In article
,
AndyS wrote:

Andy writes:

I have a need for a quarter-wavelength transmission line at 25 ohms
impedance.

My SPICE simulator tells me that I can connect two 50 ohm, quarter-
wavelength
transmission lines in parallel and get the exact same performance
as the needed
25 ohm line.

However, I have not seen anyone try this, nor seen it used in
equipment.

Does anyone here have any experience with doing this, or have any
learned
opinions as to why it might, or might not, be a good idea ??


In article
,
steveeh131047 wrote:

It works well.

It has to work well.

Suppose first that you connect a load with impedance Z to the far end of a
quarter wave transmission line of 25 ohm surge impedance. At the near end of
the quarter wave transmission line this will look like 25^2/Z ohms, or 625/Z
ohms.

Now start over, but instead of the load with impedance Z suppose you have
two identical loads, each of impedance 2*Z, which if they had been connected
in parallel would have had an impedance Z. Take two different quarter wave
transmission lines, each of surge impedance 50 ohms, and connect the two
loads (separately) to the far ends of the two transmission lines. At the
near ends of each of the transmission lines this will look like 50^2/(2*Z)
ohms, which simplifies to 1250/Z ohms. Connect the near ends of the two
transmission lines in parallel. You will then see 625/Z ohms, the same as
you saw with the Z ohm load at the end of the quarter wave 25 ohm line.

By symmetry the voltages across, and the currents through, the two 2*Z ohm
loads have to be the same. So nothing will change if they are connected in
parallel. Do that and you will have a single Z ohm load at the far end of
the two paralleled quarter wave 50 ohms lines.

David, ex-W8EZE

--
David Ryeburn

To send e-mail, use "ca" instead of "caz".