Antennas

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Hello all,

I am trying to set up a link, about 1/4 mile long.  I have two radios,
each producing 200mw of power.  I also am thinking about purchasing one
of two antennas -- one is 15dBi and the other is 24dBi.  Different
people in the area have told me that for this link, all I need is a
15dBi antenna, but I am wondering if there would be any speed advantage
(or any advantages at all) to using a 24dBi antenna with this relatively
short link.

Thanks in advance,
Kevin Brown.


Re: Antennas
Kevin Brown wrote:

> Hello all,
>
> I am trying to set up a link, about 1/4 mile long.  I have two radios,
> each producing 200mw of power.  I also am thinking about purchasing one
> of two antennas -- one is 15dBi and the other is 24dBi.  Different
> people in the area have told me that for this link, all I need is a
> 15dBi antenna, but I am wondering if there would be any speed advantage
> (or any advantages at all) to using a 24dBi antenna with this relatively
> short link.
>
> Thanks in advance,
> Kevin Brown.

If you have two radios producing 200mw and the power isn't watered down
by a bunch of connector and cable runs, you should be able to do a
quarter mile with far less than either 18 or 24dbi antennas. The main
advantage of the 24dbi I can think of is focusing the signal so that the
radios receive less interference from outside sources and radiate less
interference to surrounding areas. No speed advantage I would think, not
unless something is wrong.


Re: Antennas
On Fri, 03 Dec 2004 19:12:58 -0600, Kevin Brown

>I am trying to set up a link, about 1/4 mile long.  I have two radios,
>each producing 200mw of power.  I also am thinking about purchasing one
>of two antennas -- one is 15dBi and the other is 24dBi.  Different
>people in the area have told me that for this link, all I need is a
>15dBi antenna, but I am wondering if there would be any speed advantage
>(or any advantages at all) to using a 24dBi antenna with this relatively
>short link.

15dBi will work just fine.  There are two benifits to using 24dBi
antennas.  Thanks for not specifying the hardware so I have to do
guesswork.

1.  Assuming you have enough signal strength, the signal level
necessary to get "full" speed from what I'm guessing is 802.11g
hardware.  This is from a DI-624 but is probably close enough to your
unspecified hardware.
    * 54Mbps OFDM, 10% PER, -68dBm)
    * 48Mbps OFDM, 10% PER, -68dBm)
    * 36Mbps OFDM, 10% PER, -75dBm)
    * 24Mbps OFDM, 10% PER, -79dBm)
    * 18Mbps OFDM, 10% PER, -82dBm)
    * 12Mbps OFDM, 10% PER, -84dBm)
    * 11Mbps CCK,   8% PER, -82dBm)
    * 9Mbps OFDM,  10% PER, -87dBm)
    * 6Mbps OFDM,  10% PER, -88dBm)
    * 5.5Mbps CCK,  8% PER, -85dBm)
    * 2Mbps QPSK,   8% PER, -86dBm)
    * 1Mbps BPSK,   8% PER, -89dBm)
Note that the signal level difference between tolerable speed
(9Mbits/sec = -87dBm) and the fastest (54Mbits/sec = -68dBm) is 19dB.
That's a HUGE difference in required signal.  If you want speed, go
for all the signal strength you can get.

Also, the above sensitivities are for the threshold where the system
starts to generate errors.  You don't wanna be running at the bitter
edge.  Try for 20-30dB of overhead.

Run the numbers at:
  http://www.ydi.com/calculation/som.php
using -68dBm for the receive sensitivity.  Aim for a fade margin (SOM)
of about 20-30dB.  See how much antenna gain it takes.  With:
  Distance     = 0.25 miles
  Tx power     = 20dBm
  Tx ant gain  = 15dBi
  Rx and gain  = 15dBi
  Tx cable loss = 3dB (including connectors)
  RX cable loss = 3dB
  RX sens      = -68dBm (for 54mbits/sec)
I get a fade margin of 20dB.  That's barely tolerable, but not
thrilling.  The 24dBi antennas yield 38dB of fade margin, which is
more than enough.  The magic number is somewhere between 15dBi and
24dBi.

