[Elecraft] Antennas

[Ron D'Eau Claire]

Good advice David, but the earth affects the impedance of a dipole. 

When fed at the center a dipole exhibits a 75-ohm impedance in free space.
But, when brought close to the earth (which most HF antennas are,
electrically) the impedance is lowered. At the typical heights most of us
hang our HF wires - say 30 to 60 feet above the earth - the impedance is
closer to 50 ohms, especially on the lower frequency bands.

Of course, "Dipole" refers to the fact that the length of the radiator is
such that a voltage loop (maxima) occurs exactly at each end, hence it has
two "poles". So a simple radiator is a dipole at only one frequency or, for
practical use, one Ham band. 

You can feed a dipole at any point along its length. The impedance is lowest
at the exact center (at the current loop) and will rise as the feed point is
moved toward either end. Center feed is probably the most popular because
its impedance happily coincides with the impedance of common coaxial line.
At this QTH, I have a 130 foot end fed wire in Inverted L configuration
because my "shack" had to go at one end of the only clear run for a wire. It
is a dipole on 80 meters and does exhibit very high feed point impedance. 

That requires a matching network that can handle very high voltages but has
the advantage that very little current flows into the antenna and so very
little current flows into the ground system, resulting in very high overall
efficiency. 

73,  

Ron AC7AC

-----Original Message-----

I get my elecraft mail in journal form so my response is delayed. I had to
mention that people who make broad statements about feed lines and antennas
should check the antenna book and handbook before producing the wrong
information.

Here is an example:

"The impedance of the feed line does not change the impedance of the
antenna.  A half wave length dipole at the proper height is still a 50 ohm
feed regardless of whether it's fed with 50ohm line or 600 ohm line."

Last time I checked a dipole (single wire 1/2 wavelength center fed) is 75
ohms. The part of the statement about impedance of the feed line is correct
but with swr on the feedline the apparent impedance can can different
Happy Holidays everyone.
73
Dave KD1NA