"Mike Coslo" wrote in message
...
I'm a tad confused about this "reflected power" thing.

I've heard some statements that reflected power is something like
shining a light at a mirror.

But if the power is truly reflected, would not this be an easily
quantifiable thing?

Let's say a signal goes down a wire, "sees a mismatch" and is reflected
from the end. IIRC, light travels around a foot in a nanosecond. So if
you have an 80 meter dipole, and send out a mismatched signal, should
not you get the reflection back to the source in a quantifiable time?
That the signal isn't going quite that fast is only a measurement help.
The reflected signal would be delayed significantly, no?

I can easily visualize the idea that an antenna needs to be matched to
a transmitter, either through the antenna's natural impedance, or
through a network that makes it look like it is. Deviations on either
side of the impedance will cause problems, just as they will with other
systems that are expecting a particular load and getting something
different. But the idea of signals being actually reflected seems hard
to swallow.

I'm a real dilettante in these matters, but why would a signal be
reflected if the impedance was incorrect, and not if it was correct?


Possibly I'm saying some pretty stupid things here.

- Mike KB3EIA -

Mike,
If you really want to see reflections, forget all about sine waves and
steady state. Connect a pulse generator with any output impedance, except 0,
to a piece of coax. Monitor its output with a scope. Now set the pulse
generator to put out pulses whose width is less than the delay time of the
coax. Use any convenient rep rate for the pulse generator ( 1 KHz is good),
and you will easily be able to tell forward going pulses from the reflected.
You will also be able to tell how the far end cable termination determines
how much of the pulse is reflected, and what their polarity is. You do *not*
need a storage scope.

Tam/WB2TT.

Tam/WB2TT wrote:

"Mike Coslo" wrote in message
...

I'm a tad confused about this "reflected power" thing.

I've heard some statements that reflected power is something like
shining a light at a mirror.

But if the power is truly reflected, would not this be an easily
quantifiable thing?

Let's say a signal goes down a wire, "sees a mismatch" and is reflected
from the end. IIRC, light travels around a foot in a nanosecond. So if
you have an 80 meter dipole, and send out a mismatched signal, should
not you get the reflection back to the source in a quantifiable time?
That the signal isn't going quite that fast is only a measurement help.
The reflected signal would be delayed significantly, no?

I can easily visualize the idea that an antenna needs to be matched to
a transmitter, either through the antenna's natural impedance, or
through a network that makes it look like it is. Deviations on either
side of the impedance will cause problems, just as they will with other
systems that are expecting a particular load and getting something
different. But the idea of signals being actually reflected seems hard
to swallow.

I'm a real dilettante in these matters, but why would a signal be
reflected if the impedance was incorrect, and not if it was correct?


Possibly I'm saying some pretty stupid things here.

- Mike KB3EIA -


Mike,
If you really want to see reflections, forget all about sine waves and
steady state. Connect a pulse generator with any output impedance, except 0,
to a piece of coax. Monitor its output with a scope. Now set the pulse
generator to put out pulses whose width is less than the delay time of the
coax. Use any convenient rep rate for the pulse generator ( 1 KHz is good),
and you will easily be able to tell forward going pulses from the reflected.
You will also be able to tell how the far end cable termination determines
how much of the pulse is reflected, and what their polarity is. You do *not*
need a storage scope.

This should be fun, Tam. I have some known bad cable on a mobile
antenna. I'll have to add some more cable to it because it is so short,
but it will be fun to see how close I can get to the crimp in the cable.

- Mike KB3EIA -