2.  Higher gain antennas have a more narrow beamwidth.  A little
interference from nearby 802.11b/g and microwave ovens can really
trash the thruput.  If you want to keep these things out of your link,
go for a narrow beamwidth antenna.  Also position it to block signals
from potential problem areas.



--
# Jeff Liebermann   150 Felker St #D Santa Cruz CA 95060
# 831.336.2558 voice    http://www.LearnByDestroying.com
#                           jeffl@comix.santa-cruz.ca.us
# 831.421.6491 digital_pager  jeffl@cruzio.com     AE6KS


Re: Antennas

> >I am trying to set up a link, about 1/4 mile long.  I have two
radios,
> >each producing 200mw of power.  I also am thinking about purchasing
one
> >of two antennas -- one is 15dBi and the other is 24dBi.  Different
> >people in the area have told me that for this link, all I need is a
> >15dBi antenna, but I am wondering if there would be any speed
advantage
> >(or any advantages at all) to using a 24dBi antenna with this
relatively
> >short link.
>
> 15dBi will work just fine.  There are two benifits to using 24dBi
> antennas.  Thanks for not specifying the hardware so I have to do
> guesswork.

===
> 1.  Assuming you have enough signal strength, the signal level
> necessary to get "full" speed from what I'm guessing is 802.11g
> hardware.  This is from a DI-624 but is probably close enough to
your
> unspecified hardware.
>     * 54Mbps OFDM, 10% PER, -68dBm)
>     * 48Mbps OFDM, 10% PER, -68dBm)
>     * 36Mbps OFDM, 10% PER, -75dBm)
>     * 24Mbps OFDM, 10% PER, -79dBm)
>     * 18Mbps OFDM, 10% PER, -82dBm)
>     * 12Mbps OFDM, 10% PER, -84dBm)
>     * 11Mbps CCK,   8% PER, -82dBm)
>     * 9Mbps OFDM,  10% PER, -87dBm)
>     * 6Mbps OFDM,  10% PER, -88dBm)
>     * 5.5Mbps CCK,  8% PER, -85dBm)
>     * 2Mbps QPSK,   8% PER, -86dBm)
>     * 1Mbps BPSK,   8% PER, -89dBm)
> Note that the signal level difference between tolerable speed
> (9Mbits/sec = -87dBm) and the fastest (54Mbits/sec = -68dBm) is
19dB.
> That's a HUGE difference in required signal.  If you want speed, go
> for all the signal strength you can get.
>
> Also, the above sensitivities are for the threshold where the system
> starts to generate errors.  You don't wanna be running at the bitter
> edge.  Try for 20-30dB of overhead.
>
> Run the numbers at:
>   http://www.ydi.com/calculation/som.php
> using -68dBm for the receive sensitivity.  Aim for a fade margin
(SOM)
> of about 20-30dB.  See how much antenna gain it takes.  With:
>   Distance     = 0.25 miles
>   Tx power     = 20dBm

200mw = 23dbm



>   Tx ant gain  = 15dBi
>   Rx and gain  = 15dBi
>   Tx cable loss = 3dB (including connectors)
>   RX cable loss = 3dB
>   RX sens      = -68dBm (for 54mbits/sec)
> I get a fade margin of 20dB.  That's barely tolerable, but not
> thrilling.  The 24dBi antennas yield 38dB of fade margin, which is
> more than enough.  The magic number is somewhere between 15dBi and
> 24dBi.
>
> 2.  Higher gain antennas have a more narrow beamwidth.  A little
> interference from nearby 802.11b/g and microwave ovens can really
> trash the thruput.  If you want to keep these things out of your
link,
> go for a narrow beamwidth antenna.  Also position it to block
signals
> from potential problem areas.
>
>
>
> --
> # Jeff Liebermann   150 Felker St #D Santa Cruz CA 95060
> # 831.336.2558 voice    http://www.LearnByDestroying.com
> #                           jeffl@comix.santa-cruz.ca.us
> # 831.421.6491 digital_pager  jeffl@cruzio.com     AE6KS



Re: Antennas


> 200mw = 23dbm


Compared to what?

