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Re: [LUG] Wireless LAN Antenna's



On Tuesday 18 Nov 2003 10:53 pm, Simon Waters wrote:
> Mark Cubitt wrote:
> > Just asking if anybody knows the sort of distance that can be achieved
> > with a 12dB Omni-Directional Antenna (Just a rough distance, I know it
> > depends on terrain).
>
> Depends on much more than just terrain.....
>
> My understanding is that 12dbi omni will give you around about a 4 fold
> range increase on whatever you are getting before. But you may have to
> substract off a substantial amount for losses in cables unless they are
> very short.
>
> The effect applies at both ends. So you might want to buy them in pairs.
>
> Alec can probably tell you what the correct rule of thumb is.
>
> The current pricing of commercial antennas is such that you can buy two
> cheap access points for one well made low gain antenna, so there maybe
> other ways to skin a cat. Although better antennas still make more sense
> for most situations where range is the issue.
>
> Now do the 50 quid access points support ethernet bridging yet?

Okay as few pointers:
The EIRP limit in the UK is 100mW (20dBm). The way to calculate this is as 
follows.
	Find out the loss of the cable.
	find out the gain of the antenna.
	run iwconfig to get the transmit power of the card (many are 30mW ie 15dBin 
the UK)

EIRP = (tx power) - (cable loss) + (Ae gain)      all measured in dB's

I've used 2 Antenna's (15dB parabolic mesh) from wlansource.co.uk to create a 
ptp link over 12.5 miles, and yes I did still get enough signal to get an 
11Mbps connection.
That was an illegal setup in the UK, I only have about 1m of cable so cable 
losses were very low (less than 1dB) so my EIRP was in the region of 29dB - 
at both ends.

The amount of cable losses you will get depend upon what quality of cable you 
use. Use anything less than LMR400 and you get horrendous losses. The best 
I've come accross is LCF78-50J which has a loss of 6.6dB per 100m. The draw 
back is it's 7/8" thick and not overly flexible and cost about £10 - £15 per 
meter depending on length. LMR400 is about 2 or 3 quid per meter but it's 
loss is about 6.6dB per 100 ft (note ft not m).
For most home installations LMR400 is fine.
As a rule of thumb, use as short a pigtail as you can. Pigtails are normally 
made with LMR100 and the losses in this stuff is very high - For my mobile Ae 
I think I loose about 2 or 3 dB in a 2.5m cable run.

As a foot note, no one has yet been prosecuted in the UK for using an illegal 
setup (ie over power) - I'm not even aware of any that have been ordered to 
reduce the power.

Sometimes Antennas also appear on ebay. A simple search for 'antenna' will 
show most of them.

HTH
Jon Lawrence

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