Antennas

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 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:

-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.

1. 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.

radios,

advantage

relatively

===

19dB.

(SOM)

200mw = 23dbm

signals

"Airhead" wrote

Compared to what?

Ed Cregger

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.

The reference point of 0dbm = 1mw

Moochoes Grassy-ass.

Ed Cregger

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)

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

Ed Cregger

Effective Isotropic Radiated Power

Google is your friend.

: 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.

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.

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

Ed Cregger

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

"Jeff Liebermann" wrote in message news: snipped-for-privacy@4ax.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 ohms).

Effective Isotropic Radiated Power

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

"Jeff Liebermann" wrote in message news: snipped-for-privacy@4ax.com...

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

Ed Cregger

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

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