Jon Ogden wrote:
>>>? To find out if this is really true, why not try it? All that's needed
>>>is a 50 +/- j0 ohm termination and a halfwave of 75 ohm coax (c.10.5 '
>>>for 10m). Measure the SWR at the half-wave and the quarter-wave points.
>>
>>Rich, glad you said it so I didn't have to.
>
>This is correct. Once you change the characteristic impedance of the
>transmission line, you are no longer operating in a 50 Ohm system.
>
>See the problem here is that you have this set up:
>
>A 50 Ohm load, a 75 Ohm transmission line of a half wavelength, and an
>SWR meter based on a 50 Ohm system.
>
>The 75 Ohm line now acts as an impedance transformer.
Let me explain further what I was trying to say:
One can use a length of transmission line that is some value other than
your characteristic impedance (in this case 50 Ohms) and use that to
transform the impedance of a non-50 Ohm load to 50 Ohms.
For example, in stacking 50 Ohm Yagis, the combined impedance of the
Yagis is not 50 Ohms but 25 Ohms. You can then use a 1/4 wave piece of
35 Ohm cable to transform the 25 Ohm impedance at the antenna to a 50 Ohm
impedance at the exciter. In the case of two yagis being parallel fed,
you'd then want to use a 75 Ohm (75/2 ~ 35) 1/4 piece of transmission
line to each yagi and combine them at the other end. Now you have a 50
Ohm system.
My original point was that impedance transformation always occurs with a
change in line length. However, you will never get a 50 Ohm match into
some antenna impedance Za using just 50 Ohm line no matter how long. You
need to use some other impedance and create a 1/2 wave or 1/4 wave
transformer. Once you change line impedances, you are no longer moving
on a constant VSWR circle on the Smith Chart.
The problem with this method for matching antennas, though is that it is
inherently narrow band.
73,
Jon
KE9NA
-------------------------------------
Jon Ogden
KE9NA
http://www.qsl.net/ke9na <--- CHECK IT OUT! It's been updated!!!!!
"A life lived in fear is a life half lived."
--
FAQ on WWW: http://www.contesting.com/ampsfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|