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[Ham-80211] Providing Long-range Wifi

Chriswlan2 chriswlan2 at linnixislands.com
Tue Aug 28 21:12:09 UTC 2012


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.

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...

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...

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.

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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