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Author Topic: half wave vertical dipole  (Read 893 times)
VK4TJF
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« on: March 28, 2008, 09:46:19 PM »

i was thinking of building a half wave verticle dipole for 20 meters, it would be just over 10 meters high. my questions are simple, 1) does the OD of the aluminum make the antenna shorter or longer and what about the thickness of the aluminum? and 2) will this antenna preform good DX if mounted on the ground?
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WA7BOC
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« Reply #1 on: March 28, 2008, 10:50:58 PM »

Google KV5R, go to his Big Vertical Antenna Project.
This article will give you a great start on your project! Mine goes up tomorrow.

73, Roger
WA7BOC
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KA5N
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« Reply #2 on: March 29, 2008, 02:26:35 AM »

A half wave vertical dipole should perform about the same as a quarter wave vertical with a reasonable number of radials.  If made of aluminum tubing (it could be made of wire and suspended from something) for a given frequency it will be shorter than a wire dipole.  Tubing thickness, other than mechanical strength, will make no difference.  The lower end of the antenna cannot be connected to ground as it is hot with RF (must be insulated from ground) but otherwise it can be inches away or high in the air.
One concern is that the feedline should be dressed away from the antenna at close to a right angle.
The ARRL publication "Simple and Fun Antennas" has a couple of chapters of easily understood information about building vertical dipoles.
Allen
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K2DC
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« Reply #3 on: March 29, 2008, 03:21:06 AM »

Hello Jim,

The vertical dipole at 10M should work just fine on 20M. I have no experience with ground mounted vertical dipoles, but my gut says to get it up as high as you can.

The larger OD of the aluminum will make a resonant antenna slightly shorter, and cover a little bit more of the band than just a wire, but not much. On 20M, the increased OD of the aluminum over a wire is a very small fraction in terms of a wavelength.

You might want to start with the same length as you'd use for a wire (.47 wavelengths total) and then trim as required to set it resonant where you want it. I always start my homebrew wires a little long, and then trim back. If you start where you think it's spot on and it winds up short, there's more work to do. I agree that the feed line should take off horizontally from the feed point as far as you can. That will help keep antenna currents off the shield of the coax (and out of the shack), and keep the feed line from interacting with the antenna. A few ferrites over the coax at the feedpoint, or a coil of 6 or 8 turns of coax on a 4" to 6" diameter will help as well.

The thickness of the aluminum will help make the structure sound, but it will make no difference at all in terms of performance compared to a wire dipole. RF travels on the surface of conductors rather than the core (it's called skin effect), so the thickness won't help you there. This differs from DC, AC, audio and VLF circuits, where the cross section of the conductor sets the current carrying capacity of the circuit.

It sounds like an interesting project. Good luck with it and have fun!

73,

Don, K2DC
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NI0C
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« Reply #4 on: March 29, 2008, 07:33:22 AM »

I've have some experience using several of the Force 12 Sigma series vertical dipoles.  These antennas work very well and take up minimal room. My recollection from reading some of the Force 12 literature available online is that these antennas should be mounted less than 0.1 wavelength above ground in order to achieve the good low angle radiation pattern.  

I have my antennas mounted with the lower T bars approx. 4-6 ft. above ground to minimize interactions with nearby metal objects such as neighbors automobiles, patio furniture, etc.  

Force 12 also uses a balun assembly at the feedpoint, and recommends dressing the coax at a near right angle for some distance as suggested by a previous poster in this forum.  Accomplishing the latter can be a problem, especially with heavy coax, and some compromise is required.

73,
Chuck  NI0C  
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KI4BDS
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« Reply #5 on: March 29, 2008, 10:36:08 AM »

I have decided that the 2m J-pole gets out as good and built out of 3/4 copper, won't blow down and ice won't hurt it. I am just not too hot on tall antenna as we get 70 MPH + winds here in N VA coming thru the Harpers Ferry canyon for want of a better name!!!!
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N1LO
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« Reply #6 on: March 29, 2008, 11:41:48 AM »

Hello James,

Larger diameters will make the resonant length shorter, and the aluminum thickness will have no effect.

Search on the subject of "K factor" for a first approximation of length.

I built one from wire and window line that I pulled up into a tree with great results: www.qsl.net/n1lo/antenna

Having it close to the ground will have a bit more loss but should still be FB.

Have Fun!

--...MARK_N1LO...--
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