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 Coslo wrote:
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?

Dunno why, but that's just the way it is. To see for yourself,
feed a 75 ohm TV through 1000 ft. of 75 ohm coax. Then feed
the same TV through 1000 ft. of 600 ohm ladder-line. How do
you explain no ghosting in the first case and lots of ghosting
in the second case? Seeing is believing.
--
73, Cecil http://www.qsl.net/w5dxp



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They make an instrument called a Time Domain Reflectometer,
you hook it up to transmission lines and it sends out a pulse
and times how long it takes to get back.
The time it takes, reflected pulse polarity, and height says a lot about the
line.
Used for finding faults on telco/cable lines.
Does the same thing with RF lines.
Matched well, no pulse, but you can even see connectors on some of these
TDR's
Googleit, too

"Cecil Moore" wrote in message
...
Mike Coslo wrote:
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?

Dunno why, but that's just the way it is. To see for yourself,
feed a 75 ohm TV through 1000 ft. of 75 ohm coax. Then feed
the same TV through 1000 ft. of 600 ohm ladder-line. How do
you explain no ghosting in the first case and lots of ghosting
in the second case? Seeing is believing.
--
73, Cecil http://www.qsl.net/w5dxp



-----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
http://www.newsfeeds.com - The #1 Newsgroup Service in the World!
-----== Over 100,000 Newsgroups - 19 Different Servers! =-----

* wrote:

They make an instrument called a Time Domain Reflectometer,
you hook it up to transmission lines and it sends out a pulse
and times how long it takes to get back.
The time it takes, reflected pulse polarity, and height says a lot about the
line.
Used for finding faults on telco/cable lines.
Does the same thing with RF lines.
Matched well, no pulse, but you can even see connectors on some of these
TDR's
Googleit, too

Fascinating stuff! Why don't we use them more for ham radio? this page:
http://www.epanorama.net/circuits/tdr.html even has a diagram for a
homebrew pulse source and diagrams for hooking up same to an
oscilloscope. While not giving SWR, it would show up any connector
faults and crimps in the cable.

- Mike KB3EIA -

"Mike Coslo" wrote in message
...

* wrote:

They make an instrument called a Time Domain Reflectometer,
you hook it up to transmission lines and it sends out a pulse
and times how long it takes to get back.
The time it takes, reflected pulse polarity, and height says a lot about
the
line.
Used for finding faults on telco/cable lines.
Does the same thing with RF lines.
Matched well, no pulse, but you can even see connectors on some of these
TDR's
Googleit, too

Fascinating stuff! Why don't we use them more for ham radio? this page:
http://www.epanorama.net/circuits/tdr.html even has a diagram for a
homebrew pulse source and diagrams for hooking up same to an
oscilloscope. While not giving SWR, it would show up any connector
faults and crimps in the cable.

- Mike KB3EIA -

i use one regularly in my station here. most people don't use them because
a simple swr meter is adequate for testing most antenna./feedline
combinations in use by amateurs.

Dave,

What happens when you ping your antenna? Clearly the spectrum of the pulse
is wider than the bandwidth of a typical antenna, but is there any useful
information?

Tam/WB2TT
"Dave" wrote in message
...

"Mike Coslo" wrote in message
...

* wrote:

They make an instrument called a Time Domain Reflectometer,
you hook it up to transmission lines and it sends out a pulse
and times how long it takes to get back.
The time it takes, reflected pulse polarity, and height says a lot
about
the
line.
Used for finding faults on telco/cable lines.
Does the same thing with RF lines.
Matched well, no pulse, but you can even see connectors on some of
these
TDR's
Googleit, too

Fascinating stuff! Why don't we use them more for ham radio? this page:
http://www.epanorama.net/circuits/tdr.html even has a diagram for a
homebrew pulse source and diagrams for hooking up same to an
oscilloscope. While not giving SWR, it would show up any connector
faults and crimps in the cable.

- Mike KB3EIA -

i use one regularly in my station here. most people don't use them
because
a simple swr meter is adequate for testing most antenna./feedline
combinations in use by amateurs.

"*" wrote in message ...
They make an instrument called a Time Domain Reflectometer,
you hook it up to transmission lines and it sends out a pulse
and times how long it takes to get back.
The time it takes, reflected pulse polarity, and height says a lot about
the
line.
Used for finding faults on telco/cable lines.
Does the same thing with RF lines.
Matched well, no pulse, but you can even see connectors on some of these
TDR's
Googleit, too

Sorry, *, won't work. TDR only good on wide band systems. Too messy
for this kind of antennas.
--
Steve N, K,9;d, c. i My email has no u's.

"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 -

Mike, KB3EIA wrote:
"But the idea of signals being actually reflected seems hard to
swallow."

It`s true that antennas are measured in degrees. A 1/4-wave antenna is
90-degrees. This is based on how many degrees the signal has changed at
the antenna input by the time the signal reaches the tip end of the
antenna.

When signal reaches the end of a 1/4-wave whip, there is an abrupt
discontinuity. Only displacement current flows without conductors. The
tip of the whip accepts only a small fraction of the current available
in the forward power traveling on the whip. The only path for the
rejected power is back toward the feedpoint. Direction and phase of the
current reverse in the reflected power, but the voltages of the incident
and reflected waves have the same phase (see 1955 Terman, page 89).

Best regards, Richard Harrison, KB5WZI