Amplifier, should I buy?

Based on earlier posts in this group, I had sworn off amplifiers until I
found one that claims to amplify BOTH the transmit and receive sides of the
signal. Here are the specs,what do you think?

RX Max. Input Power -10 dBm

RX Power Gain 20 dB


TX Input Power 0 ~ +20 dBm

TX Power Gain 13 dB

TX Linear output Power 500 mW / +27 dBm

On Thu, 29 Jun 2006 20:59:36 +0000 (UTC), JayJay <(E-Mail Removed)>
wrote in <Xns97F18F1F865F6adaef9ae09r9w000@81.174.50.80>:

>Based on earlier posts in this group, I had sworn off amplifiers until I
>found one that claims to amplify BOTH the transmit and receive sides of the
>signal.

Say what? A receive amplifier boosts noise along with signal, which
isn't helpful.

>Here are the specs,what do you think?

What you probably really need is a better antenna.

--
Best regards, FAQ for Wireless Internet: <http://Wireless.wikia.com>
John Navas FAQ for Wi-Fi: <http://wireless.wikia.com/wiki/Wi-Fi>
Wi-Fi How To: <http://wireless.wikia.com/wiki/Wi-Fi_How_To>
Fixes to Wi-Fi Problems: <http://wireless.wikia.com/wiki/Wi-Fi_Fixes>

Thanks for the answer, more inline-

John Navas <(E-Mail Removed)> wrote in
news:(E-Mail Removed):

> On Thu, 29 Jun 2006 20:59:36 +0000 (UTC), JayJay <(E-Mail Removed)>
> wrote in <Xns97F18F1F865F6adaef9ae09r9w000@81.174.50.80>:
>
>>Based on earlier posts in this group, I had sworn off amplifiers until
>>I found one that claims to amplify BOTH the transmit and receive sides
>>of the signal.
>
> Say what? A receive amplifier boosts noise along with signal, which
> isn't helpful.

Does the noise cancel out the total gain on receive? Or does that
depend on the design of the amp? I agree a better antenna would be a
better solution, but not as convenient in this case.

>
>>Here are the specs,what do you think?
>
> What you probably really need is a better antenna.
>

JayJay <(E-Mail Removed)> hath wroth:

>Thanks for the answer, more inline-
>
>John Navas <(E-Mail Removed)> wrote in
>news:(E-Mail Removed) :
>
>> On Thu, 29 Jun 2006 20:59:36 +0000 (UTC), JayJay <(E-Mail Removed)>
>> wrote in <Xns97F18F1F865F6adaef9ae09r9w000@81.174.50.80>:
>>
>>>Based on earlier posts in this group, I had sworn off amplifiers until
>>>I found one that claims to amplify BOTH the transmit and receive sides
>>>of the signal.
>>
>> Say what? A receive amplifier boosts noise along with signal, which
>> isn't helpful.
>
>Does the noise cancel out the total gain on receive? Or does that
>depend on the design of the amp? I agree a better antenna would be a
>better solution, but not as convenient in this case.

Ok, let's start with the receiver only. If you have a receiver with
about -85dBm sensitivity (number selected arbitrarily), and you have a
long length of coax cable with 6dB of loss, your receiver sensitivity
at the end antenna end of this coax will be -79dBm. You're range will
be half of what it would be if the receiver were located next to the
antenna with no coax loss.

Now, if we install a perfect receive amplifier at the antenna, with a
perfect 0dB NF (noise figure) and perhaps 15dB of gain (also selected
arbitrarily), your receiver sensitivity will be back to the previous
-85dBm sensitivity and your range will be twice as good as with the
coax. By adding an amplifier, the receive sensitivity can never be
better than the basic sensitivity of the receiver. In effect, all the
amplifier can do is compensate for the effects of the coax cable loss.

However, there's no such thing as a free lunch or free gain. In order
to insure that the receiver sensitivity is determined by the added RF
amplifier, the amplifier gain has to be at least 10dB or more. 12 to
15dB is fairly typical for rx amplifiers. However, that also reduces
the dynamic range of the receiver front end by:
15dB gain - 6dB coax loss = 9dB
The bottom of the dynamic range is the receiver noise floor and that's
not going to change much with an added amplifier. However the top end
is not that great on most commodity receivers. Making it 9dB worse is
going to make the receiver much more susceptible to interference
problems, overload from strong signals, and possibly blocking from
adjacent services.

