[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
> (http://shop.duxtel.com.au/product_info.php?products_id=45) 
> and commTenna
> (http://shop.duxtel.com.au/product_info.php?products_id=46) 
> 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 :)
> Cheers!
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