[Antennas] Feedpoint resistance vs. antenna height

2003年2月24日 22:44:54 -0800

In a side-thread of a side-thread of a thread (happens all the time!) on one
of the other antenna discussion groups, someone brought up a question
concerning the feedpoint resistance of a 40 meter horizontal dipole when
erected at various heights above ground. In order to create some pictures
to go along with all the words I modeled the scenario with MultiNEC.
I thought perhaps folks on this list might be interested as well, partly
because the results point out a small error in a chart that appears in the
ARRL Antenna Book and in other antenna reference books. Take a look at
www.qsl.net/ac6la/adhoc/dipole.html . Then compare the chart on that web
page against Fig 1 (pg 3-2) from the 19th edition of the Antenna Book, or
Fig 16 (pg 3-11) from the 17th edition, or Fig 6.17 (pg 115) from Moxon's HF
Antennas for All Locations, or similar figures from other books. The
reference books show the curves for real ground and perfect ground merging
at heights above approximately 0.2 wavelengths (for a horizontal dipole).
But the NEC-2 modeling shows this is not quite true.
Of course this matters not a whit for any real-world dipole you might care
to construct. It's just an interesting little technical tidbit.
73,
Dan AC6LA