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> Antenna Balun Positioning
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Posted: May 04, 2012 11:53 am
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I need to connect a VHF dipole to a transmitter using coaxial cable, using a balun made from a quarter wavelength of coax inside a grounded copper pipe. The antenna is made from two lengths of wire which run along a wooden boom, joined to the feedline in the center. Ideally I'd like to fix the balun to the back of the boom, parallel to the cable. Will this affect the radiation pattern or input impedance of the antenna? Or would it be better to have the balun at 90 degrees to the antenna?
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Geek
Posted: May 04, 2012 01:12 pm
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As close to the antenna is where the balun belongs!

Radiation pattern can only be improved with a balun. Just hang the stub 90 degrees (of flatside, let it droop. If vertical, orient it with a string or something) to the elements and you're set smile.gif

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Posted: May 04, 2012 04:45 pm
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Maybe a diagram will clarify things

user posted image

What I'm asking is is it OK to have the balun parallel to the antenna elements? It would be easier to build and smaller, but I'm concerned that it might skew the radiation pattern.
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Sch3mat1c
Posted: May 04, 2012 09:04 pm
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In principle, the connection is supposed to be perpendicular to the elements. Running them both in parallel on the same beam suggests the lower element will induce voltage on the cable, which will be current another quarter wavelength along, and so on. It might be okay if you place a big ferrite bead on the feedline 1/2 wave from the feedpoint (keep the balun as close to the feedpoint as possible).

Tim


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Posted: May 04, 2012 10:50 pm
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Use the one on the right.


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tekwiz
Posted: May 05, 2012 12:27 am
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Yep, the balun must be at 90 in order to prevent interference with the elements. The antenna is balanced & placing the balun in any other position would upset the balance. This would indeed skew the pattern; not to mention the match. wink.gif


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CWB
Posted: May 05, 2012 01:09 am
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yeppers ...
90 degrees in relationship to the radiating elements .

if you are making a horizontal dipole configured like a "T" , just drop the balun section vertical .

if you are going for a vertical omni pattern you might try a simple vertical ground plane instead .
a colinear antenna made from several sections of coax is an option as well .

what frequency ?


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Posted: May 05, 2012 06:56 pm
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QUOTE (tekwiz @ May 04, 2012 11:27 pm)
Yep, the balun must be at 90 in order to prevent interference with the elements.

Yeah that's what I expected. Thanks all.
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CWB
Posted: May 06, 2012 06:11 am
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make sure to run the coaxial line straight back for at least 1 wavelength before dropping it to vertical .
a few wavelengths would be better .


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circuitfella11
Posted: May 08, 2013 08:06 am
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QUOTE (atom @ May 05, 2012 12:45 am)
Maybe a diagram will clarify things

user posted image

What I'm asking is is it OK to have the balun parallel to the antenna elements? It would be easier to build and smaller, but I'm concerned that it might skew the radiation pattern.

i wondered on that too. from experience it really screws up the radiation pattern. the signal from the perpendicularly built balun gives way better reception rather than the parallel one.

despite the space consumed by the antenna, you're left with two options, perpendicular to the elements, or parallel, more than a wavelength from the elements.

i think putting it perpendicular saves the space, saves also the materials to build it.
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