Ron J wrote:
I was curious. If a matching network was designed to make the SWR at a
band of frequency less than 1.4 to 1, then what would happen if I
inserted a power sensor on the line with a rated SWR of 1.05 to 1 at
this frequency band?

Would that make my overall system SWR 1.4 + .05 = 1.45 to 1?

SWRs, like power, cannot be superposed.
--
73, Cecil http://www.qsl.net/w5dxp

Hi Cecil,

Thanks! I try to explain the affect of the power sensor even if it is
ideally very minimal. My main concern is that at the band of design
frequency the transmitter is only designed to handle 1.5 SWR max. I'm
afraid that I might break it since our matching network already changes
the SWR to a peak 1.40 to 1 at certain frequencies on the band.

I may have other idea. If I add the return loss of the devices, would
that allow me to somehow find the SWR?

Thanks!

Ron J wrote:
My main concern is that at the band of design
frequency the transmitter is only designed to handle 1.5 SWR max. I'm
afraid that I might break it since our matching network already changes
the SWR to a peak 1.40 to 1 at certain frequencies on the band.

I may have other idea. If I add the return loss of the devices, would
that allow me to somehow find the SWR?

Measure the impedance of the system (using low power). That's the only
way to know for sure.
--
73, Cecil http://www.qsl.net/w5dxp

On 20 Dec 2005 10:42:14 -0800, "Ron J" wrote:

Hi Cecil,

Thanks! I try to explain the affect of the power sensor even if it is
ideally very minimal. My main concern is that at the band of design
frequency the transmitter is only designed to handle 1.5 SWR max. I'm
afraid that I might break it since our matching network already changes
the SWR to a peak 1.40 to 1 at certain frequencies on the band.

I may have other idea. If I add the return loss of the devices, would
that allow me to somehow find the SWR?

As others have told you, no, the system isn't analysed as simply as
you suggest.

Some thoughts:
- your concern that your transmitter will suddenly degrade at VSWR=1.5
is probably unfounded;
- if your power sensor is intended for inline monitoring, it should be
designed to "monitor" with insignificant impact on the load presented
to the transmitter.

You didn't tell us enough to give you a definitive answer, but it is
likely that you are unnecessarily worried about the sensitivity of the
transmitter to VSWR, and the cumulative effect of the monitoring
sensor which should be insignificantly small in the real world with
suitable / practical components.

Owen
--

On Tue, 20 Dec 2005 18:28:31 GMT, Cecil Moore wrote:

Ron J wrote:
I was curious. If a matching network was designed to make the SWR at a
band of frequency less than 1.4 to 1, then what would happen if I
inserted a power sensor on the line with a rated SWR of 1.05 to 1 at
this frequency band?

Would that make my overall system SWR 1.4 + .05 = 1.45 to 1?

SWRs, like power, cannot be superposed.

Because the worse case SWR is often the concern, it can in fact be
found, not by addition but by multiplication.

In the case given, the worse case SWR = 1.4 * 1.05 = 1.47:1, an
insignificant change from the original due to the insertion of, what I
believe is an in-line sensor.

Of course, it is equally likely that the insertion of the power sensor
will -improve- the SWR.

Consider both SWRs in terms of their equivalent reflection
coefficients, rho.

swr - 1
rho = -----------
swr + 1

Letting swr = 1.4, rho = 0.1667

With swr = 1.05, rho = 0.0244

Worse case = 0.1667 + 0.0244 = 0.1919

Converting back to SWR = 1.473, as given above.

Best case = 0.1667 - 0.0244 = 0.1423

Converting back to SWR = 1.331

Without Gamma, which has phase info, all we know are the error limits,
the SWR is somewhere between 1.331 and 1.473.

Note: If we could make the swr of the sensor = 1.4 and put it in the
right location in the line, the net swr = 1:1. This is commonly
called a -matching network-. [g]