The right design for a receiver amplifier is to have only a little
more gain than the losses. Losses include everything betweent the
receiver front end including diversity switches, pigtails, coax
adapters, the coax cable, and whatever I forgot. Add about 1dB for
the NF of the amplifier. Most of this is in the coax cable so the
rest can be just estimated. So, for 6dB of coax, the rx amplifier
should have no more than about 8-10dB of gain. Any more doesn't
improve the sensitivity and just reduces the dynamic range.

If you have a very short length of coax cable between the receiver and
the antenna, an rx amplifier is a total waste of effort and is
seriously detrimental to the dynamic range.

It's just as bad for the tx RF amplifier, but that can wait until
after dinner. Stay tuned. (Quiz on this tomorrow).


--
Jeff Liebermann (E-Mail Removed)
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

On Fri, 30 Jun 2006 00:13:58 +0000 (UTC), JayJay <(E-Mail Removed)>
wrote in <e81qc6$opq$(E-Mail Removed)>:

>Thanks for the answer, more inline-
>
>John Navas <(E-Mail Removed)> wrote in
>news:(E-Mail Removed) :
>
>> On Thu, 29 Jun 2006 20:59:36 +0000 (UTC), JayJay <(E-Mail Removed)>
>> wrote in <Xns97F18F1F865F6adaef9ae09r9w000@81.174.50.80>:
>>
>>>Based on earlier posts in this group, I had sworn off amplifiers until
>>>I found one that claims to amplify BOTH the transmit and receive sides
>>>of the signal.
>>
>> Say what? A receive amplifier boosts noise along with signal, which
>> isn't helpful.
>
>Does the noise cancel out the total gain on receive? Or does that
>depend on the design of the amp? I agree a better antenna would be a
>better solution, but not as convenient in this case.

A good receive amp (pre-amp) might help if the radio doesn't have enough
sensitivity on its own, but that's rarely a problem in my experience.
In general, an amp can add noise, but not remove it -- thus if S/N is
too low (rather than just too weak a signal), amplifying it won't help.

I've not been able to find any good reviews of such products that
demonstrate any real benefit on receiving. For example, the review at
<http://www.tomsnetworking.com/2004/06/08/linksys_wireless_signal_booster_review/>
mostly looks at the output side as measured by a Wi-Fi PC Card, and even
those results were inconsistent. While the throughput tests showed some
modest benefit at maximum range, that probably wasn't due to the receive
amplification.

This is part of why I think a better antenna is much more likely to be
helpful than a signal booster. It's also likely to be less expensive,
and won't necessarily be less convenient; e.g., Linksys HGA7S, 7 dBi
screw-on replacement for 2 dBi "rubber duck" with about 2-3x the range.

--
Best regards, FAQ for Wireless Internet: <http://Wireless.wikia.com>
John Navas FAQ for Wi-Fi: <http://wireless.wikia.com/wiki/Wi-Fi>
Wi-Fi How To: <http://wireless.wikia.com/wiki/Wi-Fi_How_To>
Fixes to Wi-Fi Problems: <http://wireless.wikia.com/wiki/Wi-Fi_Fixes>

Jeff Liebermann <(E-Mail Removed)> hath wroth:

>It's just as bad for the tx RF amplifier, but that can wait until
>after dinner. Stay tuned. (Quiz on this tomorrow).

OK, on to the transmit power amplifier. I've previously demonstrated
that the receiver sensitivity is NOT going to improve with the
addition of an RX ampflifier. At best, it will eliminate the effects
of coax cable losses.

However, the transmitter power amplifier will certainly increase the
signal level. This is commonly known as an alligator, which is an
animal with a big mouth and small ears. The xmit amplified access
point can be heard over a much larger area than it can hear the
replies from the clients. Unless the client radios have a similar
power amplifier, the system become asymmetrical, with more range in
one direction than the other.

For example, the typical wireless access point delivers +15dBm to the
antenna. Add on power amplifiers output from 250mw (+24dbm) to 1 watt
(+30dbm). Assuming 1 watt output, that's a gain of 15db. Since range
doubles for every 6dB of signal increase, this 15dB gain is good for:
10^(dB/20) = 5.6 times
the range of the unamplfied xmitter. If plugged into an omni antenna,
that's 31.4 times the coverage area where the transmitter can be
heard, but it can't hear any of the laptops and low powered clients.
In effect, this makes an amplified xmitter no better than a jammer.

There are also timing problems. The power amplifier has an input RF
detector that senses when the access point transmits, and switches the
power amplifier from receive to xmit (and back again). It takes a
finite amount of time to detect and switch, during which time, the
access point is simply not functioning. If it doesn't mangle the
inter-symbol interference, it will trash the preamble. Truncating the
preamble is only a problem with some diversity systems, which use the
preamble to measure the S/N ratio.