Ed Cregger




Re: Antennas

>
>
> > 200mw = 23dbm
>
The reference point of 0dbm = 1mw


>
> Compared to what?
>
> Ed Cregger
>
>




Re: Antennas
Moochoes Grassy-ass. <G>

Ed Cregger


>
> message
>>
>>
>> > 200mw = 23dbm
>>
> The reference point of 0dbm = 1mw
>
>
>>
>> Compared to what?
>>
>> Ed Cregger
>>
>>
>
>




Re: Antennas
On Sat, 4 Dec 2004 09:58:50 -0500, "The Amazing Seismo"

>
>> 200mw = 23dbm

>Compared to what?
>Ed Cregger

Commandment #11:  Thou shalt not abrev.

The most common ones in use for wireless are:
dB  = decibels as in a ratio.  
  For power, it's:  dB = 10 * log(ratio)
dBm = decibels above 1 milliwatt into 50 ohms.
dBi = decibels above an isotropic (antenna) radiator.

There are plenty of other dB references (i.e. dBw, dBu, dBv, dbV, dBm,
dbVU, dBFS, dBspl, dBrnCO) and impedance references (50, 75, 600
ohms).


--
# Jeff Liebermann   150 Felker St #D Santa Cruz CA 95060
# 831.336.2558 voice    http://www.LearnByDestroying.com
#                           jeffl@comix.santa-cruz.ca.us
# 831.421.6491 digital_pager  jeffl@cruzio.com     AE6KS


Re: Antennas
It has been a while since I studied for the ancient FCC
commercial ticket exam. The old license, with radar
endorsement, still hangs upon the wall, somewhere in the
house.

In the "olden days", any spec referring to isotropic, point
source, antennas was considered marketing hype with copious
amounts of snake oil stirred into the mix. All serious
references were to a 1/2 wave dipole. Funny how time changes
everything.

Thanks for the memory stimulators.

Ed Cregger


> On Sat, 4 Dec 2004 09:58:50 -0500, "The Amazing Seismo"
>
>>
>>> 200mw = 23dbm
>
>>Compared to what?
>>Ed Cregger
>
> Commandment #11:  Thou shalt not abrev.
>
> The most common ones in use for wireless are:
> dB  = decibels as in a ratio.
>  For power, it's:  dB = 10 * log(ratio)
> dBm = decibels above 1 milliwatt into 50 ohms.
> dBi = decibels above an isotropic (antenna) radiator.
>
> There are plenty of other dB references (i.e. dBw, dBu,
> dBv, dbV, dBm,
> dbVU, dBFS, dBspl, dBrnCO) and impedance references (50,
> 75, 600
> ohms).
>
>
> --
> # Jeff Liebermann   150 Felker St #D Santa Cruz CA 95060
> # 831.336.2558 voice    http://www.LearnByDestroying.com
> #                           jeffl@comix.santa-cruz.ca.us
> # 831.421.6491 digital_pager  jeffl@cruzio.com     AE6KS




Re: Antennas
On Sun, 5 Dec 2004 00:50:42 -0500, "The Amazing Seismo"

>It has been a while since I studied for the ancient FCC
>commercial ticket exam. The old license, with radar
>endorsement, still hangs upon the wall, somewhere in the
>house.

That must have been quite a while ago.  The current incantation is the
GROL (General Radio Operators License):
  http://www.narte.org/h/fccabout.asp
  http://www.w5yi.org/page.php?id=35
It's now issued for a lifetime.  It's basically the same as elements 1
and 3 that you probably took for the old 2nd class license, with a bit
of the old element 4 that was the old 1st class license.  In other
words, it's easier than your previous ticket.  The catch is that they
ask the questions in a rather odd manner so that you must know the
answer they are expecting, which is not necessarily correct or even
close.  I had to borrow the study guide and temporarily revert to
college exam style memorization because of the odd question style.

>In the "olden days", any spec referring to isotropic, point
>source, antennas was considered marketing hype with copious
>amounts of snake oil stirred into the mix. All serious
>references were to a 1/2 wave dipole. Funny how time changes
>everything.

