K6XK/0:
> First, it's 25 ohms. And a simple shunt fed inductor at the feedpoint
> is very simple, easy and cheap to construct and adjust there. Also
> known as a Z match, it connects across the feedpoint with the inner
> conductor of the coax at its top. Just trim for 1 to 1 SWR. Ball park
> is maybe 5 turns of 3 inch diameter air core or a few turns on a T-200
> toroid core.
Hi Roy,
That only works with a unity match when the antenna is shorter
than 1/4 wl long, so it presents capacitive reactance at the antenna
terminals.
The antennas capacitive reactance forms the "C" part of an L
network (series C, shunt L), with the impedance step-down towards
the antenna.
Unless he is willing to shorten the antenna, he will need to use a
series capacitor of 3500 pF between the antenna and a shunt coil
of 4.35 uH across the feedline. Loss will be 0.5% with an inductor
Q of 200 and a normal capacitor.
He could invert the sign of the components and use a series
inductance (or make the antenna longer) and use a shunt
capacitance across the feedline. The series inductance would be
2.17 uH and the shunt capacitance (across the feedline) would be
1740 pF. If the same Q components are used the "no free lunch"
rule kicks in and loss is again the same 0.5%!!
How we match the system would be mostly a matter of what we
have laying around, or can get easily. The end result is always
about the same, the is no free lunch or magic.
73, Tom W8JI
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