[MLB-WIRELESS] Antenna heights ... what's your practical experience like?

[Denis Cheong]

Hi David

This is very similar to what Dan (KOJ) and I (KOK) have been working on
over the last few months.  These nodes are 900m apart, but there are
various trees and other obstacles between us (no LOS).

5GHz we tested initially did not work (Mikrotik SXTs).

We are now up and running at 2.4GHz using a pair of Bullet M2s, one with a
24dBi parabolic and the other an 18dBi yagi, however after some
experimentation (photos soon) we clearly need another few metres of height
to get full transfer rates, which are currently wildly varied between 1 and
11 Mbit (real TCP throughput).  Signal strength is bouncing up and down all
over the place.  We're now looking at replacing the Yagi with another grid,
and going up a couple of metres at KOJ which we now know will give us an
extra 10dB and hopefully a more stable connection.

After our experience, I would suggest the following:

   1. Use Google Earth, and create a direct path between the exact proposed
   mount point at the two sites,
   2. Do a 'map terrain' against that path - you will see the approximate
   relevant heights between the two sites - and see whether it really is dead
   flat or not.  If you elevate the path to your expected height, you can then
   follow the approximate path along using street view to see if there is
   anything else in the way (trees, 2 storey houses, etc).  You might be
   surpsised that what looks dead flat probably really isn't, unless you are
   in a flood plain.
   3. Use Google Earth to get the exact bearings between the two points,
   and map them relative to either (a) your roofline, or (b) other markers.
   We used a nearby mobile phone tower and some identified trees.
   4. Each of you go up on the two roof and stand where you will mount your
   poles, and with high power binoculars see if you can see each other (at 2km
   it may be a stretch; we can't at 900m).  At night you can use a really
   bright torch or spotlight (flash it on and off) to see if you can see it it.
   5. There ends the planning ... from there on in you need to start the
   practical stuff.  Get your hands on some radios.  5GHz is better for many
   reasons, but as Greg says it's intolerant of any interference from trees,
   etc.  Starting with 2GHz is a safer route if you think there could be
   anything in the way.  Higher gain antennas the better - 24dBi is the max
   you will get at 2.4GHz (a 0.6 x 0.9m grid) but even better if you can get
   away with it is a 20dBi Nanobridge 2.4 or 25dBi Nanobridge 5 - currently
   king of the hill in price/performance because it will give you dual chain
   (Horizontal/vertical polarisation) MIMO @ up to 300Mbit whereas a grid will
   only give you one chain @ up to 150Mbit.  A couple of 20dBi Nanobridges
   should set you back under $250.
   6. For experimentation with your signal some suggestions:
      1. Get the two radios connected on the ground in one house first.
      Make sure they're getting a nice strong signal and paired AP/client.
      2. Figure out a temporary way to mount them at each end - depends
      entirely on your situation.  Flat roof - stick a ladder up and
mount it on
      it, or climb up it with a broom handle, or gaffa tape it with a
broom.  Any
      other type of roof - more tricky.
      3. Point them at each other, using your reference points, and see if
      they connect - it's a bit of black magic as to whether it works,
but if you
      did all the above planning and nothing showed up as a showstopper (e.g. a
      3-storey block of flats between you) then it should work.
      4. Once you have got your link, and are satisfied with the settings,
      and signal strength, etc., then start figuring out how to mount it.
      There's lots more options there - and they all depend on how
high you then
      need to go, and what types of roofs you have.

Of course, two other considerations are - if you want to connect to
Melbourne Wireless, where are your nearest nodes and do you have line of
sight to them?  And if all else fails, you can always use those nodes as
intermediaries if necessary.

To source equipment, you can't go past our Melbourne Wireless sponsors:
City Technology (~10% discount on Nanobridges) or Duxtel (limited to
Mikrotik gear however ... that being said, at 2km if you do have clear line
of sight then a couple of SXTs could be enough).  Also if you can wait
until the next Melbourne Wireless meeting/swapmeet send a note out saying
what you are after - from what I gather, most of the members have ended up
accumulating equipment that is now just gathering dust.

Good luck and keep us posted!

Denis.

2011/10/31 David Tomic <david at tomic.com.au>

> Hi guys,
>
> So I'm new to this game, and I'm looking to setup a link between two
> houses that are just shy of 2km LOS from each other.
>
> I know I can find equipment that'll span this sort of distance without any
> problems at all, but my main concern now how high I'll have to mount
> everything in order to get a decent signal?!?
>
> The terrain is close enough to dead flat, but given that both houses are
> only single story, I'm not really sure exactly where I stand on this front?
>
> Does anybody have any practical experience / advice / rules of thumb for
> this sort of scenario?
>
> Is there any particular equipment that I should be looking for [or looking
> to avoid] with this setup?
>
> I'd really appreciate any help / advice / suggestions you might be able to
> give me ...
>
> Thanks a lot,
> --David
>
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> Melbwireless at wireless.org.au
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>
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