It travels exactly the same distance in both directions.
That is why you can measure exactly the same SWR at any point on the
line. That is except for the loss that the line introduces. But the
relative phase of forward and reflected waves remain constant.

Regards
Gary


On Thu, 20 Nov 2003 19:08:37 -0500, "Doug Dotson"
wrote:

Wouldn't phase be a problem? Hard to believe that a signal can
be reflected back from the antenna and then reflected back from the
transmitter to the antenna and will be in phase well enough to actually
do any good.

Doug, k3qt
s/v Callista

"Gary Schafer" wrote in message
news
SWR in itself is not necessarily bad. Power reflected back toward the
transmitter is not lost as a result of the reflection itself. When
that reflected power hits the transmitter it is re-reflected back up
to the antenna.

So a 3:1 swr with 6.25 watts of reflected power and 25 watts of
forward power, still delivers 25 watts to the antenna to be radiated.
That is of course when there is no feed line loss.


With feed line loss involved (as there always is) you will get a false
SWR reading. The more loss your cable has the better your SWR will
look.
This is because not only is there less power to reach the antenna that
is causing the reflection, but also there is less of the reflected
power that gets back to your SWR meter. It gets lost in the coax both
ways.

So if you have 3 db of line loss and your antenna has a 3:1 SWR, you
will only read it as about 1.4:1.

WHY:
With 3 db of line loss only 12.5 watts will make it to the antenna.
With a 3:1 SWR at that point 25% of the 12.5 watts (or 3.125 watts)
will be reflected back towards the SWR bridge. But the 3.125 watts
coming back down the cable will also undergo the 3 db of cable loss so
only .78 watts will show up as reflected power back at the SWR bridge.
That gets compared with the full 25 watts that the input of the SWR
bridge sees. The bridge will tell you that you have an SWR of only
about 1.4:1 when it is really 3:1!

Further, most SWR bridges are quite inaccurate at low readings. In the
area of 10 to 40%.

Even the revered Bird watt meter is not very accurate when reading on
the low end of the scale. It has a published accuracy of +- 5% of FULL
scale. Measuring a 25 watt radio, a 50 watt slug is normally used. 5%
of 50 watts is +-2.5 watts. Trying to read that 1.4:1 reflected power
level of .78 watts with a meter that has an error of +- greater than 3
times the level that you are trying to read leaves you guessing at
best!

Users of watt meters measuring reflected power often make the mistake
of seeing a few watts of reflected power and thinking things are "ok"
and not really calculating what they have, as the "reflected part is
rather low compared to the forward power". They equate it to an SWR
meter position comparison.

Bottom line is when using a watt meter on VHF to look at reflected
power, if the reflected indication is more than a division or two on
the meter you probably have to high an SWR.

With a short antenna cable, 20 or so feet, 1.5:1 SWR is about the
limit. With a longer cable, depending on its loss, The acceptable
limit may be much lower as seen above.

Some transmitters start shutting down their output at around 1.8:1
SWR.
So the problem with high SWR is not so much one of added loss as it is
a problem of the mismatch that the transmitter sees and reduces its
output power. Although with a high loss feed line you end up with less
of the reflected power to be re-reflected back up to the antenna.

Regards
Gary