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Author Topic: Why high voltage at antenna ends?  (Read 19796 times)
KJ4RQV
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Posts: 130




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« Reply #15 on: March 05, 2010, 09:22:18 AM »

This has been so interesting to me. I had no idea it was so involved but I have been able to wrap my head around it due to the great discussions.
Thanks to all.
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KL7AJ
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« Reply #16 on: March 05, 2010, 09:52:33 AM »

Well, as I'm fond of saying, an hour of experimentation saves a decade of debate.

If you can get hold of an R.F. current probe (W8JI's site has a simple construction project for one) you can see the current distribution of a dipole for yourself.  Also, for my class, I built a dipole with miniature Christmas tree lights.  You can see the different high and low current points...very cool and graphic demonstration.


Eric
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K3JRU
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« Reply #17 on: March 12, 2010, 05:24:21 AM »

I would like to expand on the question raised by Don.

My question deals with placement of fan-dipole in an attic and what measures can be taken to either eliminate 'sparks' or direct them safely to a ground.

My presumptions (which I am asking for your help with) are that low power helps, and using 1/2-wl dipoles also helps (e.g., the shorter the antenna, the greater the voltage build-up??).

I appreciate your input on the aspect of the high voltage question...

73, Jim
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VK1OD
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Posts: 1697




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« Reply #18 on: March 14, 2010, 01:03:01 AM »

I would like to expand on the question raised by Don.

My question deals with placement of fan-dipole in an attic and what measures can be taken to either eliminate 'sparks' or direct them safely to a ground.

My presumptions (which I am asking for your help with) are that low power helps, and using 1/2-wl dipoles also helps (e.g., the shorter the antenna, the greater the voltage build-up??).

I appreciate your input on the aspect of the high voltage question...

73, Jim

Jim,

I don't know what the risk is of placing an antenna in the roof space. Are there statistics on how many roof fires were cause by ham radio antennas? I suspect very few, the risk is probably quite low.

Having seen pictures of insulated antenna wire draped over timber and sarking, the worst practices seem to be followed, but you don't hear of roof fires attributed to antennas.

I suspect the risk from high temperature luminaires such as QI downlights is a bigger issue.

However, it would be prudent to make sure that your antenna is supported on quality insulators, clear of combustible material.

Unfortunately, roof spaces are often dusty places, depending to some extent on the type of roofing, insulation etc.

You know that an antenna in the roof space is one of last resort. If you have a conductive roof material, or insulation materials (eg sarking, foil insulation blankets etc), you should not expect good performance at all. BTW, wet concrete tiles and wet timber are hardly transparent to radio waves.

Good luck with your project.

Owen
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K7BDD
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« Reply #19 on: June 07, 2013, 11:33:16 AM »

As Wernher von Braun was attributed to have said, "Rocket science 'aint rocket science."

He was responding to someone in awe of his supposed "genius" design of the F-1 engine.  His matter of fact reply was to assert that if you know a subject well enough, then all the mystery disappears.

K7BDD (2nd)
K7BDD@arrl.net
www.K7BDD.com
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W5DXP
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« Reply #20 on: June 07, 2013, 01:19:39 PM »

If one looks at the ends of a single-wire dipole, a standing-wave antenna, in terms of the distributed-network/wave-reflection model, the reflected voltage at the end of the dipole is in phase with the forward voltage such that the total voltage is double the forward voltage (or double the reflected voltage). The reflected current at the end of the dipole is 180 degrees out of phase with the forward current and the total current is therefore zero.
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My antenna says, "What makes me happy is when the tuner is adjusted for maximum available current through my radiation resistance!" 73, Cecil, w5dxp.com
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