? Do we just measure the apparent inductance? All inductors must have distributed capacitance, does the formula take that into account? Certainly squeezing/stretching the coil must alter the capacitance? 73 Alan G4ZFQ


geoff M0ORE
Inductance is a difficult parameter to measure accurately. A coil has all three components which affect the measurement. L, C and R. The formula only caters for the number of turns and the C and R components are in the lap of the Gods. Hence we can alter the perceived inductance by compressing or spreading the turns. G3BIK wrote a very good explanation of coil construction in the
RSGB magazine RadCom Dec 1994 if anyone wants to read up on it.
On 12/09/2020 21:49, Alan G4ZFQ wrote:
My understanding is that all one changes by compressing and expanding the turns is that one changes the distributed capacitance and not the inductance. Looking at the formula for the inductance of a toroid I see inductance being dependent on the number of turns squared, the cross


@CurtisM
Yes nice explanation that physical arrangement of the coil adjusts inductance.
One can measure inductance by resonating with a capacitor, measuring frequency, and calculating inductance from that. Here in US the njqrp once offered a little kit that measured inductance and capacitance, in era before cheap lcd. Read them out in morse. I don't know what's out there now, but an LC meter would be a nifty offering, yeah take advantage of a market here too focused on their coils. 73 curt


Timothy Fidler
Perhaps my workings are a parallel of what was in the Radcom magazine article . can't comment on that. I came to them independently based on reading between the lines of the standard text Buck and Hayt .
....and guess what you little possums.. someone on this very site, an Italian no less , sent me that as an Ebook for nix cost . Ya don't look a gift horse in the mouth..nor talk about it too much if you have any brains.. :))


ajparent1/KB1GMX
There is a interaction with turns and capacitance and it shows up when measuring Q.
The assumption is the mu of a toroid is higher than it really is. So if you concentrate the winding your also concentrating flux and therefore the inductance. Te easy test is to measure at a much lower frequency where the capacitance small as it is is less significant as compared to the inductance. The problem with that is mu of most magnetic materials is also related to many factors including frequency. So the calculation used most often is an approximation. The error is not linear. Just like 468/f (142.64M/F) is rarely the exact length of a dipole. They do not take into account height above ground or diameter of the conductor or if there is dielectric to name a few factors. Allison  Please reply on list so we can share. No private email, it goes to a bit bucket due address harvesting


geoff M0ORE
The late W4ZCB described a simple L & C oscillator to which
you added the unknown component and then measure the frequency
and by using a simple bit of maths or a spreadsheet calculate the
unknown parameter. You needed a good reference capacitor to do
the initial calibration, I was lucky to be able to borrow a 0.1%
tolerance to make my own standard. Not perhaps as good as a lab
standard but I am aware of the limitations.
On 12/09/2020 23:53, wb8yyy via
groups.io wrote:
Yes nice explanation that physical arrangement of the coil adjusts inductance.


N3MNT
Had some time today so I pulled all the LPF out of my U3S and stuck them on my VNA and measured insertion loss while adjusting the windings. The results showed that there was no one size fits all and the changes were not huge. On some filters compressing L1 and L3 while expanding L2 produced best results. On some compressing all was best, while on one compressing the windings produced the worse results. For all six filters, the max insertion loss was 0.4db. at the operating freq.


Fred Piering
DK:
You are absolutely correct. Most inductance meters measure the "Apparent" inductance which includes the self capacitance of the windings. There are simple methods to measure the true inductance and self capacitance. However, from the simple circuit standpoint, I would think the only thing it cares about is what it looks like, not why. I have performed measurements that have shown the change in self capacitance with windings from full distribution to 270 and 180 degrees. 73 Fred, WD9HNU


Timothy Fidler
Re  sparse wound toroids and the drop off of measured L, there is no black magic here 
Re T506 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
subject.
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
Micrometals website.
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.


Timothy Fidler
Errattum 
"NB ten turns so pitch per turn is 19.6 mm." read NB ten turns so pitch per turn is 1.96mm I shall flagellate myself with a 240 Vac wire for a few minutes for that clanger (I think it is aback up version I had in Wordpad that caused the strife). TEF

