[Antennas] Loading a shortened dipole

2003年10月22日 07:07:53 -0700

 >The best suggestion is to take your loaded 40 meter dipole [coils at 
each end] and feed it with 450 ohm open wire into a tuner. >This will 
operate just fine on all bands between your lowest frequency [40 meters] 
all the way up to 2 meters. It's cheap, it >works well and you have a 
clean looking multiband antenna.
>>My understanding is that as a general rule, leaving the loading coils <br>near the ends of the antenna should be more efficient. However the <br>coils will have to be larger (more turns) as you move away from the <br>center and toward the ends.
>
 This topic is the subject of an interesting article in October, 2003 QST.
The article entitled "Designing a shortened antenna" is on page 28
and is authored by Luiz Lopes, CT1OJ
.
 The writer sindicates that one should not place loading coils any 
closer to the
center of the dipole than 30 degrees each side of the feedpoint
because that's where the curent is highest (of course) thus it would be
excessively lossy, as mentioned here. Also placing them too close to the 
ends
demands too much inductance to so that isn't a good option.
(it would seem to cause the antenna to be more narrowbanded for the
 same reason, as well.)
 He breaks the half wave dipole down into six 30 degree elements
(half the 360 degrees of a fullwave)
and recommends placing the loading coils at 30 degrees from the center
feedpoint for a good compromise between efficiency and bandwidth.
 While I haven't seen it presented in just this way previously
that seems to have been about how most loaded antennas that I've seen
in the past have been built.
 Of course the segments located outside the coils will no longer
be 30 degrees in linear measure but will be 30 degrees electrically.
 The article is worth reading for anyone anticipating building
a loaded dipole.
73, Dick W0EX