Hi Ian & Carl,
Actually given only 50 ohm and 75 ohm coax and not paralleling any coax
you can match all resistive impedances between 50 and 112.5 (75*75/50) ohms.
e.g.
50 ohms ---- 29.3 deg. 75 ohm coax ----- 29.3 deg 50 ohm coax ----- 75
ohms (as shown by the 1/12 wave match)
50 ohms ---- 40.5 deg. 75 ohm coax ----- 23.8 deg 50 ohm coax ----- 85 ohms
50 ohms ---- 58.3 deg. 75 ohm coax ----- 14.9 deg 50 ohm coax ----- 100 ohms
50 ohms ---- 90.0 deg. 75 ohm coax ----- 0.0 deg 50 ohm coax -----
112.5 ohms (std 1/4 wave match)
Also many reactive impedances are also easily matched
e.g.
50 ohms ---- 42.6 deg. 75 ohm coax ----- 10.3 deg 50 ohm coax ----- 75 -
j20 ohms
For a matching over a small range of frequencies transmission lines work
well.
73,
Larry, W0QE
Ian White GM3SEK wrote:
> Carl wrote:
>
>> Does anyone remember the formula for matching 50 to 75 Ohms with 2 very
>> short lengths of coax?
>> If I remember it used a 75 Ohm section at the output of the amp, and a 50
>> Ohm section to the CATV hardline. Frequency dependent of course.
>>
>> Its been about 25 years since I built one.
>>
>>
>
> http://www.tuc.nrao.edu/~demerson/twelfth/twelfth.htm
>
> By paralleling two lengths of 50 or 75 ohm line, it can also match to 25
> or 37.5 ohms; or to even lower impedances by paralleling three lengths
> of the same line.
>
>
>
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