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
Thanks in advance,
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.
15dBi will work just fine. There are two benifits to using 24dBi
antennas. Thanks for not specifying the hardware so I have to do
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
* 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:
-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
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.
the air is much thicker here from all the bull sh*t we are given so it takes
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
have to reduce the intenional radiator by 1 db (for 2.4ghz)
: 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 :-)
That must have been quite a while ago. The current incantation is the
GROL (General Radio Operators License):
'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.
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.
Keep going. Study for the GROL so that you will have more wallpaper.
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
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
Thanks for the memory stimulators.
"Jeff Liebermann" wrote in
message news: email@example.com...
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
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.