[TowerTalk] Best Coax (Matching 50 to 75 Ohms)

Sat, 15 Apr 2000 08:59:24 EDT

Tom, N4KG has a great idea with the series transformer. To convert back to 50 
ohms at the shack end of the 75 ohm cable another transformer can be used in 
reverse. This is a very low Q solution and will easily cover the entire band 
but not can not be used on more than one band with desired results.

The series transformer has other interesting applications as well. Stacked 50 
ohm antennas/beams connected together by equal lengths (length does not 
matter) of 50 ohm coax produce an impedance of 25 ohms. This can be matched 
back to 50 ohms with a series transformer consisting of 75 degrees at 50 ohms 
plus 122 degrees at 75 ohms. Another little known fact is that there are 
actually two solutions to the series transformer calculations. In the 25 ohm 
case, the second solution is 105 degrees at 50 ohms plus 58 degrees at 75 
ohms to return to 50 ohms. If you are conserving your RG11 the second 
solution is desirable.

The loads do not have to be resistive to use a series transformer. Using 50 
ohm and 75 ohm coax for the transformer will allow a 50 ohm match for any 
load vswr not exceeding (75/50) squared which is 2.25:1. Any combination of 
r+-jx producing a vswr of less than 2.25 can be matched with the 50/75 ohm 
coax series transformer.

 I have used these techniques as high as 432Mhz with success and have found 
only one problem so far: The connectors used to connect the matching sections 
have to be accounted for. N type connectors are 50 ohms and the internal 
length has to be added to the 50 ohm part of the solution, even when 
installed on the 75 ohm part of the transformer. PL259 connectors can also be 
used but their impedance is closer to 30 ohms and must  be accounted for in 
the calculations. The designs are easy with computer software but quite an 
exercise done by manual methods!

73 de k5gw

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