Using a dipole reference was fine before the days of computah modeling
(late 1970's) when antenna patterns were measured, not calculated,
usually with a dipole reference.  This resulted in much abuse by
antenna manufactories in specifying the gain of a dipole over
isotropic.  2.15dB is the accepted figure, but that didn't stop
vendors from using other numbers or positioning their antennas so that
ground reflections would yield the highest gain.

All that changed with NEC antenna modeling.  Measurd dipole over real
ground references made no sense when all the models are based on
isotropic radiators in free space.  In my never humble opinion,
isotropic reference models are easier to deal with and do not involve
any creative number juggling.  At least everyone will agree on the
gain of an isotropic radiator (0dBi) in free space.

>Thanks for the memory stimulators.

Keep going.  Study for the GROL so that you will have more wallpaper.


--
Jeff Liebermann    jeffl@comix.santa-cruz.ca.us
150 Felker St #D   http://www.LearnByDestroying.com
Santa Cruz CA 95060    AE6KS  831-336-2558


Re: Antennas

> On Sun, 5 Dec 2004 00:50:42 -0500, "The Amazing Seismo"
>
>>It has been a while since I studied for the ancient FCC
>>commercial ticket exam. The old license, with radar
>>endorsement, still hangs upon the wall, somewhere in the
>>house.
>
> That must have been quite a while ago.  The current
> incantation is the
> GROL (General Radio Operators License):
>  http://www.narte.org/h/fccabout.asp
>  http://www.w5yi.org/page.php?id=35
> It's now issued for a lifetime.  It's basically the same
> as elements 1
> and 3 that you probably took for the old 2nd class
> license, with a bit
> of the old element 4 that was the old 1st class license.
> In other
> words, it's easier than your previous ticket.  The catch
> is that they
> ask the questions in a rather odd manner so that you must
> know the
> answer they are expecting, which is not necessarily
> correct or even
> close.  I had to borrow the study guide and temporarily
> revert to
> college exam style memorization because of the odd
> question style.
>
>>In the "olden days", any spec referring to isotropic,
>>point
>>source, antennas was considered marketing hype with
>>copious
>>amounts of snake oil stirred into the mix. All serious
>>references were to a 1/2 wave dipole. Funny how time
>>changes
>>everything.
>
> Using a dipole reference was fine before the days of
> computah modeling
> (late 1970's) when antenna patterns were measured, not
> calculated,
> usually with a dipole reference.  This resulted in much
> abuse by
> antenna manufactories in specifying the gain of a dipole
> over
> isotropic.  2.15dB is the accepted figure, but that didn't
> stop
> vendors from using other numbers or positioning their
> antennas so that
> ground reflections would yield the highest gain.
>
> All that changed with NEC antenna modeling.  Measurd
> dipole over real
> ground references made no sense when all the models are
> based on
> isotropic radiators in free space.  In my never humble
> opinion,
> isotropic reference models are easier to deal with and do
> not involve
> any creative number juggling.  At least everyone will
> agree on the
> gain of an isotropic radiator (0dBi) in free space.
>
>>Thanks for the memory stimulators.
>
> Keep going.  Study for the GROL so that you will have more
> wallpaper.


I have the GROL too. It got me a radio sales job once. 8>)

Ed Cregger




Re: Antennas
    I still have my First Class with radar endorsement around.  Of
course, I have the General Class with radar now.  I'm not sure that it
is really good for anything any more.  

    Regards,  


                    Fred



Re: Antennas

> I still have my First Class with radar endorsement around.
> Of
> course, I have the General Class with radar now.  I'm not
> sure that it
> is really good for anything any more.
>
> Regards,
>
>
> Fred
>


Yep, I studied a course for the First Class ticket and
passed it. When I called the FCC office to schedule a test,
I learned that  they had discontinued it and had introduced
the GROL. I should have taken the test in the late sixties
as I had planned, but it wasn't meant to be.

Odd that my GROL test was still mostly on vacuum tube
theory, but that was a good thing. Got the Radar endorsement
a few weeks later.

Last thing I heard was that they were using the Amateur
Extra class test for the GROL. I did get my Extra (1985)
before they lowered the code requirement. I got the first
ham ticket after the commercial ticket, by about a year.

