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Author Topic: Hamwaves.com RF Inductance Calculator  (Read 1955 times)
AA5WG
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Posts: 496




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« on: January 27, 2010, 06:12:28 PM »

Hi to all:

Located at Hamwaves.com is a RF-Inductance Calculator.

http://www.hamwaves.com/antennas/inductance.html

The bottom section of this calculator lists the Self-Resonant Frequency 1/4 (parallel) self resonant frequency of n=0 sheath helix mode.

(1)  Is it bad if the seld-resonant frequency is found to be in the ham bands or is close to the ham bands?  

(2) What about "series" self-resonance?

73,
Chuck
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AA5WG
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Posts: 496




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« Reply #1 on: January 27, 2010, 06:45:06 PM »

The example coil being built (using Hamwave RF Inductance Calculator) is for the link antenna coupler.  It is the antenna output/tank coil.
This coil will be used on 160 - 30 meters.

Is it bad if the self resonant frequency is found to be in one of these bands or close to one of these bands?

73,
Chuck
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N7WS
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Posts: 35




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« Reply #2 on: January 27, 2010, 08:07:45 PM »

The self-resonant frequency (SRF) of an inductor is that frequency where the stray capacitance across the series inductance causes parallel resonance.

Since you have no way to control this, operation at or above this frequency is normally to be avoided.

Likewise, capacitors have an SRF but this is where the lead inductance is series resonant with the capacitance.  Operation above this frequency is also to be avoided.
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WB6BYU
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Posts: 13170




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« Reply #3 on: January 27, 2010, 09:14:20 PM »

You will probably be operating the coil at parallel-resonance in your tuner.
If it is already parallel-resonant, you need zero pf of capacitance across it.
That makes it difficult to tune since any variable capacitor has some finite
minimum capacitance.

So if you want to be able to tune your tuner, you have to operate the coil
far enough below parallel resonance that adding your variable capacitor
will bring it to resonance.  Even if resonance falls between ham bands, the
coil still won't work in your tuner on the bands above resonance.
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AA5WG
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Posts: 496




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« Reply #4 on: January 28, 2010, 07:31:22 AM »

Is there a rule-of-thumb how far below self resonant frequency the tank output coil should be operated at?

73,
Chuck
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W5DXP
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Posts: 3562


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« Reply #5 on: January 28, 2010, 11:51:02 AM »

> AA5WG wrote: Is there a rule-of-thumb how far below self resonant frequency the tank output coil should be operated at? <

The ROT is probably "as far as is feasible". If one is limited to an eight foot mobile antenna on 75m, the loading coil is necessarily operated fairly close to its self-resonant frequency but what other choice is there?
--
73, Cecil, w5dxp.com
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
VK1OD
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Posts: 1697




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« Reply #6 on: January 28, 2010, 06:13:18 PM »

Is there a rule-of-thumb how far below self resonant frequency the tank output coil should be operated at?


There is a certain attraction to Rules of Thumb (RoT), but beware that they aren't just ROT.

If you were to make an air cored solenoid inductor, you could represent it as some inductance (in H) in series with some resistance which represents the loss in the non ideal conductor used to make the inductor. Note that the effective resistance at RF varies with frequency, and a reasonable first approximation is that is proportional to the square root of frequency due to skin effect.

But, this equivalent circuit fall down at higher frequencies because it does not account for the distributed capacitance between turns. So, a better approximation is to shunt the series L and R with a small value of C.

If you were to wind a solenoid that has an inductance of say 10µH, a series resistance of say 0.3 ohms at 1MHz, and an equivalent shunt capacitance of say 2pF, you could easily calculate and plot a two terminal equivalent R and X from 1 to 100MHz.

Firstly, X will not simply double for every octave of frequency, and it will not be a straight line. Secondly, R is not constant, it is not proportional to sqrt(f).

Work back from the two terminal equivalent at say 10MHz and calculate L... how can our 10µH inductor appear to have a higher inductance? What does this say about measuring inductance?

What if you could change the inductor so that it had the same L but less equivalent shunt C, what would that mean for the characteristics discussed above.

Welcome to the real world inductor... well a first approximation of it anyway.

Some of the effects described above are attributed to some to ferrite, but no, they occur with an air cored inductor.

Add a lossy ferrite core, and now the series L varies with frequency, and series R varies with frequency, but not as simply as described above.

Owen
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AA5WG
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Posts: 496




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« Reply #7 on: January 29, 2010, 07:20:04 PM »

W5DXP, Wb6BYU, Owen:
Thank you.
Chuck
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