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Author Topic: 40 - 10 Meter Dipole Length  (Read 4768 times)
K9BAY
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Posts: 144




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« on: May 29, 2010, 06:26:52 AM »

I just want to confirm.  For the 40- 10 M dipole  instead of doing two 33 feet exact length segments of wire (66 feet total),  I could use the ground side of the balun be at 23 feet and the plus (positive) side at 43 feet or vice versa.  This might make it better for running the coax to the balun.   I am trying to offset it, so the feed point isn't in the exact center of the yard.

Thanks!
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K0BG
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« Reply #1 on: May 29, 2010, 06:49:41 AM »

OCF antennas, by their very nature, are plagued with common mode current, the use of a balun notwithstanding. Why do you need to offset it?
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K9BAY
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Posts: 144




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« Reply #2 on: May 29, 2010, 08:21:31 AM »

Thanks for the information.  The offset will allow me to not have to run the feedline from the center of my back yard.  I measured 23' from the tree I want to use and it is almost parallel with the end of the house and would allow me make my run upwards more cleanly. 

But I am finding out that I will need a 4:1 balun  (I currently have a 1:1) and that the antenna is much more directional?

I may have to reconsider and do some remeasuring.

Any ideas?  I wish there was some way to run the coax higher off the ground over to the feed point.

Thanks!


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WX7G
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Posts: 6028




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« Reply #3 on: May 29, 2010, 08:36:18 AM »

K9BAY your proposed antenna simulates like this with it mounted 30' above ground and fed through an ideal 4:1 balun:

7.1500 MHz, 2.3:1 VSWR
14.175 MHz, 2.5:1 VSWR
21.225 MHz,  17:1 VSWR
28.400 MHz, 4.7:1 VSWR

With a suitable antenna tuner it should work well on 40, 20, and 10 meters. There will be higher line loss on 15 MHz but it will still work. Yes there will be some feedline radiation due to the offset feed but that may or may not be an issue. You'll just have to build it and give it a try.
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K9BAY
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Posts: 144




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« Reply #4 on: May 29, 2010, 10:06:52 AM »

I think it is going to be better center fed. Insted of OCF.  I do have an auto tuner.

Thanks for the info!

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WX7G
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Posts: 6028




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« Reply #5 on: May 29, 2010, 11:16:27 AM »

Center fed the VSWR on 50 ohm t-line will be quite high on all but 40 meters. Using a 1:1 balun:

7.1500 MHz, 1.5:1 VSWR
14.175 MHz,  81:1
21.225 MHz,  10:1
28.400 MHz,  64: 1

Using a 4:1 balun:
7.1500 MHz,   3:1 VSWR
14.175 MHz,  20:1
21.225 MHz,    5:1
28.400 MHz,   16:1

Your original plan to use the OCF is a good one. Or feed it with ladder line to a tuner having a balanced line output. Line radiation? Don't worry about it. Try it and see.

If you don't like the antenna build a different one. This is the fun thing about wire antennas; you don't have to get it right the first time. Build and experiment.  

    
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K9BAY
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Posts: 144




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« Reply #6 on: May 29, 2010, 11:50:29 AM »

I'll probably go with center fed for now.  I have re-measured and I am hoping I can do a clean run to the feed point.  Right about the fun building and experimenting!

Thanks!
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K7UNZ
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Posts: 691




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« Reply #7 on: May 29, 2010, 07:32:09 PM »

There is another option, if you have the room.

Make your antenna a full wavelength, and feed it in 1/4 wave from one end.  You will have 1/4 wave on one side, and 3/4 wave on the other, and you can feed it with 50 ohm coax, no problem.

73, Jim/k7unz
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N2EY
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Posts: 3879




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« Reply #8 on: May 29, 2010, 07:55:40 PM »

For the 40- 10 M dipole  instead of doing two 33 feet exact length segments of wire (66 feet total)

Such a dipole, if fed with coax, will work well on 40 meters and possibly 15 meters. But it won't work well at all on 20, 17, 12 or 10 meters, because on those bands the feedpoint impedance isn't anywhere close to 50 ohms.

73 de Jim, N2EY
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N4HRA
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Posts: 280




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« Reply #9 on: May 30, 2010, 04:29:05 AM »

Why place the tuner at the radio to tune the Coax to 50 ohms and not the antenna?

Get an AUT and some ladder line, Connect the ladder line to the Dipole and to the ATU.
Now you will be using the L & C of the ATU to tune the antenna for a match. The input to the ATU will be 50 ohms back to your rig

I have a dipole up 30 ft fed with ladder line to a SGC ATU in the attic,
Apply 12Vdc and RF and I work 10 - 80 meters with 1:1.3 max SWR.

QST has a real good article on ATU page 47 this month
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