[Antennas] dipole question

[Don Havlicek]

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.
> 
> 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.
> >
> > 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.]

> 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!

> 
> 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.
> >
> > 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.

Don
N8DE

> 
> 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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