Longer legal links?

This appeared on the Consume mailing list over the weekend.  Looks like we
could get a good few miles within legal limits if we use slightly more
expensive cards.  Will be almost £100 for the cards alone :-/

Andy

--------------

> > > This in turn suggests that a better approach is to build a
> > > bi-directional amp with little or no gain in the receive path, instead
> > > focusing on achieving an optimal noise figure, and a LOSS in the TX
> > > path such that the amplifier can be coupled with a highly directional
> > > antenna without breaking the EIRP rules.
> >
> > I think i get what you mean here.....
>
> I wonder if it wouldn't be easier to get similar legal performance using
> Cisco's manually configurable antennae diversity and tx power?
>
> For tx @ +7dBm on antennae 1 only and rx on 1 & 2 (using BSD's ancontrol):
>
> ancontrol -v0 -d3 -v1 -d1 -p5
>
> Take Malcolm's example, recalculate using lower radio output, higher tx
> antennae gain, rx on both antennae and higher receive sensitivity (Cisco
> claim -85 dBm @ 11 Mbps) and you get impressive values at legal EIRP:
>
>     radio output = +7dBm (5mW)
>     radio sensitivity = -85dBm at 11Mbps
>
>     Transmit antenna gain = +13dBi
>     Receive antenna gain = +20dBi + +13dBi = +33dBi
>
>     output to air = +7dBm + 13dB = +20dBm(i) i.e. legal
>     off-air sensitivity = -85dBm - 33dB = -118dBm
>
>     difference = +20dBm - -118dBm = 138dB
>
> Assuming I got the figures right, there should be an additional benefit
due to
> the tx antennae's improved rejection of off-axis noise when rx'ing.
>
> Am I way off base or does this look feasible?

As I understand it, Peter is implementing exactly the configuration I
have proposed- using a high gain antenna on an RX only port and a lower
gain antenna on a TX / RX diversity port [which I will call the TX port
for brevity]. I don't know the Cisco/Aironet products too well, so this
is interesting.

The gain of the TX port antenna is limited by the difference between the
radio output and the EIRP limit (20dBi). So as a first estimate the TX
power isn't very important because we simply fit an antenna to boost
whatever power it gives to the 20dBi limit.

In theory there is, as Peter says, a benefit in adopting a lower TX
power and higher TX antenna gain when this port is used to receive,
although this should only occur for signals that are out of the main
area of coverage as defined by the high gain receive antenna and so is
probably not that significant. In fact having asymmetric receive and
transmit patterns does lead to the possibility of being able to hear
stations that you can't talk back to for this reason.

However, typical diversity switches at 2.4GHz provide around 20dB of
isolation, so if the TX power were +20dBm, there would be 0dBm of
feedthrough to the RX port antenna, hence any RX antenna gain of more
than 20dBi would exceed the EIRP limit through spurious transmissions
from the RX antenna alone. The figure of maximum possible RX antenna
gain will obviously increase by 1dB for every 1dB that the actual TX
power is dropped. i.e.

   TX power  max RX gain
     20dBm     20dBi
     15dBm     25dBi
     10dBm     30dBi
      7dBm     33dBi

So dropping the TX power has a secondary benefit of allowing higher gain
receive antennas for long range point to point links, although remember
that the output from the RX antenna as calculated above is IN ADDITION
to the normal TX output, and so a practical RX antenna gain should be at
least 6dB less than this so as to raise the actual TX output to air by
no more than 1dB.

Taking the specific example from Peter below, a receive antenna gain of
33dBi (why '20dBi + +13dBi'? ) is too high as illustrated above.
Although possible, 33dBi is also a very high gain - a quick calculation
suggests about a 2.5meter dish would be needed!

Other than that, the figures are fine and would enable a theoretical
link of some 9 miles under ideal conditions.


malcolm C