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Old January 24th 06, 08:52 PM posted to rec.radio.amateur.antenna
K7ITM
 
Posts: n/a
Default Help with J antenna design

Careful about assuming that a 1/4 wave stub will do the trick...

To match a 50 ohm line to a 2500 ohm load using a shorted stub at the
transmission line end and a section between the 50 ohm line and the
2500 ohm load requires (using sections shorter than 1/4 wave for each):

50 ohm stub & section: 8.2 degrees & 81.9 degrees
75 ohm: 8.3 & 82.1 degrees
100 ohm: 8.5 & 82.2 degrees
150 ohm: 9.0 & 82.6 degrees
200 ohm: 9.8 & 83.3 degrees
300 ohm: 15.0 & 85.7 degrees
350 ohm: 45 & 88.9 degrees
353.55 ohm: 90 & 90 degrees

Note that the last on is the case of not using a stub (90 degree stub
looks like an open circuit). The 1/4 wave line impedance should be
sqrt(Zin*Zout).

You can adjust the length to tune out reactance, too.

But if you're matching resistive load to resistive line, the total
length of the stub part and the series part will be longer than 1/4
wave, and more so as the impedance of the stub and series part rise.
You can't use this arrangement if the stub and series section
impedances are higher than sqrt(Zin*Zout). SO--beware of spacing the
two too far apart!

Cheers,
Tom