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[Ham-80211] Providing Long-range Wifi

Marlon K. Schafer (509-982-2181) ooe at odessaoffice.com
Mon Sep 10 15:12:41 UTC 2012


Comments inside....


----- Original Message ----- 
From: "Chriswlan2" <chriswlan2 at linnixislands.com>
To: "TAPR Mailing List for Ham Radio Use of 802.11" <ham-80211 at tapr.org>
Sent: Tuesday, August 28, 2012 2:12 PM
Subject: Re: [Ham-80211] Providing Long-range Wifi


>
> That's what I read about V vs H as well...  Sounds like the difference, at
> best, is not supposed to be great.  I did only one test, over 7 miles, and
> it takes for ever to go to the other end spin the antennas, come back...
> Readings change a few dBs all day long, with temp etc, so I got not much
> help with this.... Really need to spend much time to draw a statistical
> conclusion.

Check the antenna specs for this.  The higher the gain on the antenna and 
the better the quality (think panel vs. grid) the more the cross pol. (xpol) 
will be.

20 to 30 dB are pretty typical cross polarization losses.  It's a great 
technique to use when you are stuck using the same band for backhaul and 
distribution (our first 10 years as a WISP were spent mostly that way).

Remember, the elcheapo wifi based radios do NOT stop transmitting at the 
edge of a channel.  They have a lot of leakage that's 10 to 20 dB below 
where they are rated at.  Xpol helps a lot of near channel colocation.

>
> On the other hand, I vaguely remember that some years ago I carefully
> checked into Circ Pol, and the theory is that when your obstruction is
> perfectly conducting, the worst case scenario, for the Linear pol 
> situation,
> is for the reflected signal to perfectly cancel the direct path signal, at
> the other end antennna -depending on path geometry.
>
> While (really circular) CP is reflected with the opposed CP circularity. 
> So
> if the CP antenna at the other end is perfectly circular, it will 
> completely
> ignore the reflected stuff that comes in with the wrong pol.  Real life 
> the
> rejection is not perfect, but still is a major help, opposite the Linear
> situation. Seems essential for say a path grazing a water surface. If your
> path is a reflection on dry lossy sand, dry lossy tree bark, nothing makes
> much of a difference IIRC...

Not sure about any of that.  It makes some sense though.

The hardest part is to make sure that you have clockwise on one end and 
counter clockwise on the other.  Otherwise you are down 30db of signal again 
:-).  We use a lot of omni and sector antennas and I've not seen any 
circular ones.  I looked into them years ago though, seem pretty cool.

I wish I could remember who it was, but there was a guy that had an antenna 
that claimed to use both pols and circular all at the same time.  People 
seemed to have pretty good luck with them, but they were kind of spendy.

>
> When planning real long paths with 8ft TV mesh dishes etc, one should 
> start
> with circ pol, as the wifi feed has to be designed and build anyways...
> Still looking for time to do it here! Oh: if anybody knows of CP 2.4
> antennas at 24-so dB for under $100...

Call Lee at wlanmall.com  Tell him Marlon sent you.

If that doesn't work try Titan Wireless.  (We get most of our gear from 
Titan.)

>
> Concerning Fresnel zone: can't remember where, but I came accross some 
> graph
> that showed that with an obstruction coming up into a Fresnel zone, the
> signal actually first starts to increase a bit, slowly, and IIRC   (?) 
> it
> maxes out at +3dB with 50% of Fresnel obstructed, and then of course, 
> above
> 50%, starts to dramatically decrease a whole lot....    IIRC.... May be it
> was the extreme case with a sharp edged diffracting obstruction?   I freak
> out if I have a single coconut tree frond swinging in >my< microwave
> Fresnel, 50ft away, but take (some) obstructions midpath with a little 
> grain
> of salt.

I'll confirm that small movement of an antenna if multipath is an issue. 
I've seen a gain of 10 to 15dB of receive signal with a 2' move in antenna 
height.  Could be up OR down.  I've also had cases where 3' down and 6' 
sideways has completely cured a link that I fought for a couple of years.

Oh yeah, you also want to use the LEAST amount of power that you need to 
make your link.  The new radios are sensitive down to -92 to 096dB.  Some 
might even be better than that.

Most if not all of the handshake stuff takes place at that 1 meg speed, 
so -94ish dB of sensitivity.  If you have a main signal of, say -60 and a 
powerline, metal roof, road etc. giving you a typical 30dB down multipath 
signal your system will be able to see the multipath just fine as the radios 
do the handshake thing at -90.  But if you run at say, -70 or -75 you'll 
force that reflection down below the receiver sensitivity.

You'd be amazed at how many times I've droped output and seen speeds go up, 
sometimes double.  It can take as little as 3 dB or as much as 10.  It is 
easy to experiment with today's gear.

Let us know if any of htis helps!
marlon

>
> Time to work....
>
> Christian
>
>
>
>
>
> Robert Dixon wrote:
>> Jim - Thanks for the reminder about polarization.  I recall from a
>> former life reading that horizontal polarization is better.  I can
>> think of these reasons:
>>
>> Trees are predominantly vertical structures, hence horizontal
>> penetrates better. Ditto for man made structures like poles etc.
>> Horizontal propagates better in general? Is that why VHF/UHF work is
>> normally H?
>> QRM is less, especially if you are in the wifi band.  Even is
>> outside, but adjacent, the splatter should still be less.
>>
>> I read that the ham wifi band plan calls for circular polarization,
>> but I don't understand why, and it would be more difficult to
>> generate.
>>
>> Bob W8ERD
>>
>>
>> On Aug 28, 2012, at 10:16 AM, Jim Tarvid wrote:
>>
>>> Struggling with a 14km link myself. I have a shot down a valley
>>> which almost works (or works much of the time). Using a 39"
>>> parabolic with a Mikrotik Groove. Suspect multipath issues and a few
>>> trees in the fresnel zone. Getting closer on identifying the trees.
>>> Switching polarity may help.
>>>
>>
>>
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>
>
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