[Antennas] dipole question

[George, W5YR]

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.

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). 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.

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.

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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