"billcalley" wrote in
oups.com:

What I gleaned from the excellent answers for the original "VSWR
Doesn't Matter?" thread is that high VSWR doesn't really matter in a
lossless transmission line environment between a transmitter's antenna
tuner and the antenna, since any reflected RF energy will simply
continue to "bounce" back and forth between the tuner's output
impedance and the antenna's input impedance until it is, finally,
completely radiated from the antenna without loss.

Well, there will be tuner losses, depending on how good the tuner's
components are.

But then why does the concept of "mismatch loss" exist in
reference to antennas? I have quickly calculated that if a
transmitter outputs 100 watts, and the TX antenna has an impedance
that will cause a VSWR of 10:1 -- using lossless transmission line --
that the mismatch loss in this "lossless" system would be 4.81dB!
(Reflected power 66.9 watts, RL -1.74).

Since mismatch loss is the "amount of power lost due to
reflection", and is as if an "attenuator with a value of the mismatch
loss where placed in series with the transmission line", then I would
think that VSWR would *definitely* matter, and not just for highly
lossy lines either. But here again, I'm probably not seeing the
entire picture here. What am I missing??

Confused!


Yes, you're confused.

If the lossless transmission line (obviously no such animal exists) were
tuned with a lossless tuner, then VSWR would not matter at all.

The loss due to mismatch in any real system will depend upon frequency,
VSWR, type of feedline, length of feedline, and the quality of the tuning
circuits used to match the system to the transmitter.

Let's take your example. 100 watt transmitter into, let's say 100ft. of
feedline at 10:1 VSWR and assume tuner losses are negligble (they often
aren't). Here are the losses for some different kinds of 50 ohm coax at
10mhz:

Belden 8237 2.19db
Belden 9913 1.63db
Belden 9258 3.19db
Belden 8240 3.71db
Belden 9201 3.83db

So, what's obvious here is that different coaxes have different losses at
high SWR. Why is that? Because as power is reflected back and forth in
a transmission line, the losses accumulate. So line that is very low-
loss to begin with will be less affected by high SWR than line that has
moderate to high losses when flat.

If 10 percent of the power in a line is lost travelling from the
transmitter to the antenna, and if the antenna only radiates half that
power, sending the rest back down the line, then 45 percent of the
transmitter power is radiated immediately, while 45 percent is reflected.
But only 40.5 percent reaches the tuner or transmitter. If ALL of that
is re-reflected, then only 36.45 percent of the power is available at the
second reflection to the antenna. The antenna will radiate 18.225
percent of the transmitter's power at this point, making the total 63.225
percent of the transmitter's output. Another 18.225 percent will be
reflected again and, of that 16.4025 percent of the transmit power will
live to be re-reflected from the tuner and 14.76225 percent will arrive
at the antenna on the next bounce. Of that we can expect 7.381125
percent of the transmitter's total power to end up radiated while an
equal amount starts its way back to the tuner. Anyway, it becomes a
pretty simple bit of limit math to predict exactly how much will be
radiated and how much lost in the coax under these conditions.



--
Dave Oldridge+
ICQ 1800667