Hi all,in addition to the examples Larry offered, the scheme will also
match impedances less than 50 ohms, down to 22.2 ohms if I remember correctly.
A two way power divider where two 50 ohm loads are in parallel requires a
25 to 50 ohm transformer which is easily handled with the series matching
method.
The only down side is the difficulty in adjusting the matching sections. On
the other hand, transmission line calculations via Smith Chart or computer
program will give the exact lengths in degrees needed so tuning is not a
real requirement anyway.
If the sections are joined via coax connectors, you have to take into
account where 75 ohms ends and 50 ohms begins. The connectors are invariably of
a 50 ohm type and their internal length must be added to the 50 ohm part of
the matching section.
The last issue is velocity factor of the coax and connectors. A serious
builder will be prepared to confirm these quantities before cutting the
matching sections to length.
It is a real joy to go through the numbers and have the transformer turn
out right on frequency.
73,
Gerald K5GW
In a message dated 8/1/2009 10:59:13 A.M. Central Daylight Time,
xxw0qe@comcast.net writes:
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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