A lot of hams use the balanced output from their T-net antenna
tuner to drive their balanced transmission line. This is not
an optimum configuration since the 4:1 balun in tuners like the
MFJ-949, tries to divide the impedance by 4.
Let's say we have a 102 ft physical-dipole fed with 450 ohm ladder-
line. At the Imax (current loop) we see 25 ohms. Our balun is designed
for 200 ohms. It tries to transform 25 ohms to 6.25 ohms which is not
an efficient point for the balun or tuner.
Away from the Imax point, reactance starts to mount, something our
balun is not designed for. So in this common antenna system, how do
we solve the problems? (This example is on 7.2 MHz with a 100 ft
feedline.)
If we put a 285 pf parallel capacitor across the balanced terminals
of the tuner, we will achieve a resistive 200 ohms because we are
placing it 100 ft from the antenna at a PC-200 point. The balun likes
the 200 ohms and the tuner likes the resulting 50 ohms. We can do this
for our favorite frequency and use the tuner to achieve a match when
we tune away from this center frequency as the SWR slowly rises. But
the impedance seen by the tuner around this center frequency is well
within its efficient matching range.
What we have accomplished with a single capacitor makes the tuner see
the same impedance that it would see with a 50 ohm half-wave antenna
fed with 50 ohm coax, i.e. the tuner cannot tell the difference
between our non-resonant length antenna and the resonant dipole.
Actually, our 102 ft physical-dipole is better because the half-wave
dipole impedance is not likely to be exactly 50 ohms but we force our
102 ft system to be exactly 200 ohms AND we can do it for all the HF
bands.
73, Cecil, W6RCA, OOTC
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