> 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.
Since the systems we use are all normalized to 50 ohms, the
SWR on a length of 50 ohm cable is constant along its entire
length except for loss. The SWR, normalized to 50 ohms, would be
2:1 no matter how long the cable is (ignoring losses, which we can
do on 160).
A 50 ohm radio, which virtually all radios are designed to be, would
still see a 2:1 SWR normalized to its output impedance. The worse
thing you can do to a rig is underload the output by terminating it
with a higher-than-design impedance.
Correct matching using transmission lines would occur with a pair
of 75 ohm 1/4 wl lines in parallel. That would give a line impedance
of 37.5 ohms, with a line-to-antenna SWR of 1.5:1 normalized to
37.5 ohms. The transmitter-end SWR would be the same, but
impedance is inverted (1/4 wl away) to 56.25 ohms. The 50 ohm
normalized SWR would be 1.13 :1, providing an excellent match.
There are other transmission line transformers that will work, but
the pair-of-parallel 75 ohm lines is the most simple and has
excellent bandwidth. Coax is also readily available.
Other than transmission lines, a 2:1 impedance ratio transformer
could be used or an "L" network. Any of the three methods are
good.
73, Tom W8JI
w8ji@contesting.com
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