On Sat, 31 Aug 2013 15:44:50 +0100, Jeff wrote:
On 31/08/2013 15:15, Jerry Stuckle wrote:
On 8/31/2013 5:41 AM, 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
He is correct. That 0.35 db loss exists even if you have zero feet of
coax. It is a "point loss", unrelated to coax length.
The loss in the coax is separate.
The loss may be 'separate' but that coax does *get warmer* as the
reflected power also experiences loss in the cable, so he is not correct.
Jeff
Ok, let's try a different approach. Assumptions:
1. Only resistive losses generate heat. Reactive loads and
transmission lines do not generate any heat.
2. Below about 1GHz, the dominant loss mechanism in coax cable
is I^2*R heating losses in the copper conductors.
3. The coax is assumed to be non-radiating.
4. Coax looks resistive because the distributed capacitance
and inductive reactances cancel, leaving only the I^2*R losses.
Therefore, if I replace a length of 50 ohm coax, with a physically
similar length of 75 ohm coax, the I^2*R losses do not change. What
does change are the standing waves along the coax, which will cause
mismatch losses. However, the basic coax loss, as controlled by the
I^2*R losses, remains unchanged. Therefore, since the mismatch losses
are all inspired by changes in reactance, there is no additional
heating losses produced by the mismatch losses, since reactive loads
and transmission lines do not generate any heat.
Anyway, please note my use of the forms at:
http://vk1od.net/calc/tl/tllc.php
to calculate the mismatch loss for various cable lengths. I
previously demonstrated that the mismatch loss is constant, no matter
how long or short the transmission line. I'm fairly sure the
calculations are correct. I'm not so certain of my explanation.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558