Re: RX amp required - NOT TX

On Sun, 12 Sep 2010 01:54:13 +0100, "Gemma" <noreply@xxxxxxxxxxx>
wrote:

What I am trying so hard
to find is an inline device which offers perhaps 9-12 dB gain for the
receiver and (possibly) 3dB gain for the transmitter just to get rid of the
insertion loss. Ideally I want a unity gain for the transmitter side after
insertion of the RX amp.

Ugh... That's not going to be easy. It sounds like you want a T/R
switch built into an inline amplifier. Those are either keyed by the
TX RF, or have a wire running to the AP. The gain of the TX amp is
usually not fixed to some number and is controlled by an ALC
(automatic level control). This is a strange beast indeed.

To keep myself busy, I install Hotspots and although I have tried a
selection of makes over time, the most common problem is that the laptop
client can see the AP but the AP is deaf to them. As it is I use moderate
gain aerials (about 8dB) and usually limit the TX power to around 50mW. This
as a rule tends to mean that MOST clients that can see the AP can also be
SEEN by the AP.

Good thinking. If you can hear it, you can talk to it. 8dBi seems a
bit high for an indoor antenna, but might work as the signal bounces
around off the walls. It's still better than an omni.

I don't see the point in bellowing out hundreds of milliwatts when the
client is whispering back with perhaps 30, of which 20 is going into their
thigh.

Try about 20mw for many PDA's and cell phones.

Already googled myself senseless and checked Ebay repeatedly. The only time
I can find a pre-amp for the receiver it is almost an afterthought for some
gizmo that will pump out 500mW - 1500mW and as an afterthought, increase the
RX gain by perhaps 6dB

Well, there's a reason that the RX gain is so low. It's purpose is
NOT to improve the receiver sensitivity. The GaAs FET front end of
most access points is fairly close to the limit of what can be done
with receivers. The best the RX amp can do is compensate for the coax
cable and connector losses. Add too much gain, and the dynamic range
of the receiver is reduced by the amount of the gain increase. The
noise floor remains the same, while the overload point drops with the
gain increase. Put a proper 20dB gain RX amp on a typical low power
AP front end, and all you'll see is intermod and blocking problems.

So far the only thing I can think of, is to buy one with a decent RX gain -
ignore the transmitter gain totally and then just under-drive the unit so
that AFTER the transmitter has been amplified it is still only pumping out
around 50 - 80mW - but in the process has some 6 - 9dB gain for the
receiver.

Maybe. If you really want more RX gain, without affecting the
xmitter, dive into your unspecified model favorite access point, break
the T/R switch chip, and bring the xmitter and receiver cables out to
separate connectors. Might as well send some DC (through a 2.4Ghz
choke) to the RX amp for powering it. The xmitter goes to one antenna
connector, while the TX amp goes to the other. Of course, this
requires two antennas, but those are cheap and easy.

You could also just break the trace between the T/R switch and the
receiver front end, to insert the RX amp, but that would not help to
reduce the effects of your unspecified length of coax cable.

Please - I cannot be alone in wanting to improve the receiver sensitivity
more than the transmitter "oomph" - has anyone found a product suitable yet?

I don't think you're going to improve receiver sensitivity much with
an external amplifier and certainly not without trashing the dynamic
range. I like numbers. What manner of receiver sensitivity were you
expecting for a 10% PER (packet error rate). Do you have suitable
test equipment needed to measure the before and after sensitivity?
That's not a rhetorical question as you won't really know if the
additional amplifier is doing any good without measurements.
<http://www2.rohde-schwarz.com/file_1344/1GP56_2E.pdf>
You don't really need the expensive test equipment as most of the
setup can be faked using client radios and AP's as a test generator
and receiver.

Question: What problem are you trying to solve by adding an RX amp?
Reducing coax loss is a solution, not a problem that needs solving.
Are you having reliability, coverage, thruput, interference,
multipath, or other problems at your hotspots?

--
Jeff Liebermann jeffl@xxxxxxxxxx
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
.

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