Hello,

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?

Thanks!

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

"Ron J" wrote in message
oups.com...
Hello,

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?


No, SWR does not do a simple addition. If it is a power sensor that uses a
short piece of transmission line then you have to know the frequency , the
length in wavelengths, the inpedance of the line, and then do some
calculations. It could be slightly beter or worse than it was to start
with. Probably not enough to show on a simple swr meter.

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 10:18:22 -0800, "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?

Hi Ron,

You leave too many issues unsaid. For one, it seems hardly likely
that you are substituting the load with the sensor, so the sensor must
be on a branch. The branch is going to have to be inserted inline,
and it is going to have its own issues.

You also say nothing of frequency. It doesn't particularly matter as
far as SWR goes, but it goes a great distance in your ability to
flatten the SWR.

You also ask "what would happen?" and that is so open-ended as to be
worthy of a 600 posting thread. Do you really want to be so vague?

You later add that your source can only tolerate a SWR of 1.5 max.
This is too too fragile by half. No doubt you are speaking of
something other that will be perturbed, but I know of no sources that
collapse to their knees under such a mismatch.

And to get back to the nut of the matter, SWRs don't add, at least not
linearly. They would be associated through a root-sum-square process.
However, with only these two numbers, the errors injected by mismatch
are trivial at ±1%.

73's
Richard Clark, KB7QHC

I note the subject matter of this thread is "Insertion Loss"

Insertion loss is the ratio of power delivered to a load, to the power
delivered to the same load when a network is inserted between the load
and the same generator.

With insertion of a passive network, such as a tuner, the insertion
loss can even be a gain.

Calculation or measurement of insertion loss ALWAYS involves or
implies knowledge of the generator impedance. In the present context
the generator is the transmitter.

As nobody knows the internal impedance of the transmitter this thread
will rapidly degenerate into nonsensical argument if it hasn't already
done so.

You have been warned!
----
Reg, G4FGQ

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]