On Sun, 3 Aug 2014 08:31:59 +0100, Ian Jackson
wrote:
In message , Jeff writes
Jerry does not seem to take on board that this is an international
group, not a group of ex-US cable technicians who slovenly misuse dBm
to mean dBmV, when the rest of the world uses dBm correctly to mean dB
relative to a milliwatt.
I think only a small minority of US cable TV people would misuse dBm
when they really mean dBmV. I worked in the industry all my life, 20
years of which were for an American company - and I never found this to
be a common practice.
A few do. The problem is not the cable distribution system levels,
but rather the accessories attached to it. The RF levels on the cable
will probably be in dBmV @ 75 ohm forever. OTA signals will probably
remain as dBuV @ 75 or 300 ohms. However, the Wi-Fi signal levels
from the cable modem/router/wireless box is in dBm. The MoCA bridge
signal levels are also in dBm[1]. This is not really a problem
because each technology usually involves different cable techs,
engineers, and support groups.
At one time, I worked on a project to distribute 2.4GHz Wi-Fi via the
inside RG-6/u coax system. It rapidly increased in complexity and
didn't work very well. It was quickly and mercifully killed. We
initially has some confusion due to the different units, but the
project manager simply demanded that we specify the full units of
measure and equivalent units where appropriate, which largely
eliminated the problem. That left the problem of people using 50 ohm
test equipment on 75 ohm systems without a 50-75 ohm impedance
matching pad.
Impedance matching L pads:
http://www.microwaves101.com/encyclopedias/380-l-pads
50/75 dB conversion formulas and cheat sheet:
http://www.maximintegrated.com/en/app-notes/index.mvp/id/808
http://pdfserv.maximintegrated.com/en/an/AN808.pdf (106 KBytes)
1] Note that dBm and dBmW are independent of the impedance since xx
milliwatts produce the same amount of smoke no matter what the
impedance. However dBmV and dBuV require that the impedance be
specified.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558