Jerry Martes wrote:

Gary

I have very little confidance in any calculations I do without outside
help. Maybe you can help me. I understand the 100 foot long center fed
dipole antenna is horizontal to the earth and is 35 feet up above it.
At the operating frequency of 3.8 MHz, its input impedance is 17 -j343.
Although I didnt realize the real part of the dipole fell that rapidly with
frequency and nearness to effective ground, I assume these numbers are
accurate.
I'll consider the 39 foot long shorted stub made from 300 ohm twin lead to
be about 0.166 wavelength long (if the velocity of propagation for that line
is 0.9). That makes the length of 300 ohm TV antenna twin lead from the
antenna to the place where the 50 ohm line gets connected, to be about 0.153
lambda. And the remaining 3 feet of 300 ohm line to be 0.012 lambda long.

Would the antenna's impedance plot at 0.057 -j1.14 on a 300 ohm Smith
Chart? Thats what I calculated.

Nope, 29.5 feet (0.127 lambda) around the 300 ohm SWR circle from the 17-j343
antenna, the impedance will be 7-j17 without the stub. In parallel terms, that
is 48 ohms in parallel with 2000 pf (~21 ohms). Neutralize the 2000 pf with an
inductive stub and you have a feedpoint impedance of 48 ohms. You seem to need
about one microhenry for that. Matching can be accomplished with a coil instead
of a shorted stub.

IMO, a better way would be to extend the feedline to 33.8 feet (0.145 lambda)
where the impedance is the conjugate of the above, about 7+j17, and a capacitive
stub will cause the match to 48 ohms. Or a parallel cap of about 0.002 uf will
accomplish the same thing.

These matching methods make use of the 1/50 conductance circles on the Smith
Chart. These easy matching methods will work for any SWR greater than Z0/50,
i.e. for Z0=300, for any SWR above 6:1. At the point where the SWR circle
crosses the 1/50 conductance circle, simply install an appropriate parallel
inductance or parallel capacitance and it twists the impedance at that point
to 50 ohms (48 ohms above). Since that is also a voltage minimum point,
relatively cheap low voltage capacitors can be used, e.g. 600v micas.
--
73, Cecil, W5DXP



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