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Author Topic: Inductance valuation question  (Read 2236 times)
VE7EPX
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« on: January 22, 2012, 03:25:24 PM »

There is probably a simple answer to this question, but at the moment, its not apparent to me. How are values assigned to commercial inductors? By that I mean, what criteria is used to value or rate an inductor by commercial producers. I have been winding my own coils for various antennas and use the formula:

L = 25400 / (freq * freq) * C

From this formula it is clear that a coil will have a different inductance values depending on the frequency it is used with. So the question that comes to mind is how do manufacturers arrive at a single value when they produce and mark a coil, knowing of course that the coils will be used with various frequencies? Thanks.
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M0HCN
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« Reply #1 on: January 22, 2012, 04:12:41 PM »

Not quite, the inductance is (to a first order) independent of frequency.

What you have there appears to be some kind of 'calculate the required inductance for a given C and resonant frequency' equation, not the same thing at all. Use it to figure out what inductor you need, or what inductor you have from the resonant frequency and C,  but that is not quite the same thing as saying that that eqn. means that the inductor varies with frequency.

Inductors are usually thought of as L = mu * N * N, where mu is the permittivity of the core ((micro)Henries per turn squared) and N is the number of turns. Mu is dependent on core material, core geometry, diameter and spacing. 

Now inductors are about the least ideal of all components so there are some second order effects, particular with very high values where inter winding capacitance can cause additional resonances (Think  supply chokes in tube linears), but to a first order inductance is independent of frequency.

Regards, Dan.
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WB6BYU
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« Reply #2 on: January 22, 2012, 04:40:31 PM »

Your formula calculates the inductance required to resonate with a capacitor at a
specific frequency.  If you think about tuning a circuit, of course either the capacitor
or inductor (or both) have to change when you want a different frequency.  But
that doesn't mean that the inductance of a coil changes.

Commercial coils (and most home-wound ones as well) are rated for the nominal
inductance (or sometimes the range of inductances for an adjustable coil.)  As the
previous poster pointed out, inductance is generally independent of frequency
(until you get high enough that the self-capacitance has to be considered.)  That
also assumes that the core permeability is constant, and it isn't always.

But for common air-wound coils that are well below the self-resonant frequency,
you can say that a coil with 20 turns on a 1" former and 2" long will have an
inductance of 0.41uH anywhere in the HF range - it doesn't vary significantly
with frequency.
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KA4POL
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« Reply #3 on: January 22, 2012, 10:09:30 PM »

Your conclusion that there might be a frequency dependance of inductance is not completely wrong. Commercial coils are measured at certain frequencies which you can see from data sheets like http://www.fastrongroup.com/image-show/25/VHBCC.pdf?type=Complete-DataSheet&productType=series
It is therefore always a good idea to check these data before using a coil simply based on the inductance value.
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G3RZP
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« Reply #4 on: January 23, 2012, 03:12:05 AM »

Strictly 'inductance' is frequency independent. 'Apparent inductance' is frequency dependent: the apparent or measured inductance will be frequency dependent because of self capacity. The self capacity also affects the Q, and for inductors in the 100 or so microhenry up range (sometimes even as low as 25 uH) alternative winding methods to straight multilayer solenoids are needed. The most common is the 'wave wound' or 'universal winding' method which minimises self capacity: that needs a machine, but if space isn't too much of a problem, winding a 'basket weave' or a 'duo-lateral' coil can be done without a machine. Both of those are pretty good at reducing self capacity and improving Q.
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W5DXP
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« Reply #5 on: January 23, 2012, 04:48:27 AM »

L = 25400 / (freq * freq) * C

Another way to write that equation is freq = 1/[2pi*SQRT(L*C)]

Given a fixed L and C (independent variables) the frequency is the dependent variable.

Here's an inductance calculator that indicates how inductance varies with frequency.

http://hamwaves.com/antennas/inductance.html
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WA3SKN
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« Reply #6 on: January 23, 2012, 05:05:54 AM »

The inductor value does not change with frequency.  The inductors effect, or reactance will vary with frequency.
And there is a good description of how this all works in the ARRL Handbook... any year will cover it, you don't have to buy new!
73s.

-Mike.
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VE7EPX
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« Reply #7 on: January 23, 2012, 12:06:21 PM »

Thanks a lot folks. Much of the 'fog' has lifted and the valuation question is much less confusing.
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W5DXP
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« Reply #8 on: January 23, 2012, 07:55:41 PM »

The inductor value does not change with frequency.

An ideal inductance does not change with frequency. Unfortunately, real world inductors do change value with frequency. For instance, at the frequency where the inductor shifts the EM wave by 180 degrees, it's impedance is purely resistive because it is self-resonant.
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STAYVERTICAL
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« Reply #9 on: January 24, 2012, 12:51:49 AM »

Capacitors also are in the same bucket as inductors in this regard.
Capacitance is just a function of plate area, spacing and the dielectric in between the plates.
There is no frequency function in the formula.
But, as others have said, the A.C. resistance called reactance varies with frequency.
In the case of a capacitor the reactance (or AC resistance) decreases with increasing frequency, the exact opposite of inductive reactance.

This is for perfect capacitors and coils, but in the real world both capacitors and inductors are going to diverge from the perfect.
For example as you increase frequency a capacitor can appear as a coil because its leads inductance rises above its own capacitance.
The frequency would have to be high, but it happens nevertheless.
This is why components in VHF/UHF equipment are used with very short leads for example.

If you have a capacitor and coil in series for example, since capacitive reactance goes down with increasing frequency and inductive reactance goes up with increasing frequency - at some point they are going to both equal.
At this point the combined capacitor and inductor are deemed to be in resonance.
In a capacitor the current leads the voltage and in a coil (inductor) the current lags the voltage.
So when the two values are the same, the leading and lagging cancel and you get a simple resistance without any reactance.
This is resonance.

Antennna manufacturers typically make antennas which are shorter than they should be, and this translates to having capacitive reactance in the antenna. To counter this capacitive reactance they add the opposite which is inductive reactance - in the form of a coil.
The value of this coil depends on the antenna length and frequency of operation.
This is a fairly simple design for a single frequency/band antenna, but where you have to design an antenna for multiple bands, you can see that the reactances change quite radically from band to band.
In this case, antenna designers use difference strategies, including traps.
A trap is just a parallel tuned circuit which acts like a switch at a particular frequency.
At other frequencies both lower and higher the parallel tuned circuit acts as either an inductor or capacitor and this is taken into account when designing the location for further traps along the antenna.
This is how the familiar trap verticals work for example.

Hope this shines a bit of light in some dark corners.

73s
« Last Edit: January 24, 2012, 12:57:57 AM by STAYVERTICAL » Logged
WB6BYU
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Posts: 13482




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« Reply #10 on: January 24, 2012, 08:29:41 AM »

Quote from: STAYVERTICAL
...
For example as you increase frequency a capacitor can appear as a coil because its leads inductance rises above its own capacitance.
The frequency would have to be high, but it happens nevertheless...


I regularly use 10pf SMD 0201 capacitors for bypassing in the 5 to 6 GHz range because that
is about where they are self-resonant (in series mode), giving the lowest impedance.  Any
larger than about 12pf (depending on the manufacturer, etc.) and they become inductive.
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