From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from smtp121.iad3a.emailsrvr.com (smtp121.iad3a.emailsrvr.com [173.203.187.121]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (Client did not present a certificate) by huchra.bufferbloat.net (Postfix) with ESMTPS id C545821F1FC for ; Thu, 19 Dec 2013 21:18:25 -0800 (PST) Received: from localhost (localhost.localdomain [127.0.0.1]) by smtp8.relay.iad3a.emailsrvr.com (SMTP Server) with ESMTP id C58332480B9; Fri, 20 Dec 2013 00:18:23 -0500 (EST) X-Virus-Scanned: OK Received: from app41.wa-webapps.iad3a (relay.iad3a.rsapps.net [172.27.255.110]) by smtp8.relay.iad3a.emailsrvr.com (SMTP Server) with ESMTP id A250524809F; Fri, 20 Dec 2013 00:18:23 -0500 (EST) Received: from reed.com (localhost.localdomain [127.0.0.1]) by app41.wa-webapps.iad3a (Postfix) with ESMTP id 88D8E280041; Fri, 20 Dec 2013 00:18:23 -0500 (EST) Received: by apps.rackspace.com (Authenticated sender: dpreed@reed.com, from: dpreed@reed.com) with HTTP; Fri, 20 Dec 2013 00:18:23 -0500 (EST) Date: Fri, 20 Dec 2013 00:18:23 -0500 (EST) From: dpreed@reed.com To: "Theodore Ts'o" MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_20131220001823000000_42568" Importance: Normal X-Priority: 3 (Normal) X-Type: html In-Reply-To: <20131220005058.GE9130@thunk.org> References: <52AF797E.6030600@imap.cc> <18972.1387302855@sandelman.ca> <1387319157.48330794@apps.rackspace.com> <20131217154345.0e91b65f@nehalam.linuxnetplumber.net> <1387379970.401720581@apps.rackspace.com> <20131220005058.GE9130@thunk.org> Message-ID: <1387516703.557832747@apps.rackspace.com> X-Mailer: webmail7.0 Cc: "cerowrt-devel@lists.bufferbloat.net" Subject: Re: [Cerowrt-devel] =?utf-8?q?treating_2=2E4ghz_as_-legacy=3F?= X-BeenThere: cerowrt-devel@lists.bufferbloat.net X-Mailman-Version: 2.1.13 Precedence: list List-Id: Development issues regarding the cerowrt test router project List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Fri, 20 Dec 2013 05:18:26 -0000 ------=_20131220001823000000_42568 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable =0AThe tower is a slightly different situation. There you are not between = the antenna and ground - the ground is between the antenna and you....=0A|/= =0A| 0=0A| -= +-=0A| /\=0A=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=0A =0AIf the element is at th= e top of the mast at left, the path from it to your phone (e.g.) is not clo= se to the ground,=0Aso the losses along the path from you to the element ar= e small. But as the path gets closer to the ground, the electric field ten= ds to be dragged toward zero. So the higher the antenna the better, becaus= e if it were close to the ground, the energy along the path would tend to b= e absorbed by the ground because it is closer to the antenna than you.=0A = =0ABut if the situation looks like this:=0A =0A__ [antenna]=0A =0A\=0A\=0A= \=0A0=0A-+-=0A/\=0A=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=0A =0AThere is no ground between you and the= antenna, but the field is forced to zero at the earth-ground, and also ref= lected away from you back towards the sky as the slanted "ray" between you = and the antenna will reflect off the ground to the right. So you will find= that since you are close to the ground, the field is zero near your head a= t the spot there, too. There is no absorption of energy by the air/wood be= tween you and the antenna, but the zero boundary condition at the ground ma= kes the field weaker around you by attenuating the field and reflecting it = away from you.=0A =0AThe differential equation solutions that describe the = time varying EM fields in both situations are, of course, far more complica= ted (Maxwell's equation with fixed boundary conditions). But what I'm sayi= ng is a rough characterization of the fields' energy structure, given the e= arth-ground being a roughly flat surface that is conductive enough to hold = a near constant zero voltage.=0A =0AHope this helps.=0A =0A =0AOn Thursday,= December 19, 2013 7:50pm, "Theodore Ts'o" said:=0A=0A=0A= =0A> Thanks for the detailed explanation about antenna issues. One=0A> que= stion, if I might:=0A> =0A> > (and don't put your AP in the attic and expec= t a good signal near=0A> > the ground.... or in the basement. Physics will= make sure that the=0A> > signal is zero at any ground, so being closer to = the ground than the=0A> > antenna weakens the signal a lot!)=0A> =0A> I tho= ught the opposite was true? That is, ground loses go up when the=0A> anten= na is closer to the ground, so it was good to have bigger, taller=0A> atenn= a towers? If what you say is correct, how do antennas on cell=0A> towers m= itigate this particular issue?=0A> =0A> =09=09=09=09=09=09- Ted=0A> ------=_20131220001823000000_42568 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable

The tower = is a slightly different situation.  There you are not between the ante= nna and ground - the ground is between the antenna and you....

=0A

|/

=0A

| &nbs= p;                     &n= bsp;         0

=0A

|=                     &nbs= p;          -+-

=0A=0A

| &= nbsp;                    =           /\
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D

=0A

 

=0A

If the element is a= t the top of the mast at left, the path from it to your phone (e.g.) is not= close to the ground,

=0A

so the losses = along the path from you to the element are small.  But as the path get= s closer to the ground, the electric field tends to be dragged toward zero.=  So the higher the antenna the better, because if it were close to th= e ground, the energy along the path would tend to be absorbed by the ground= because it is closer to the antenna than you.

=0A

 

=0A

But if the situati= on looks like this:

=0A

 

=0A

__  [antenna]

=0A

 

=0A

\

=0A

\

=0A

\

=0A<= p style=3D"margin:0;padding:0;">0

=0A

-+= -

=0A

/\

=0A

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D

=0A

 

=0A

There is no ground between you and the antenna= , but the field is forced to zero at the earth-ground, and also reflected a= way from you back towards the sky as the slanted "ray" between you and the = antenna will reflect off the ground to the right.  So you will find th= at since you are close to the ground, the field is zero near your head at t= he spot there, too.  There is no absorption of energy by the air/wood = between you and the antenna, but the zero boundary condition at the ground = makes the field weaker around you by attenuating the field and reflecting i= t away from you.

=0A

 

=0A

The differential equation solutions that describ= e the time varying EM fields in both situations are, of course, far more co= mplicated (Maxwell's equation with fixed boundary conditions).  But wh= at I'm saying is a rough characterization of the fields' energy structure, = given the earth-ground being a roughly flat surface that is conductive enou= gh to hold a near constant zero voltage.

=0A

 

=0A

Hope this helps.

=0A<= p style=3D"margin:0;padding:0;"> 

=0A

 

=0A

On Thursday, December 19, = 2013 7:50pm, "Theodore Ts'o" <tytso@mit.edu> said:

=0A=
=0A

> T= hanks for the detailed explanation about antenna issues. One
> que= stion, if I might:
>
> > (and don't put your AP in the = attic and expect a good signal near
> > the ground.... or in the= basement. Physics will make sure that the
> > signal is zero a= t any ground, so being closer to the ground than the
> > antenna= weakens the signal a lot!)
>
> I thought the opposite was= true? That is, ground loses go up when the
> antenna is closer to= the ground, so it was good to have bigger, taller
> atenna towers?= If what you say is correct, how do antennas on cell
> towers miti= gate this particular issue?
>
> =09=09=09=09=09=09- Ted>

=0A
 ------=_20131220001823000000_42568--