On Fri, 30 Aug 2013 11:03:22 -0400, "Ralph Mowery"
wrote:
"Jeff Liebermann" wrote in message
.. .
However, what was referring to was mismatch loss.
Voltage_reflection off of a 75 ohm input:
Vr = (50-75)/(50+75) = 0.2
Voltage transmission into 75 ohm input:
T = 1 - (0.2^2) = 0.96
Converting to decibels, the loss will be:
Mismatch_Loss = 20 Log 0.96 = -0.35 dB
mismatch loss. Note that the mismatch loss is independent of the
length of coax cable.
I am thinking that that .35 db loss due to the mismatch is in adition to
the loss per unit length. That is if you have 100 feet of coax with a loss
of say 3 db when matched, you will have a loss of 3.35 per 100 feet, and if
you go to 200 feet you will have a loss of 6.70 db and not 6.35.
http://en.wikipedia.org/wiki/Mismatch_loss
I've always assumed that it was independent of length because the
mismatch loss can only occur at two points (source and load) and is
not a "bulk" phenomenon. Note that this is a 75 ohm system, not a 50
ohm system, where both the 50 ohm source and load are mismatched to
the 75 ohm transmission media. That makes things a bit easier to
visualize. My
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.
Sanity check:
http://vk1od.net/calc/tl/tllc.php
Plug in:
Belden 1530A (RG-6/u)
100 meters
150 MHz
Zload = 50
which results in:
Line Loss (matched) 7.924 dB
Line Loss 8.097 dB
VSWR(50)in 1.59
Mismatch loss = 8.097 - 7.924 = 0.1730 dB
Now, changing on the 100 meters to 500 meters should produce 5 times
the mismatch loss if your method is correct. It doesn't:
Line Loss (matched) 39.622 dB
Line Loss 39.799 dB
VSWR(50)in 1.50
Mismatch loss = 39.799 - 39.622 = 0.01770 dB
which is almost identical to the 100 meter caculation.
Note that this is for the load end of the coax only. A mismatch at
the source would produce an additional 0.1730 dB loss or:
2 * 0.1730 = 0.3460 dB
total mismatch loss, which corresponds nicely to my original 0.35 dB
loss calculation.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558