It is sad to see the license for a vocation die. I remember
when having an FCC ticket was as good as gold.

Ed Cregger




Re: Antennas
On Sat, 4 Dec 2004 06:00:38 -0600, "Airhead"

>200mw = 23dbm

Oops.  There went my aura of perfection.  Add 3dB to the fade margin.
That would give 23dB for 15dBi antennas, and 41dB for 24dBi antennas.
15dBi would work, but I would still go for the 24dBi antennas.


--
Jeff Liebermann    jeffl@comix.santa-cruz.ca.us
150 Felker St #D   http://www.LearnByDestroying.com
Santa Cruz CA 95060    AE6KS  831-336-2558


Re: Antennas
: Oops.  There went my aura of perfection.  Add 3dB to the fade margin.
: That would give 23dB for 15dBi antennas, and 41dB for 24dBi antennas.
: 15dBi would work, but I would still go for the 24dBi antennas.

I'm just wondering guys what are you doing.
Here, in Poland, we're making 11Mb (802.11b - DSSS) links for distances of few
kilometers using 30mW radios and 15dBi yagi's.

You're lucky that you can use more that our 100mW of EIRP :-)

cheers,
m.


--
Marcin Lukasik
Milea Wireless Communications, http://milea.pl
phone/fax/mobile: (++48) 13 4480070 / 13 4481148 / 509 390 490

,,the only difference between men and boys is the price of their toys''




Re: Antennas

>: Oops.  There went my aura of perfection.  Add 3dB to the
>fade margin.
> : That would give 23dB for 15dBi antennas, and 41dB for
> 24dBi antennas.
> : 15dBi would work, but I would still go for the 24dBi
> antennas.
>
> I'm just wondering guys what are you doing.
> Here, in Poland, we're making 11Mb (802.11b - DSSS) links
> for distances of few
> kilometers using 30mW radios and 15dBi yagi's.
>
> You're lucky that you can use more that our 100mW of EIRP
> :-)
>
> cheers,
> m.


What does the "I" stand for in EIRP? TIA

Ed Cregger




Re: Antennas
>What does the "I" stand for in EIRP? TIA

Effective Isotropic Radiated Power

Google is your friend.



Re: Antennas

<William P.N. Smith> wrote in message
>>What does the "I" stand for in EIRP? TIA
>
> Effective Isotropic Radiated Power
>
> Google is your friend.
>


I was afraid that you were going to say that. Thanks.

Ed Cregger




Re: Antennas

>
> >: Oops.  There went my aura of perfection.  Add 3dB to the
> >fade margin.
> > : That would give 23dB for 15dBi antennas, and 41dB for
> > 24dBi antennas.
> > : 15dBi would work, but I would still go for the 24dBi
> > antennas.
> >
> > I'm just wondering guys what are you doing.
> > Here, in Poland, we're making 11Mb (802.11b - DSSS) links
> > for distances of few
> > kilometers using 30mW radios and 15dBi yagi's.
> >
> > You're lucky that you can use more that our 100mW of EIRP
> > :-)
> >
> > cheers,
> > m.
>
>
> What does the "I" stand for in EIRP? TIA
>
Effective Isotropic Radiated Power

 isotropic.
   exhibiting properties (as velocity of light transmission) with the same
values when measured along axes in all directions





Re: Antennas

> : Oops.  There went my aura of perfection.  Add 3dB to the fade margin.
> : That would give 23dB for 15dBi antennas, and 41dB for 24dBi antennas.
> : 15dBi would work, but I would still go for the 24dBi antennas.
>
> I'm just wondering guys what are you doing.
> Here, in Poland, we're making 11Mb (802.11b - DSSS) links for distances of
few
> kilometers using 30mW radios and 15dBi yagi's.
>
> You're lucky that you can use more that our 100mW of EIRP :-)

the air is much thicker here from all the bull sh*t we are given so it takes
more power.
Is 100mw really the max for a PTP in Poland?
Here it is 1000mw into a 6 dbi antenna and for every 3dbi over the initial
6dbi you
have to reduce the intenional radiator by 1 db (for 2.4ghz)




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