[aprssig] What a diffeence an antenna makes

[Stephen H. Smith]

jdw at eng.uah.edu wrote:

>
> On Oct 31, 2005, at 11:43 AM, Stephen H. Smith wrote:
>
>> A half-wave's maximum radiation lobe is straight out toward the 
>> horizon (perpendicular to the wire) while a quarter wave's maximum 
>> lobe is about 15-20 degrees above the horizon,  actually making it 
>> better for things that are in the air rather than on the ground.
>
>
> what does the J-pole radiation pattern look like?
>
> We've been flying roll-up J-poles on balloons recently.  They've 
> performed well, but there is a noticeable null directly under the 
> balloon.
>
> -Jason
> kg4wsv
>
>

A J-pole is simply a half-wave dipole fed from the end, with a stub 
matching system,  instead of the center.  It has exactly the same 
radiation properties as any other 1/2-wave dipole, meaning the max 
radiation is straight out horizontally toward the horizon, with a 
substantial null directly downwards.       The only difference is that 
mechanically, it is easier to support and feed a vertical dipole from 
the end instead of the center,   and you don't need an insulator in the 
center of the dipole as required for the classic direct center feed. 

Actually an UPSIDE down quarter-wave ground plane (either a 1/4-wave 
whip and 4 radials, or a 1/4-whip agains a metal plate or maybe the 
bottom of a metal electronic equipment box) will actually work better 
than a 1/2-wave, since the 15-20 skew from horizontal will now be 
downward rather than upward.   However, you MUST provide a decent ground 
plane for this to happen.     (This is exactly the reason why VHF/UHF 
quarter-wave whips are mounted on the belly of aircraft instead of the 
upper side of the fuselage. )



Stephen H. Smith    wa8lmf (at) aol.com
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