On Sat, 31 Aug 2013 10:41:24 +0100, Jeff wrote:
Note that the 0.35 dB loss is not converted to heat or dissipated. The
antenna (or coax) does not get warmer because of mismatch loss. All
that happens is that some of the power gets reflected around and does
not get radiated out the antenna.
So the reflected wave is somehow mysteriously exempt from the loss/m of
the coax then!!
Jeff
Coax attenuation is part of a separate loss calculation that does not
involve matching. Mismatch loss is in addition to the coax losses.
Actually, that's not quite right. Mismatch loss is not a real loss,
where RF is converted to heat. It's simply the amount of additional
power that could have been delivered to the load had the system been
properly matched.
Let's try the boundary conditions and see what breaks. The coax
attenuation (in both directions) changes the measured VSWR and
therefore the mismatch loss. For example, if you had a ridiculously
long length of coax, with plenty of attenuation, the reflected RF at
the source is sufficiently attenuated so that the VSWR looks very
close to 1:1. Therefore, there's no mismatch, and therefore no
mismatch loss.[1]
At the other extreme, very short lengths of coax cable, have almost no
effect on the end point VSWR's. For this example, we have a 50 ohm
source, 75 ohm coax, and 50 ohm load. Reduce the 75 ohm coax cable to
near zero length, and there's no coax attenuation. Since the source
and load are matched, there's no mismatch, and therefore no mismatch
loss.
So, by your interpretation, there's no mismatch loss at the boundary
conditions (short coax and very long coax), while there's allegedly
mismatch loss for coax cable lengths in between? I don't think so.
More likely that the mismatch loss is unchanged, no matter how long or
short the cable.
[1] In the distant past, I wired 10base2 ethernet (cheapernet) at
several customers using existing 75 ohm CATV coax cables. 50 ohm
transceivers, 75 ohm coax, and 50 ohm resistive terminators. No
problems (other than crappy crimps). I also played with two 1000ft
rolls of RG-6/u and RG-58a/u. The 75 ohm RG-6/u was better because of
much lower losses (6dB versus 14dB at 10 Mhz).
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558