A fine gentleman did an experiment building two inductors. The coils were wound with 420/46 litz. Both inductors have 40 turns. One inductor is close wound (215uh 7.7pf distributed capacitance). The second has a 0.625/wire diameter space between wires, (187uh and 6.9 pf distributed capacitance). The inductor with spacing has a higher Q, except at the low end
where the Q's are equal.
The reason for spacing the wire is to reduce Proximity effect.
Here is the graph, each coil was tested twice (two lines each)
formatting link
What would be the reason that the Q does not start out higher, but moves up with frequency?
Mikek
Re: the difference in inductance, both 40 turns. The inductance, 215uh vs 187uh = 86.9% The capacitance 7.7 vs 6.9 = 89.6% The capacitance of 40 turns did go down 10.3% with spacing. The inductance went down 13% with spacing. FWIW
But note: the coils have near identical Q's and then the spaced coils Q rises quickly, why didn't it just start out higher as it should have less loss because of less proximity effect. That only shows up at higher frequencies, why?
Assuming they were wound with equal lengths of wire (give or take a modest margin), and wound for the same inductances, then at low frequency, DCR is equal and inductance is equal.
At high frequencies, ACR rises for the close-spaced one, due to proximity effect, while the spaced one rises less.
Here's paper, The self-resonance and self-capacitance of solenoid coils:
It's mostly over my head, has some neat pictures showing nodes with a mercury tube. Pages 39, 41, and 98 discuss self cap. page 77 is on SRF and page 101, is a summary.
That's a good thought, I'm leaning against it though, I suspect my fine gentleman measured his inductors with either a Boonton
260A or an HP 4342A, both of these use very high Q* capacitors, so even if the cap had more loss at low frequencies, I don't think it would make that much difference in the Q curve.
ISTR reading the Boonton 260A tuning capacitor has a Q near 20,000. I recently saw some silver air variable caps with a Q of 15,000. Run of the mill air variables run from 1,000 on up.
Any C (an actual C or parasitic C) across an L increases the Z and therefore increases the "effective" value of L. Since the R is unchanged, the Q of the effective increased L is increased also. This works up to the SRF, but of course not above SRF.
The Q factor of a unloaded LC circuit is determined by the following factors: series resistance in the circuit, parallel resistance across the circuit and magnetic losses.
The series resistance in the LC circuit, the lower the series resistance the higher the Q.
The total series resistance in the circuit (Rs) is the sum of:
The wire resistance of the coil, thicker wire or litzwire with much strands helps to reduce wire resistance.
The resistance of the capacitor plates, silvered plates gives the lowest resistance. Plates with oxide gives more resistance than clean plates.
Contact resistance between rotor and frame of the variable capacitor, preferable the variable capacitor has a spring connected to rotor and frame, this provides a low resistance. When the contact is made with a slidercontact at the rotor, this must be clean and free of oxide.
The parallel resistance across the LC circuit, the higher the parallel resistance, the higher the Q.
Parallel resistance across the circuit (Rp) is caused by dielectric losses.
There are: Dielectric losses in the coilformer Dielectric losses in the insulation of the coilwire Dielectric losses in the insulators of the tuning capacitor Dielectric losses in materials placed near the coil or tuning capacitor, look here for more information about this subject. If the capacitor plates are not clean: dielectric losses in the dirt and oxide on the capacitor plates.
Also there can be a leaking resistance in insulators, e.g. by moisture.
All these losses together makes a parallel resistance (Rp) across the LC circuit.
Magnetic losses This occurs when a magnetic material (iron) is placed near the coil. Non magnetic materials (plastic, wood, aluminium etc.) don't give magnetic losses.
Why is the Q factor not constant for all frequencies?
The series resistance Rs gives a reduction of Q. If we leave other losses out of consideration, the Q will have a value of:
Q= 2.pi.f.L / Rs
If the value of Rs is constant, the value of Q will increase with increasing frequency (f). A series resistance in the circuit will especially give a reduction of Q at lower frequencies.
On the other hand: if we only look at the losses caused by the parallel resistance Rp, the Q will have a value of:
Q=Rp / (2.pi.f.L)
Here we see, that at a constant value of Rp, the Q will decrease with increasing frequency (f). A parallel resistance across the circuit will especially give a reduction of Q at higher frequencies.
The value of Q will both depend on series and parallel resistance, it is possible that a LC circuit gives a increasing Q at increasing frequency (because of series resistance) and than Q will reduce (because of the parallel resistance). In this case we have a peak in Q somewhere in the frequency band.
Often the losses caused by the parallel resistance are the highest, and we only see a reduction of Q at higher frequencies in the medium wave band. Than there is also a peak in Q, but this occurs at a frequency lower than we use.
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.