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Author Topic: Link Tank Coil Air Gap Spacing Voltage Breakdown  (Read 910 times)
AA5WG
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Posts: 496




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« on: May 07, 2013, 06:56:14 PM »

Hi to all,

Is there any documentation, recommendations or rules of thumb that can advise me how much air gap (spacing) is needed between coil turns to help prevent high voltage arcing/breakdown between adjacent coil turns for a given voltage, i.e. 1000 volts, 2500 volts, 5000 volts, 7500 volts etc.?

I read about air gap spacing recommendations to help prevent high voltage breakdown/arcing between air variable capacitor rotor and stator plates. 

Are there similar recommendations for the output tank coil for the link antenna coupler? 

Thank you,

Chuck
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N3QE
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Posts: 2203




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« Reply #1 on: May 07, 2013, 07:10:25 PM »

It would be unusual for the 50-ohm link to have insane voltages across it, and the voltage is across the entire inductor.

Typically the voltage across the output tank coil will be far far higher - but it typically has more turns and more length, making for a natural balance.

If you are trying to design a broad-range tuner with a single coil set, one of the limits you will hit is the parasitic capacitance of the inductor setting a maximum usable frequency. It is possible with switched taps etc. to use a single coil set over all of HF (e.g. Johnson Matchbox) but it's not easy. And when you are switching taps like this, you find that as you move up in frequency, you're using a smaller and smaller section of the coil, thus the volts per inch is going up and up and up.

Len Cebik's site had some nice examples of link-coupled tuners. I'll see if I can post some of mine.
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K2DC
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« Reply #2 on: May 08, 2013, 03:05:00 AM »

Chuck,

   The ule of thumb should be the same, regardless of the component type.  The breakdown voltage of air is 76,200 volts per inch, give or take altitude, humidity, air impurities, etc.

73,

Don, K2DC
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AA5WG
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Posts: 496




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« Reply #3 on: May 08, 2013, 06:34:20 AM »

Don, N3QE and all,

Thank you for your inputs. N3QE I look forward to seeing your material.

I am not sure about the voltage breakdown vs spacing gap for air single layer wound coils.

I am talking about link antenna coil types.  Some say the voltage changes along the length of
the coil.  I don't know.

Chuck
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WB6BYU
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Posts: 13248




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« Reply #4 on: May 08, 2013, 06:48:23 AM »

The voltage between adjacent turns of a coil can be estimated by dividing
the total voltage by the number of turns:  usually this isn't a problem at ham
power levels.

More often arcs are between one end of the coil and surrounding objects,
because then we are dealing with the full voltage across the coil.  (This may
be a particular problem with tapped coils when the unused turns are not
shorted.)

The voltage to ground changes along a coil much more than the voltage
between turns.  This is one reason why link coupling coils are usually wound
around the coil at a point that is closest to ground potential (the grounded
end, or the center of a balanced coil.)  This also reduces capacitive coupling
of harmonics through the tuner.
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