[MLB-WIRELESS] Long distance links

mw at freenet.net.au mw at freenet.net.au
Thu Oct 30 22:26:32 EST 2008

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

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

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

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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