[Antennas] dipole question

[George, W5YR]

Thanks, Don - thought something looked fishy when I typed that 2400! I
think that my mind took an early Sunday afternoon nap while I was typing
that!  <:}

Don Havlicek wrote:
> 
> Comments inserted below:
> 
> "George, W5YR" wrote:
> >
> > rb wrote:
> > >
> > > Consider a 140' dipole fed with ladderline.  This antenna is OK on 80 and
> > > above, but very marginal on 160, insofar as loading is concerned.
> >
> > OK . . . it's about 100 ft short . . .
> > >
> > > Now, consider the same length dipole, but with a folded horizontal element
> > > made from 450 ladderline, and 450 ohm ladderline feed.  How will this effect
> > > loading down on 160 in relation to the above?
> >
> > It will electrically "lengthen" the physical antenna.
> 
> Are we assuming that the 'folded' portion will simply be a longer dipole
> with the ends folded back to, but not connect with, the feedpoint?  That
> will be a very tricky item to tune on ANY frequency .. considering the
> voltage potentials at the feedpoint, I'd be very wary of it.

It will lengthen it in the manner that K4VX use the 450-ohm line in the
article I referenced. That avoids the problems you refer to . . . 
> >
> > Read the article on PP. 40-42 in the July QST for a good treatment of this
> > configuration. He describes a 45 ft loaded dipole for 40 meters using
> > ladder line for loading. 

I believe there is enough information there to size a 160-meter version.
> > >
> > > OK, let's go to a 140' folded dipole made with 600 ohm ladderline for the
> > > horizontal element, and fed with 450 ohm ladderline.  Will this one load 160
> > > much easier than the simple dipole?
> >
> > First, a folded dipole does not behave any differently from a conventional
> > wire dipole. Adding the second parallel element and connecting them
> > together at the ends just raises the driving-point impedance by a factor of
> > four (number of conductors^2).
> 
> CORRECT [assuming the two wires are identical .. if not, then the
> impedance transformation can be much higher.]

Agreed  . . .he described a two-same-wire situation, but my little formula
looks general when it isn't.
> 
> > So, you don't want to use 600 ohm line for
> > the flattop since that would give you a 2400 ohm input Z with you trying to
> > feed it with 450 ohm ladderline.
> 
> WHOA !!! .. the feedpoint impedance is four times the NORMAL feedpoint
> impedance .. irrespective of the feedline used!  For the 'average'
> folded dipole, the feedpoint imipedance is a nominal 300 ohms .. no way
> is it 2400 ohms!

Absolution duly requested above.
> 
> >
> > Instead make the antenna out of 450-ohm line and that will match 450 ohm
> > line, but nothing magic happens except that the feedline now is terminated
> > at the antennas resonant frequency. You would still need a tuner to get to
> > 50 ohm coax for the transmitter.

And this is off-base, too. The antenna would require 300 ohm line for a
matched termination. 
> > >
> > > Does going from a simple dipole of a given length to a folded dipole of the
> > > same length give us a better flat bandwidth than the simple dipole has?
> >
> > Slightly, but only because of the larger physical "diameter" of the folded
> > dipole. You get slightly more bandwidth but no change in radiation
> > properties from a simple wire the same length.
> 
> CORRECT .. the VSWR curve for the folded dipole is not as acute as for a
> 'regular' dipole.

73/72/oo, George W5YR - the Yellow Rose of Texas         
Fairview, TX 30 mi NE of Dallas in Collin county EM13qe   
Amateur Radio W5YR, in the 56th year and it just keeps getting better!
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