Topband: Inverted L Matching

[Tom Rauch]

> 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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