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Voltage hazard at end of dipole wires?

K

K7RDN

Member
Dec 2, 2010
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0
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I have made a 80m dipole (12 gauge insulated wire) and run 100W into it now. Because of my limited space it goes through a couple trees (not touching any branches unless the wind blows) and attaches to another tree with an insulator and kevlar rope. The end of the wire is about 2 feet from where it attaches to the tree. I am getting a Heathkit SB-221 amp that will put out about 1300 watts. Should I be concerned about how far I should keep the end of the wire from the tree? Any chance of doing damage to the tree and worst case, catching it on fire? How about the wire that goes through the trees?

73's
Bob
K7RDN
 

Bob,
Is there any possibility of danger from being that close to a tree running that kind of power? Sure. But the danger isn't very great at all and it's done all the time. Is stuff touching that antenna going to catch on fire? Same answer. Is your SB-221 liable to blow up? I very, very much doubt it. At least not from what you've described. Am I absolutely positive about all that? Of course not! But I know that I haven't seen/heard of any huge/important problems directly attributable to antennas running through trees. Still worried? So rearrange something to make it 'better'.
- 'Doc
 
I have the end of my 80m dipole about 3 feet from the tree that it's tied to. I've run legal limit to it with no problems. Remember, unless the tree/branch is dead, it's pretty difficult to actually light it on fire. ;)
 
I think I would be more concerned about a n y nearby objects that can offer capacitance and detune your antenna - than I would about the fire hazard. With a proper install; that fire issue is a non-starter.

As a thought, keep this in mind. Before I installed the Alpha Delta DX-CC 10-80m dipole, I called them for tech support. They called me back and the tech went over the optimal conditions to install a dipole. The engineer at Alpha Delta products instructed me to make sure that my dipole was no closer than 10 ft from any object when installed. Not just metal objects; ANY objects. They didn't want me or anybody to keep their antenna from getting a bad reputation because I didn't install it in the best/optimal environment and become compromised. Detuning effects can make it hard to get the full potential from ANY dipole.

Some food for thought . . .
 
Robb said:
I think I would be more concerned about a n y nearby objects that can offer capacitance and detune your antenna - than I would about the fire hazard. With a proper install; that fire issue is a non-starter.

As a thought, keep this in mind. Before I installed the Alpha Delta DX-CC 10-80m dipole, I called them for tech support. They called me back and the tech went over the optimal conditions to install a dipole. The engineer at Alpha Delta products instructed me to make sure that my dipole was no closer than 10 ft from any object when installed. Not just metal objects; ANY objects. They didn't want me or anybody to keep their antenna from getting a bad reputation because I didn't install it in the best/optimal environment and become compromised. Detuning effects can make it hard to get the full potential from ANY dipole.

Some food for thought . . .
Click to expand...

Trees (wood) will have almost zero effect at hf frequencies if you use insulated wire.. Trees will not detune an antenna. I have my 20m dipole (insulated wire) over the top of two trees with no effect on the resonance of the antenna. Even in a rainstorm I don't notice any increase in the swr. Actually, varying the height above ground has more noticible effects. Put the antenna in the trees and have fun. They won't burst into flames or destroy your rig. Just don't put your lips on it... :)
 
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Dry leaves, dead branches or repeatedly cooked live branches can catch on fire. There's a lot of available voltage at the ends of any antenna even at 100 watts. Much more than THHN can handle. A few feet in from the end things become safer.

Remember antennas longer than 1/2 wave can have multiple voltage maxima.
 
Kamikaze said:
Dry leaves, dead branches or repeatedly cooked live branches can catch on fire. There's a lot of available voltage at the ends of any antenna even at 100 watts. Much more than THHN can handle. A few feet in from the end things become safer.

Remember antennas longer than 1/2 wave can have multiple voltage maxima.
Click to expand...

Antennas have voltages and currents running through them...agreed. Now, let's hear the story of a tree bursting into flames when you keyed up your 100 watt xceiver on 20 meters.

My story: 25 years using dipoles in trees...in them, on top of them and when I'm lucky in btween two..alive dead and otherwise. I have run full legal limit into a 20 meter dipole touching the branches of a loblolly pine when I lived in VA. Never once did I cause a fire or even seen smoke. I've never even talked to anyone who has seen this happen.
 
While it is highly unlikely to have a problem with only 100 watts it can and certainly has happened when running legal limit. Arcing can occur if the end of the antenna is in contact with wet leaves. Check out some of the guys running "big iron" on AM at full legal limit and a lot of them will tell you of the times they burned off their share of leaves, branches, and even end insulators.
 
Captain Kilowatt said:
While it is highly unlikely to have a problem with only 100 watts it can and certainly has happened when running legal limit. Arcing can occur if the end of the antenna is in contact with wet leaves. Check out some of the guys running "big iron" on AM at full legal limit and a lot of them will tell you of the times they burned off their share of leaves, branches, and even end insulators.
Click to expand...


For those that don't believe, here is the thread on QRZ.com about it. the OP looks familiar too. :whistle:

http://forums.qrz.com/showthread.php?294163-Voltage-hazard-at-end-of-dipole-wires
 
So how far should the end of the wire be away from the tree to be safe? I will be running between 1000 & 1500 watts.
 
A foot or better would be a good guess.

Hams have been running wires through trees, over top of trees for years.

Antennas will not harm trees, but trees will sure harm antennas.

Put the antenna up and start making contacts, most of all have fun.
 
Captain Kilowatt said:
While it is highly unlikely to have a problem with only 100 watts it can and certainly has happened when running legal limit. Arcing can occur if the end of the antenna is in contact with wet leaves. Check out some of the guys running "big iron" on AM at full legal limit and a lot of them will tell you of the times they burned off their share of leaves, branches, and even end insulators.
Click to expand...

I can see a windy AM'er running the limit causing smoke but I'd bet the risk is nil using SSB or CW.
 
Use a good, transmitting-quality end insulator at each end of the dipole. Don't tie the wire itself around the branches, even if it's "insulated". That insulation is rated at power frequencies, not RF, and it will wear through after a few breezy days.

Don't use just a little egg-shaped plastic or porcelain insulator if you're running more than a couple hundred watts. True "end insulators" are anywhere from 6 to 12 inches long, and often with a corrugated surface to present a longer leakage path. Hy-Gain used to make these. I have several of them and they work just fine.

I HAVE seen dry trees in flames from just 250 watts output to a poorly installed dipole. It can and will happen.
 

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