Topband: Inverted L Matching

[k6se@juno.com]

Re Jack, W4NF's inverted vee:

An EZNEC model of the antenna with a good ground system like Jack's
yields a feedpoint resistance of about 25 ohms.  This agrees with his
observation of a 2:1 SWR at resonance on a 50-ohm line.

If the transmitter pi-network output can tune into a 75-ohm load, then an
easy method to lower the SWR at the transmitter would be to use a
1/4-wave length of 50-ohm line between the antenna feedpoint and the
transmitter.  Such an arrangement would yield a 75-ohm SWR at the
transmitter of about 1.3:1 at resonance and the 2:1 SWR bandwidth  would
be from about 1.800 MHz to 1.870 MHz.

If the main object is to get a low SWR when running barefoot (i.e., at
the transeiver rather than the amplifier), be sure to include the total
length of coax from the tranceiver to the antenna (via the amplifier).

If the distance to the antenna is greater than 1/4-wavelength of coax
(about 89 feet for poly, 110 feet for foam), then 3/4-wavelengths will
yield the same results.

73, de Earl, K6SE


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