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Author Topic: distance between feed points of antenna  (Read 817 times)
KJ6TSX
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Posts: 116




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« on: January 11, 2013, 09:47:36 PM »

This is probably a dumb question but I have to ask. What is the minimum distance (air space) between the feed of the two elements of a dipole or other balanced antenna?  would a 1/2 inch be sufficient at 1kw
Thanks
George
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WB6BYU
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Posts: 13239




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« Reply #1 on: January 11, 2013, 10:25:05 PM »

At 1kw across 50 ohms, Ohm's law says the voltage would be the square root of
1000 * 50, or 224V RMS, or 314V peak.  So whatever spacing is used in a 240VAC
power circuit should be adequate in this case.  I don't remember the breakdown
voltage of air:  while half an inch would probably work, I'd give it a bit more just
to be sure.

But that's at the low end of the voltage range for balanced antennas.  Say you
built a 40m dipole and wanted to feed it on 20m where the feedpoint impedance
is 2000 ohms.  Now the voltage is SQRT( 1000 * 2000 ) = 1400V, or 2000V peak.
In some conditions it could be even higher.  So you have to know more about
the antenna you are building to determine a safe spacing.
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KJ6TSX
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Posts: 116




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« Reply #2 on: January 11, 2013, 10:34:36 PM »

I am using this on 10 & 12 meters only, I think I will take your advise and make a 1 inch gap. I think the old saying goes a ounce of prevention is worth a pound of cure.
Cheers
George
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K2DC
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Posts: 1361


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« Reply #3 on: January 12, 2013, 03:30:45 AM »

The breakdown voltage of free space is 30,000 volts per inch.  The breakdown voltage of air is less, but how much less depends on altitude, humidity, particulate content, and probably several other factors.  Suffice it to say that an inch is as good as a mile, as long as it's properly weatherproofed like any other outdoor connection.

73,

Don, K2DC
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WB2WIK
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Posts: 20595




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« Reply #4 on: January 12, 2013, 04:47:33 PM »

1/2" is actually plenty; however most commercial designs add a bit more spacing (like an inch or maybe 1-1/2") to allow for that gap being bridged by rain, snow, ice, etc. which are not nearly as good dielectrics as air is.

If the antenna was indoors and protected from weather, even 1/4" would be plenty.
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