[MLB-WIRELESS] Long distance links
mw at freenet.net.au
mw at freenet.net.au
Thu Oct 30 22:50:31 EST 2008
Just wanted to clarify a couple of points after reading that over again :-}
It's not ME that has the 'other' melb-geelog link, of course, but it kinda
looks like I'm suggesting that in the first sentence ;-) That's Peter's
link, and how it is set up: I have no idea (but I'd like to talk about
Secondly, that's 100Km in my discussion about the 24dBi antenna, not 50Km!
> -----Original Message-----
> From: melbwireless-bounces at wireless.org.au
> [mailto:melbwireless-bounces at wireless.org.au] On Behalf Of
> mw at freenet.net.au
> Sent: Thursday, 30 October 2008 10:27 PM
> To: melbwireless at wireless.org.au
> Subject: [MLB-WIRELESS] Long distance links
> Hi Folks,
> I have been thinking about how to set up a second
> Geelong<->Melbourne link, and have considered a few options.
> One idea is to set up a half-way point somewhere, and I found
> a prospective spot in portarlington that looked very good. I
> have this kit (like I brought to the last meeting) made up of
> Mikrotik Crossroads
> and commTenna
> which from my loungeroom window, I could connect to the
> hotspot service at the caravan park at Portarlington
> foreshore. I have a friend whi lives a few blocks away, and
> the lay of the land looked perfect to get a bead on my place
> back the other way.
> Unfortunately I was defeated by trees (!) and my buddy there
> isn't willing to set up a pole on his roof, so I'm back to square 1.
> So I'm still on the lookout for some intermediary location -
> if there's anyone with a look over the bay to geelong (check
> out node KBN
> http://www.melbournewireless.org.au/maps/view?id=KBN) I'd
> like to hear from you.
> For distances greater than about 25Km, I have a plan that
> should work well and still keep us insidethe legal
> transmission limits too.
> As you all know, I'm sure, we can't go over 36dB EIRP (4W) so
> with a 600mW radio, and maybe a cable loss of 2, we can only
> use a antenna: 28dBm radio
> - 2dBi cable loss + 10dBi antenna = 36 (and a 400mW radio
> will need 12dBi antenna, right?)
> Now according to my calculations, if we have a receive
> sensitivity of -93 at the other end of a transmitter running
> at max output, then we can only get about 25Km before we run
> out of signal. If we wanted to go 50Km, then we need a
> receiver with an antenna gain of up around the 24dBi mark,
> which, of course, ends up producing 46dB EIRP on the transmit
> cycle - oops, over limit!
> Ok, so I'm telling you what you already know I s'pose, so
> I'll get to the point! ;-)
> These Mikrotik devices that I use are able to use the two
> antenna connections to transmit on one and receive on the
> other. That means that if the receive antenna is connected
> to a 24dBi grid antenna and the transmit antenna has just a
> 12dBi panel or something, then we should be able to get a
> nice solid long distance link happening, and keep within the
> legal limits no probs!
> Now, there's also the other issue that affects stability of a
> Melb-Geelong link, and that's the bay. Most of you probably
> also know that when shooting over water, relfections from the
> surface can end up causing interference on the direct
> transmissions, especially when the path between the two
> antennas is exactly a half wavelength longer (or 1.5, 2.5,
> ... etc) than the reflected path - thus cancelling out the
> signal waveforms.
> So you probably also know that two antennas set up the right
> distance apart will help overcome this effect using the
> default diversity switch mode, but if we use the above
> technique to have two antennas with different gains, then we
> run into EIRP limit troubles again.
> There *is* a solution! ;-)
> Once again, the Mikrotik systems come to the rescue with a
> special 'Nstreme Dual' mode. In this mode, a router with two
> radios installed can be set up to transmit on one radio and
> receive on the other. The usual benefit of this technique is
> to get a full duplex radio link, but the other cool benefit
> is that we can also use dual high gain antennas on the
> receive circuit for diversity, and one (or two) lower gain
> antennas on the transmit.
> Sound cool? I sure think so :-)
> So anyone interested in some experimentation? I'm quite keen
> to give it a go, and I'll even help you out by offering some
> of this gear at or below cost. If you're game, let me know -
> on or off list is fine by me :)
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