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Author Topic: Dipole that is to long  (Read 1389 times)
N7GCO
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Posts: 146




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« on: August 31, 2012, 05:23:25 PM »

I am putting up a 80 meter dipole antenna at about 75' between two trees. The trees are only 100' apart so I will need to bend the ends of the dipole.

1) How do you bend them in such a way as to be able to connect the joint to the support?
2) DO you need to trip off the excess wire to bring it into frequency, or can you just wrap it back on itself (I don't know if this really electrical shortens the antenna or not)?

Thanks
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W9GB
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Posts: 2626




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« Reply #1 on: August 31, 2012, 05:48:40 PM »

You let the ends droop.
http://www.wcerc.org/Projects/dipole.asp

Tie your support insulator (I prefer ceramics) to spot on driven element ... Equally on both sides
.
The excess (too long, ~ 15 feet) drops at both ends ....
Due to potential HV at those wire ends .... must NOT be close to humans, trees, etc.
« Last Edit: August 31, 2012, 05:54:37 PM by W9GB » Logged
WB6BYU
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Posts: 13336




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« Reply #2 on: August 31, 2012, 05:50:02 PM »

I just tie the end insulator in from the end of the wire.  The support ropes then tie
to the insulators, leaving the ends of the wires hanging down.

Folding back the ends isn't exactly the same as snipping them off, but will also raise
the resonant frequency.  The exact amount depends on whether the wire is insulated
or not and a few other details, but it is a common approach.
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N4JTE
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Posts: 1158




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« Reply #3 on: September 01, 2012, 05:05:58 PM »

Sounds like your gonna have to let around 25 ft at each end droop down, I would tie a masonry string/insulator to the excess wire and tie off straight down if possible. You are trying to get all the wire up in the air you can 1/2 wl long so folding anything back is counterproductive. Also no mention of actual height so hopefully your at least 66 ft or so up in the air with the shortened version you describe.
Bob
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N4JTE
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Posts: 1158




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« Reply #4 on: September 01, 2012, 05:07:39 PM »

Sorry misread post, height good rest of answer might help.
bob
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WA4FNG
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Posts: 162




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« Reply #5 on: September 02, 2012, 07:03:47 PM »

I use wire rope clips (google it...) when I need to attach a support at a midpoint in a wire antenna. Just make a small loop with the wire and attach a small clip. Tighten the nuts and then attach to support rope or insulator.
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K0ZN
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Posts: 1553




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« Reply #6 on: September 02, 2012, 07:32:14 PM »

"(I don't know if this really electrical shortens the antenna or not)?"

 If you are using UN-insulated wire it will shorten the antenna. If you wrap/twist or whatever, two piece of wire together you have ONE conductor.

 Think about it for a second..... when two conductors touch, there is a connection between them.

 If you are using INSULATED wire, that would not be the case, but it would still be a bad practice and likely not give resonance where you expected as that
would be kind of linear loading.

Respectfully, pick up a copy of the ARRL handbook and put in a little study time. You will be well rewarded with reduced frustrations and problems in your
antenna system(s). The hard fact is that this is a technical hobby and certain parameters have to be met or else things won't work. Antennas are not complex but they are not forgiving of assumptions, guesses and application of many of the myths floating around out there.  Knowledge is power in this game.

73,  K0ZN
 
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KC4MOP
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Posts: 743




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« Reply #7 on: September 03, 2012, 05:19:32 AM »

I am putting up a 80 meter dipole antenna at about 75' between two trees. The trees are only 100' apart so I will need to bend the ends of the dipole.

1) How do you bend them in such a way as to be able to connect the joint to the support?
2) DO you need to trip off the excess wire to bring it into frequency, or can you just wrap it back on itself (I don't know if this really electrical shortens the antenna or not)?

Thanks

You'll be ok with your approach. The drooping wire attached to a support rope at each end will have to be trimmed to get the dipole resonant to the particular part of the band. Hop[e you got an analyzer (MFJ259 or 269)
And easier approach is leave everything alone with the wire between the two trees and whatever droops down and feed it with open ladder line and have a balanced tuner in the shack and now you have a multiband antenna.
It may even load on 160M with little problems.

Fred
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AF5FH
Member

Posts: 31




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« Reply #8 on: September 03, 2012, 09:39:10 AM »

I have an antenna similar to the one you are discussing. About 80 feet across the top, but only about 28 feet above ground, short trees, sigh! Mine is a DX Engineering DXE-WA-135 kit, I used insulated tie wraps to secure the insulated antenna wire to the end insulators, and brought the end wires down towards the ground, using tie wraps to attach the antenna wire end to support polyester rope on each end. Ended up with a 125 feet long dipole, fed with 300 ohm ladder line, loads 1:1 SWR through a MJF-941D manual tuner on all bands 80 through 10 meters.

If you don't want to go the balanced feed line and tuner approach, leave the vertical stubs long by a couple of feet, borrow an analyzer (like KC4MOP said), and adjust the length. You can wrap the excess wire back on itself and secure it with tie wraps.

Jim, AF5FH
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WB6BYU
Member

Posts: 13336




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« Reply #9 on: September 03, 2012, 10:01:21 AM »

You can adjust the antenna with an ordinary SWR meter, as most of us did for many
years before SWR analyzers were available.  It isn't that difficult.
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