[Elecraft] Slinky Question

[Vic Rosenthal]

Mike McCoy wrote:
> 
> > If you need to
> > make a dipole shorter than 1/2 wl one good way to do it would be to use
> 'linear
> > loading':
> >
> >    ------------------------+   +-----------------------
> >              +-------------+   +------------+
> >              +------------->   <------------+
> >                          feedpoint
> >
> > You can make the folded sections out of ladder line.
> >
> 
> I just tried building such an antenna out of 450 ohm 'windowpane' ladderline
> per the above and it doesn't work as designed/expected either. I fed the
> center matched to some RG8X with a 4:1 balun, insulated the ends (at the
> middle) of the 'top' wires and shorted the far ends. I cut it for 40 mtrs
> (about 33' overall) and it worked best on 17mtrs (1.4:1 SWR)!

You can't just use the same overall length.  Someone with a modelling program
can work out the exact dimensions, but I would guess that if your antenna is
near resonant at 17 meters, it is probably too long -- since the 17 meter
resonance is most likely at the third harmonic.  You would probably find the
antenna resonant at 6 MHz as well.  If you shorten the 'folded' section equally
on both sides, you will end up with an antenna that works on 40 and 15 meters.

> The lesson I (think I) learned was if you are going to make a folded dipole
> the wires need to be at least 6-8" apart (and not the 1" or so of 450 ohm
> 'windowpane' type ladderline). 

This isn't a folded dipole!  The top section is a regular dipole, with inserted
'folds' to make it longer.  You could let the folds hang vertically, like this:

  --------------------+  +----> <----+  +----------------------
                      +  +           +  +
                      +  +           +  +
                      +  +           +  +
                      +--+           +--+

Vic K2VCO