Antenna

[Ged Haywood]

Hello all,

On Tue, 24 Feb 2004 hostap-request at shmoo.com wrote:

> 8. RE: Antenna (Johnson, Michael1 [IT])
>
> I am looking for something with a 300 meter ranger or more.

There's more to it than just the Antenna.  Most important is radio
line of sight, which means a path clear of obstructions in a zone
around the line joining the two points.  For radio, it's not really a
straight line but it's near enough at 2.4GHz and a few hundred metres.
The clear zone (look for Fresnel Zone on Google) needs to be something
of the order of ten metres diameter for this sort of length of link.
You will easily get 300m using a couple of patch antennas even if you
only have the 32mW or so that we get on wireless cards here in the UK.
It would be a struggle using the little whips that come with most gear
but I've heard of that kind of range being achieved.  At the extremes
of range (signal-to-noise ratio) you start to see the cards throttling
back the data rate, so you want to have plenty to spare.  If you need
to run the signal down long a cable to the antenna, watch out for poor
cables, which will sap your signal.  Long runs of _any_ cable will do
the same.  Try to keep your cables less than a few metres long even if
it means siting the transceiver somewhere inconvenient.  All kinds of
safety issues crop up if you're going to consider that.

As has been said, there are truckloads of useful stuff on the Web just
waiting for you to read it.  I'd been meaning to put a power budget on
my site for a while, your message prompted me to quickly do that.  It's
very rough might help you a bit:

http://www.jubileegroup.co.uk/JOS/radio/powerbudget.html

> 10. Re: Antenna (Andrea G. Forte)
>
> What about antennas for centrino laptops? How can I connect the homemade
> antenna to the laptop? I do not want to disassemble the whole laptop.

No need to do that.  I use repeaters to avoid that kind of problem.
There are two radios at the top of my mast, one for the 700 metre link
to the remote site, one for a link to the local offices which is only
about 50 metres.  This has the added advantage that there are no
cables directly linking the computers in the offices to the gear up
the mast.  The mast is the highest thing for quite a long way in most
directions, so it might be more at risk from lightning than I'd like.
If it does suffer a strike and the lightning protection doesn't work
(I really don't believe those things:) then I've lost a few bits of
relatively cheap kit, not an entire network.  Just the hardware would
cost orders of magnitude more money to replace, and that's before you
even think about the system software, configuration and data.  I don't
like to think about replacing them.

73,
Ged.