Archived from groups: alt.internet.wireless (
More info?)
"Jac" <amhirsohail@yahoo.com.au> wrote in message
news:4131b083$0$27223$61ce578d@news.syd.swiftdsl.com.au...
> HI All,
>
> Is the gain and loss associated with antennas an cable the same? For
> example, if I run 10 metres of RG58 1db loss cable to a 18 db antenna, not
> accounting for connector loss would the net result be 8 db gain? Or does
it
> get to the stage where enough cable loss negates an gain on the antenna?
> Thanks
> Jac
Jac,
The answer, sad to say, is "Yes and No".
Yes, a db is a db is a db, so a cable that lost 10 db would be cancelled out
by an antenna that gained 10 db, all other things being equal.
However:
It doesn't work the way we'd like. Comparing one db to another is like
comparing a dollar in the pocket of a millionaire with a dollar in the
pocket of a laborer: they get a lot more precious when you haven't got as
many, so we have to be sure that we're using meaningful figures since WiFi
transmitters usually produce only 1/10 of a watt.
First, antenna manufacturers express their products gain in "dBi", which is
a measure of gain relative to an "Isotropic" radiator: it's a theoretical
point source that doesn't exist in practice. Meaningful figures are
expressed in dBd, i.e., gain relative to a reference dipole, which is a more
realistic number since a reference dipole, although seldom seen in the
field, can actually be constructed and measured. Since gains expressed in
dBi are always higher than those in dBd, salesmen prefer dBi. Long story
short: deduct 2.5 db from dBi in order to get a dBd figure, which is the
highest "ideal" gain you can get in an actual antenna.
See
http://www.softwright.com/faq/engineering/FIELD%20INTENSITY%20UNITS.html
for more information on dBi vs. dBd figures.
Second, the gain of a new antenna must be compared to the gain of the
_existing_ antenna: if your current antenna has 3 dBi of gain, and the new
one has 14 dBi, the _difference_ is what counts, and you'll have to deduct
any added connector and cable loss from that difference in order to get a
"real world" figure.
Third, cable losses must be justified in terms of the path gain they
provide: I assume you used "RG-58" as an generic designator for coaxial
cable but even Heliax will take 1/2 your power in the first 100 feet. Unless
that 100 feet buys you an unobstructed path to the other end of the link and
relieves you of _at least_ that much loss in the original path, it's not
money well spent: see
http://www.bawia.org/coax.html for a list of coaxial
cable loss figures at 2.4 GHz. This assumes, of course, that you're able to
move the original antenna to the same spot as the new one, but even if it's
attached to the AP, you might do better to move the whole AP up to the top
of a pole and feed it power over the Ethernet cable than to suffer the cable
losses which are otherwise necessary to benefit from the new antenna's
potential gain.
HTH.
William
--
William Warren
(Filter noise from my address for direct replies.)
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