|[AMRadio] Inverted V antenna|
k4kyv at charter.net
Mon Jan 25 14:41:34 EST 2016
A half wave dipole in free space has a feedpoint Z of about 75 ohms. Ground
reflections may affect impedance with a real world antenna a finite distance
above the ground, but the impedance of an inverted vee will always be lower
than that of a straight dipole.
Imagine you have a half wave horizontal dipole some distance above ground.
It will exhibit a certain impedance at the feed point, maybe not exactly 75
ohms depending on its height above ground, but somewhere in that ball park.
Now, imagine you have a quarter-wave section of open wire line, suspended
horizontally at the same height with nothing connected to the far end.
Measure the impedance at the near end. The exact value will vary according
to the characteristic of the line, but it will measure very low Z.
Fan the two conductors out, from being close-spaced and parallel (OWL) until
they are spread out completely horizontally. In doing this, you are
changing the configuration from a quarter-wave section of parallel line, to
a vee-shaped dipole, and finally to a straight line dipole. The Z will
start off at a very low value, and increase as the wires are fanned out. It
stands to logic that at some angle between zero degrees and 180 degrees, the
Z will measure exactly 50 ohms.
This same phenomenon explains the increase in Z with a ground plane, when
drooping the radials down from 90° off the radiator to 45° from the
From: AMRadio [mailto:amradio-bounces at mailman.qth.net] On Behalf Of CL in NC
My understanding of the inverted VEE in addition to changing the pattern
angles is that it lowers the feed point towards 50 ohms. I don't know if
the first fellow to figure it out calculated that or he experimented by
putting up a single support antenna, let the ends droop and said, "Hey,
neat, it matches coax perfectly". If the ground is a determining factor for
the Z change, then making it vertical may do something strange. It may act
like a two direction sloper. Ground plane antennas do the same thing, they
present a 35 ohm load like a vertical when the radials are 90 degs off the
radiator, but let them droop down to 45 degs from horizontal and you can
feed it with 50 ohm cable.
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