[AMRadio] Inverted V antenna |
Donald Chester
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 horizontal. Don k4kyv -----Original Message----- From: AMRadio [mailto:amradio-bounces at mailman.qth.net] On Behalf Of CL in NC via AMRadio 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.
This page last updated 21 Feb 2018.