Thanks!

To add one thing about Diffraction though...  I have towers on round poles. 
I can get signal (often not too bad) behind the metal pole.  I *think* we 
must get at least some level of signal going around or through things in 
ways that don't always follow the obvious rules.

Nice explanations though!  Much more detailed than my simpleton version.
marlon

----- Original Message ----- 
From: "Patrick Shoemaker" <shoemak...@vectordatasystems.com>
To: "WISPA General List" <wireless@wispa.org>
Sent: Friday, May 22, 2009 5:16 AM
Subject: Re: [WISPA] Thanking Marlon and multipath experience


Not trying to be pedantic here, but I think some clarification is needed
regarding reflection, refraction, and diffraction.

Reflection is the phenomenon that occurs when an EM wave encounters a
surface (larger than approximately one wavelength), hits it, and leaves
the surface with reduced intensity. Same thing as light hitting a
mirror. It's the big one for multipath.

Refraction is the effect that "bends" an EM wave when it transitions
between two different media with different permittivities. We see this
effect when you put a stick in a pool of water, and the stick looks bent
at the surface of the water.

Diffraction (knife-edge diffraction) is the phenomenon that allows EM
waves to travel around objects. It's the reason you can get an AM radio
signal in a valley obstructed by mountains on all sides, the
mountaintops diffract the signal and "fill" the valley with RF. It's my
understanding that this phenomenon is not really worth considering at
the frequencies we're using.

Hope that helps!

Patrick Shoemaker
Vector Data Systems LLC
shoemak...@vectordatasystems.com
office: (301) 358-1690 x36
http://www.vectordatasystems.com


Marlon K. Schafer wrote:
> Glad to be of help Forbes!  Once in a while I make a lucky guess.  grin
>
> OK, design note time.
>
> I work to keep all of my links in the -65 to -75 range.  If you look up 
> the
> specs for your radios you will likely find that this is well above the
> lowest receive signal level, but not too far above it.
>
> Why is this important?  As I've been taught over the years (Thanks Jaime,
> Bob and others) most knife edged refraction (a bounced signal off of an
> object) will be about 30 dB down below the original signal level.
>
> So if your radio has a max sensitivity of -94 and you have a -65 signal
> you'll be almost out of the possibility of the receive radio even picking 
> up
> your refracted signal.  Think of the refraction as a reflection or echo. 
> An
> echo makes it really hard to understand someone.  Multipath is the same
> thing.  But if you can make the echo so quiet that you can't hear is it
> won't hurt anything, even if it's there.
>
> MOST of my towers put out LESS than 1 WATT!!!!!  A few are still close to 
> 4
> watts, but changes in AP's allow me to run ever lower tx powers.  Believe 
> it
> or not I have customers getting over 2 megs of service from systems that 
> are
> less than 2 watts and at ranges of 18 miles!  Yes I can prove this if 
> anyone
> wants to come visit....
>
> If you are picking up your towers at levels above -60 to -65 you'll have a
> LOT more trouble on your network.  Especially tower to tower.  Turn the
> danged things down.  If you need more power at a customer's place install 
> a
> bigger antenna!
>
> Over the years I've run into many companies that try to use a bigger 
> stick.
> They systems never work well for long.  The more customers go on it the
> worse that problem gets too.  I fact I have started pulling customers from 
> a
> competitor in the area.  He's running a system near me that looks to be
> running about 42 watts (remember we're only allowed 4!!!!).  Why am I
> getting his customers?  Because his system doesn't work right.  Much of 
> that
> is due to his design flaws.
>
> OK, next question is, how do I know what he's running?  All you need to 
> know
> is what gain the antenna is at your end, what the receive signal is and 
> how
> far the transmitter is away from you.  From there it's easy with a 
> formula.
> At 42dB I have him at about a 15dB antenna and 1 or so watt TX power.  A
> VERY common config sold by some distributors.  It's too bad, these never
> work well long term and rotten wisp networks give us all a bad name.
>
> Anyway, Forbes, try turning that power down.  WAY down.  I'll bet you can 
> go
> back to g mode and have even more stability than you have now.
>
> laters,
> marlon
>
> ----- Original Message ----- 
> From: "Forbes Mercy" <forbes.me...@wabroadband.com>
> To: "WISPA General List" <wireless@wispa.org>
> Sent: Wednesday, May 20, 2009 1:47 PM
> Subject: [WISPA] Thanking Marlon and multipath experience
>
>
> I admit that most of you can dance around me in the field of RF.  I'm more
> of an installer and management type.  Today I had a Nano CPE with a -48 
> dbi
> signal which, as you know, is amazing; it should be it was a block away 
> from
> the tower.  The problem is the customer had long outages and erratic
> service.  Aiming away from the tower kept the great connection but still
> crappy pings.
>
> Marlon suggested Multipath as a possibility and advised to turn the radio 
> to
> "B" only and reduce the power on the radio. I had heard of Multipath but
> never experienced it. We changed both settings and while the signal stayed
> at -48dbi the time-outs and erratic pings went away.  90% of our system is
> two or more miles from towers, so we never had this problem before. This
> particular town is the only place where the towers are on buildings all
> within ½ mile of nearly everyone
>
>
> It answered a lot of problems for others in town we have been battling
> supposed interference with and I just wanted to publically thank Marlon 
> for
> exposing me to a side of RF I hadn't even thought of.  This is what makes
> WISPA worth it to me.
>
> Forbes Mercy
> President - Washington Broadband, Inc.
>
> No virus found in this incoming message.
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> 18:31:00
>
>
>
>
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