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Author Topic: Tree antennas  (Read 2404 times)
KA4AQM
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Posts: 59




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« on: February 17, 2014, 05:58:26 PM »

Now that the trees are without leaves, I have an idea about mounting a balun about 25 feet up next to the main trunk and running some isulated wires out for a 2 or 3 wire dipole. QUESTION: has anyone tried this and what is the real impact once the leaves start coming in? Can I expect an swr problem?
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KB4QAA
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« Reply #1 on: February 17, 2014, 08:01:09 PM »

-Running bare wires in a tree will short your antenna, give you poor performance, high impedances, and give a minor risk of fire.

-Can you please explain a 3 wire dipole?
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G3RZP
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« Reply #2 on: February 18, 2014, 12:41:06 AM »

I have a 40m sloper that ends in some bushes and has the low end about 3 feet above ground. A bush grew and touched the end, and when I did some maintenance work on Sunday, I noticed said bush was nicely burned where it had touched. Now with the amount of rain we've had in the UK in the last three months, a fire is pretty well impossible but in a lot of VK or W6 at the moment, it would be another matter. BTW, that burn is old enough that it was before I got my new amplifier last month, so that was with 100 watts.

Use a tree for support, but don't let it touch the actual antenna.
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KE4JOY
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Posts: 1384




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« Reply #3 on: February 18, 2014, 12:51:03 PM »

All my antennas use 'natural' support system.

Here have a look at this gizmo maybe it will give you a few ideas!!  Smiley

https://www.ezhang.com/index.php?PHPSESSID=llo3hspak6rv2j7ahqsujbr6l2

ps; I made my own from a cheap fishing reel and a slingshot some wood, some screws, and a few tie wraps  Cheesy

I will post a picture in a few minutes.
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VE3FMC
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« Reply #4 on: February 18, 2014, 01:34:33 PM »

-Running bare wires in a tree will short your antenna, give you poor performance, high impedances, and give a minor risk of fire.

-Can you please explain a 3 wire dipole?

A 3 wire dipole is a fan dipole. 3 bands on one feedpoint.

 I have plenty of experience with antennas in trees.

One leg of my 80 meter dipole runs right through a maple tree. The other leg ends about 2 feet from a tree branch. I have run 1000 watts into that antenna and the trees have never caught on fire. The leaves come on the maple tree and the swr does not change.

At one point in time I had a 40 meter dipole running through that maple tree. Again 1000 watts, no fires. No change in swr when the leaves arrived in the spring.

I moved that 40 meter dipole into the clear on another tower. No difference in performance from one tower to the other.

The key to running dipoles through trees is to use insulated wire. Never use bare wire.

So for the original poster if you use insulated wire your antenna will work. Remember that when you use insulated wire the legs of the dipole will be shorter than when using bare wire. Cut the lengths to the formula, then you can simply shorten them to resonate the antenna.

Good Luck
Rick VE3FMC

« Last Edit: February 18, 2014, 01:43:17 PM by VE3FMC » Logged
W8MW
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Posts: 329




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« Reply #5 on: February 18, 2014, 04:32:18 PM »

What you described is exactly what I've done with many different trees.  Also I used multiple dipoles (fan dipole) coming off a common coax-fed feedpoint.  It can be mechanically challenging to climb the tree and find a suitable limb for hanging the balun.  And then try to find workable openings through the branches for throwing out the insulated antenna wire, each rolled-up section with attached tie-off rope.  I have on occasion needed to "persuade" some limbs to provide clear openings.  My most complex tree antenna had resonant dipoles for 80/40/20/10/6 meters.

I'd recommend you use plenty of tie-off rope to give you maximum options on how to position the legs of each dipole.  It's a good thing to try and keep the included angle at the feedpoint as wide as possible.  180 degrees would be a flat top dipole which I was never able to do.  120 or so generally worked well and put me into "inverted v" territory.  My goal was resonant antennas and with patience it's possible to tune them and to achieve very reasonable SWR.

Performance wise, I've been pleasantly surprised at how well tree antennas worked for me, leaves or no leaves.  Like FMC I never experienced any arching or burning in the tree running a KW.  

Good luck with your installation.

73 Mike W8MW
« Last Edit: February 18, 2014, 04:34:25 PM by W8MW » Logged
G3RZP
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« Reply #6 on: February 19, 2014, 01:17:35 AM »

I did run a triatic stay between two trees about 20 feet apart, and hung a vertical dipole for 18 MHz from it, with the bottom 15 feet above ground. It worked well, and the leaves didn't make a difference. But it didn't work anywhere near as well my 4 element Steppir at 62 feet does!
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N3QE
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« Reply #7 on: February 19, 2014, 07:30:12 AM »

For a traditional resonant half-wave dipole, the high voltage points are at the ends of the wires. The center of the antenna has the lowest voltages which works out well for tree-center half-wave dipoles. Terminating the ends of the wires with some kind of insulator (to deal with wet support ropes etc.) is a good choice because that's where the high voltage is.

Very unlikely that leaves will affect your SWR. The most likely thing to happen that will make your SWR go up, is water ingress into the Coax. Don't sweat that too much if this is just a "trial" antenna.

Tim.
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KA4AQM
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Posts: 59




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« Reply #8 on: February 19, 2014, 05:48:18 PM »

Thanks everyone for response. I did want to use insulated wire to minimize any swr effects. And as explained, the 2-3 wire dipole should have read Fan-dipole (2-3 wires coming from each side).

I have too many man made obstructions to use dipoles otherwise so will give the tree idea a chance.

Again, thanks for the input!

Don't forget, Virginia QSO Party coming up in Mar...tune in, answer the call and have a blast working the best QSOP in the US! (biasd? yes!)
Randy
KA4AQM
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