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Voltage at center of dipole




 
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K6JEK
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« on: October 20, 2016, 09:52:29 AM »

How high is the voltage at the center of a resonant dipole? We've all see the diagrams that show voltage low, current high there. And we've heard stories about setting things on fire with the ends of a dipole. Tom, K6AD, set his fence on fire that way. At least he did it with class, with a Collins KW-1. I've always been very careful to keep the ends in the clear.

But what about the middle? When the dipole goes back up, Plan A has the center supported by a branch up in a redwood tree, pretty close to other branches. Dry leaves could land on it.  Twigs. Little birdies. Spider webs.  

I'm inclined to not worry but this is not grounded in numbers. Or maybe I need plan B.
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W1ITT
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« Reply #1 on: October 20, 2016, 01:38:15 PM »

Let's just run some quick numbers.  If the impedance at the feedpoint is 72 ohms resistive, then the voltage is the square root of the "impedance multiplied by the power".  So for a kilowatt, it's the square root of 72,000 or about 268 volts.
Let's move out along the dipole a ways.  Supposedly at 16% off center, the Windom connection impedance is around 300 ohms. It may or may not be, but let's just play along.  Then the square root of 300,000 is about 548 volts. 
The question is, how much voltage does it take to ignite the leaves, or fencepost or whatever it touches?  I don't know, but the best course of action would be to avoid the problem if possible.  Good engineering practice calls for us to insulate and separate live antenna parts from other objects.  Our RF does no good if it is shunted off to the apple tree.
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AB2EZ
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« Reply #2 on: October 20, 2016, 03:14:18 PM »

Adding to the last post by W1ITT,

Voltage is always defined between to points.

For example, as he did, one can calculate the voltage across the center insulator of a resonant dipole (which is not the voltage you are asking about), having a driving point impedance of R ohms, and driven by a balanced transmission line whose characteristic impedance is also R ohms. All you need is the power being applied to the feed point, and the value of R. The calculation that he performed is of the rms RF voltage across the center insulator. The peak voltage (amplitude of the RF sine wave) is the rms voltage multiplied by 1.4.

If the balanced transmission line has a chacteristic impedance that is not the same as the driving point impedance, then the peak voltage across the center insulator will be larger by a factor of the square root of the resulting SWR.

But, the question you are asking is: How large is the amplitude of the RF voltage between either of the two wires of the dipole (at a location close to the center insulator) and a tree located near that same point.... and could an arc be produced across either of those two gaps?

The answer to the voltage question is:

If the tree is a fairly good RF insulator, then the voltage between the tree (located  near the center insulator of the antenna) and either of the wires will be a fraction of the voltage across the center insulator. The fraction will be between 0 and 1. This is because the RF electric field between the two wires, near the feedpoint,  is approximately uniform, and parallel to the antenna. Also, this assumes that the presence of the tree does not disturb the RF fields near the center insulator.

If a conducting path forms between the part of the tree that is near the center insulator and ground... charge will accumulate on that portion of the tree, and will reduce the voltage across each gap... reducing the ability of an arc to form across either gap.

Stu

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Stewart ("Stu") Personick. Pictured: (from The New Yorker) "Season's Greetings" looks OK to me. Let's run it by the legal department
K6JEK
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RF in the shack


« Reply #3 on: October 20, 2016, 05:28:39 PM »

Damn you guys are good.  I wonder why I didn't think my way through it as you did.

It will be a reasonable match at the feed point. Everything is insulated, wire, connections, everything. The wire should not be resting or rubbing on anything but time and weather have  a way of changing things. I had a "resonactor" (loading coil) on a W9INN catch fire once because a spider built a rope from the connector on one end to the other. The web was wet from fog. It ignited and started the PVC form going. The PVC did not burn all the way through but while things were going, it was a sight to see, smoke coming out of my antenna. I think the spider may have perished.

To run the test on this antenna, somebody should give me a KW-1 don't you think?
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WBear2GCR
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« Reply #4 on: October 21, 2016, 06:18:13 PM »

Ok, same antenna... voltage at the ends? (WRT ground...)
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« Reply #5 on: October 21, 2016, 09:20:10 PM »

Mr Bear...

I assume your question is what would be the voltage at the ends of a dipole.  For a given power, that depends on the size of the antenna conductor.  For a conductor that approaches infinitely thin, the voltage would approach infinite, but not get there.  For a #12 wire, the end voltage would be higher than if the conductor were a piece of one inch tubing.
In ON4UN's excellent Low Band DXing, 5th Edition, Chapter 9 there is a chart of end impedances  for monopoles of various heights and diameters, which is informative.  We would expect the end of a half wave dipole to be the same, just that it's fed differently.  For a half wave monopole with a conductor diameter of 0.1 degrees, the end impedance is a tad over 1250 ohms., while a conductor of 0.5 degrees shows about 980 ohms.   
This tells us what we already know, that fat conductors are less apt to go into corona effects.  I have climbed hot AM broadcast towers, hopping over from a couple hardwood pallets to where the impedance, and therefore the voltage is low.  I climbed most of the way up to work on an FM antenna fed through an isocoupler.  But the rule of thumb is that one should not go all the way to the top lest one's pointed head be the place where the corona takes off.
So the answer is...it depends.

Norm
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WB2RJR
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« Reply #6 on: October 23, 2016, 06:57:05 PM »

The voltage on the dipole to the tree should be similar to the voltage near the base of a 1/4 wave vertical. Never saw anyone selling these ( hygain, hustler etc.etc) recommend putting a fence around the base so people or the dog don't touch it. The far end is another story.

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« Reply #7 on: November 16, 2016, 12:30:59 PM »

Remember transmitting from my old military landrover lightweight with a ANGRC9 and its 40 watts amp at 80 meters. The 15 foot whip antenne at the front tuned with a coil at the base. I was parked underneath a tree when suddenly a burning branch fell on top of the hood of the jeep! The top of the antenne was touching the branch of the tree and had set it on fire, and only with 40 watts AM!
The tree did not cath fire Kiss
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N5RLR
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« Reply #8 on: November 19, 2016, 03:48:07 AM »

...In ON4UN's excellent Low Band DXing, 5th Edition, Chapter 9 there is a chart of end impedances  for monopoles of various heights and diameters, which is informative.  We would expect the end of a half wave dipole to be the same, just that it's fed differently.  For a half wave monopole with a conductor diameter of 0.1 degrees, the end impedance is a tad over 1250 ohms., while a conductor of 0.5 degrees shows about 980 ohms...

Can this chart be found online, or is it still under copyright?
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Michael

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W1ITT
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« Reply #9 on: November 19, 2016, 07:09:46 AM »

Michael....It's a new book, still in print, available at the ARRRL bookstore for $45 plus inflated shipping costs.  Not surprisingly, it's mostly an antenna book with extensive coverage of both transmit as well as receive antennas.  For the 160, 80 and 40 meter crowd it's much better than the ARRRL Antenna Book. 
The hefty book comes with a CD that has the whole book on it, in addition to some good software, but I doubt that it's online anywhere.  If you get it via Amazon, be certain to buy the January 2011 Fifth Edition, as they are offering a few others.  ON4UN says that this will be his last update.  Just buy the book!
73 de Norm W1ITT
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Steve - K4HX
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« Reply #10 on: November 21, 2016, 05:08:45 AM »

I second Norm's post. It's the best antenna book you can buy for 160-40m operation, especially the coverage of receive antennas.
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« Reply #11 on: November 21, 2016, 05:32:56 PM »

Sounds good.  Smiley
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Michael

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