There's also the power amp drive level problem. Only a few power
amplifiers have AGC (automatic gain control). This makes them largely
insensitive to input drive level. Having exactly the right drive, in
the linear region of the power amplifier, is manditory or there will
be envelope (amplitude) distortion. The AGC takes care of adjusting
the level, but those without AGC must have the exact specified length
of lossy coax cable between the access point and the amplifier, or
they simply won't work. Fortunately, most power amps have AGC these
daze.

A power amplifier improves the signal in one direction only. An
antenna improves the signal in both directions. Also, 15dB of xmit
power amplifier gain costs $150 to $250. The same 15dB of antenna
gain costs about $50.

--
Jeff Liebermann (E-Mail Removed)
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

Bottom line: always put your money into the antenna.

The overall link is limited by the laptop WiFi card. Poor receive
sensitivity and low TX power. Upping the AP's TX power won't help the
overall link, only the outbound direction. Adding a pre-amp to the AP's
receiver input won't improve its noise figure significantly and could cause
degraded intercept point, wrecking its performance. The single exception to
the "no pre-amp" rule is when the pre-amp is mounted right at the antenna
driving a long coax to the AP.

But a better AP antenna will increase effective radiated power outbound and
improve receive sensitivity inbound.

Always put your money into the antenna.


"JayJay" <(E-Mail Removed)> wrote in message
news:Xns97F18F1F865F6adaef9ae09r9w000@81.174.50.80 ...
> Based on earlier posts in this group, I had sworn off amplifiers until I
> found one that claims to amplify BOTH the transmit and receive sides of
> the
> signal. Here are the specs,what do you think?
>
> RX Max. Input Power -10 dBm
>
> RX Power Gain 20 dB
>
>
> TX Input Power 0 ~ +20 dBm
>
> TX Power Gain 13 dB
>
> TX Linear output Power 500 mW / +27 dBm

Thank you, Jeff, for a very cogent answer to why one puts the
amplifier midway between the computer and the antenna, when the choice
of improved communication requires coax runs.

The next time the subject comes up in the sailing groups (which, if
that's the solution, require that the amp go halfway, on the mast, but
not at the top, typically), I'll try to paraphrase :{))

L8R

Skip

On 30 Jun 2006 05:02:25 -0700, "Skip - Working on the boat"
<(E-Mail Removed)> wrote in
<(E-Mail Removed). com>:

>Thank you, Jeff, for a very cogent answer to why one puts the
>amplifier midway between the computer and the antenna, when the choice
>of improved communication requires coax runs.

Actually at the antenna for best results, not midway.

>The next time the subject comes up in the sailing groups (which, if
>that's the solution, require that the amp go halfway, on the mast, but
>not at the top, typically), I'll try to paraphrase :{))

Better to put the amp at the top, next to the antenna. Better yet to
put the client bridge at the top of the mast, with zero loss Ethernet
down the mast.

--
Best regards, FAQ for Wireless Internet: <http://Wireless.wikia.com>
John Navas FAQ for Wi-Fi: <http://wireless.wikia.com/wiki/Wi-Fi>
Wi-Fi How To: <http://wireless.wikia.com/wiki/Wi-Fi_How_To>
Fixes to Wi-Fi Problems: <http://wireless.wikia.com/wiki/Wi-Fi_Fixes>

"Skip - Working on the boat" <(E-Mail Removed)> hath wroth:

>Thank you, Jeff, for a very cogent answer to why one puts the
>amplifier midway between the computer and the antenna, when the choice
>of improved communication requires coax runs.

Midway? No way. Both the rx and tx amplifiers work best at the
antenna. The rx amp cannot compensate for the loss of the coax cable
between the antenna and the amplifier is placed midspan. When located
at the antenna, all the coax loss is compensated for by the amplifier,
not just the piece between the amp and the receiver.

>The next time the subject comes up in the sailing groups (which, if
>that's the solution, require that the amp go halfway, on the mast, but
>not at the top, typically), I'll try to paraphrase :{))

Where did you get the idea midspan was best? The amp goes at the
antenna. I just re-read my rants and I never even mentioned midspan
installations.

What I've seen sometimes is putting the amplifier at the radio and
then running a long length of lossy coax to the antenna. That's the
worst possible location. The RF output of the tx power amplifier is
largely lost in the coax cable losses. The receive amplifier, located
at the wrong end of the same coax cable, can't compensate for the coax
loss. Minimal benifit in xmit and none in receive. Usually, someone
justifies this location on the basis of "too much weight topside",
"lighning protection", or some such rot.

--
Jeff Liebermann (E-Mail Removed)
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558