On Jan 21, 11:07*am, wrote:
I have a question about how to calculate VSWR/ReturnLoss of a hybrid
antenna/feed line sysem which include bottom jumper, pressure window,
wave guide, upper jumper and antenna? I list the parameters of all the
components as bellow for an actual instance:
* * * * * * * * * * * * * * * * *VSWR * *InsertionLoss
Antenna * * * * * * * * * * 1.06
Antenna Radome * * * *1.01 * * * *0.4
Top Jumper * * * * * * * *1.03 * * * *0.09
Waveguide * * * * * * * * 1.06 * * * *2.36
Pressure Window * * * 1.01
Bottom Jumper * * * * * 1.03 * * * *0.09

Could somebody help me get the ReturnLoss/VSWR at the end of Bottom
Jumper for the entire system? *Thanks.

The problem is that you need more information than you have here.
Let's reduce this to the case of just two components, a load that
shows a 1.10:1 VSWR relative to the system reference resistance
(commonly 50 ohms), and a transmission line that shows a 1.10:1 VSWR
at the "input" end when terminated with the system load resistance at
the other end. But the 1.10:1 can represent either of two
resistances, or any of an infinite set of reactance + resistance that
represent a constant magnitude but arbitrary phase angle reflection
coefficient.

For example, with a system reference resistance of 50 ohms, the 1.10:1
SWR load might be 55 ohms. And the transmission line might be 90
electrical degrees of 52.45 ohm line (well within tolerance for RG-8
type 50 ohm line); it would show 1.10:1 SWR with a 50 ohm load. But
if you put the 55 ohm load at the end of that line, you'd measure
1.00:1 SWR at the input end of the line. On the other hand, the line
could just as well have been 90 electrical degrees of 47.58 ohm line,
which would also show a 1.10:1 SWR with a 50 ohm load at the far end;
but if you put your 55 ohm load on that line, you'd see a 1.21:1 SWR
at the input end of the line. In this example, I've just used things
that evaluate to pure resistances; in practice, they could also
include reactance.

So in summary, without more information about each component, you
can't arrive at an unambiguous answer to your question. You really
should characterize each one-port in the system with its S11, and each
two-port with its full S-matrix; then you have enough information,
ASSUMING no other coupling than at the ports you used to characterize
each piece. But even that's not always the case--and commonly isn't
the case for antenna systems.

Cheers,
Tom