On Apr 22, 1:35*pm, Owen Duffy wrote:
Cecil Moore wrote in news:d6ea0bf4-7bb4-4089-9c88-
:

What happens when two paralleled transmission lines are of different
lengths? How does one analyze such a problem? For instance, what
impedance is seen at the source when a 30 foot run of 450 ohm ladder-
line is paralleled with a 20 foot run of the same line when the common
load is 100+j200 ohms? Will EZNEC handle such a problem? What if the
two different length paralleled transmission lines are not the same VF
and/or not the same Z0? Does superposition work for such a problem?

Here is an opportunity for you to do some leg work Cecil.

I have modelled in RFSIM99 a load of 100 ohms in series with 9.1µH at
3.5MHz with 30' of lossless 400 ohm line, vf=0.9, and 20' of lossless 450
ohm line, vf=0.8 as you describe. The input impedance is 767+j296.

Do you get the same result with a valid model in EZNEC?

Owen

Remarkably, Owen, I get exactly the same result using LTSpice.

Spice and RFSim99 are the two that came to mind when I wrote about
more appropriate modeling tools yesterday... It might be fun to find
the general solution to the set of linear equations relating the
forward and reverse voltage and current at each end of each line and
in the load, for a given excitation, to end up with a relatively
simple (?) formula for translating the load impedance to the impedance
seen at the input end of the lines, and maybe that already exists in a
text or reference book somewhere, but there are a few too many
equations in the set to get me interested in doing it by hand.

It's also interesting that in both this example and in another
different one I tried yesterday, I found that, at some frequency
(16.993MHz for Owen's example), the impedance at the input goes to
infinity. That is to say, there is no current in the load: the load
is at a point along the loop of two lines that is at a zero-voltage
point of the standing wave. It shouldn't be a surprise that this
happens, but you might not think about it when just blindly connecting
two lines of different lengths "in parallel," and end up getting
bitten by it.

Cheers,
Tom