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kvan64
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Which Omni attenna is better for AP - 9dB or 15dB?

Mon Apr 23, 2007 1:24 pm

Hi, I have an RB153 + SR2 with 15dB Omni and 6 m low loss cable + XR2 with 18 dB Yagi and 9m low loss cable. both antennas are mounted on my roof. I can't get connection pass 500m. I read in several threats and saw some one mentioned about better signals with lower power Omni. Should I ditch the 15 dB and buy a 9dB instead?? Plz help!
 
Chipi
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Mon Apr 23, 2007 2:03 pm

Suppose it´s better to use XR2 with OMNI... I think it work better with noise....

Regards
----------{Microcom Argentina SA}----------
Distribuidor Mayorista MIKROTIK en Argentina

http://www.microcom.com.ar
 
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tgrand
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Mon Apr 23, 2007 2:19 pm

SR2 is 26 dB + 15dB Omni = 41 dB
It exceeds any north american limitations.

If you are trying to link up with stock Laptop Wireless cards with no external antenna then I would recommend even smaller onmi such as a 5 dB Omni (better beam width) with multiple AP's in a WDS configuration.

Keep in mind that the laptop radio may be able to see the AP it still can not register to the AP because the laptop antenna is not strong enough for the AP to see the Laptop.

It is much easier for a deer on the road to see the headlights of a vehicle, than for the driver to see the light reflecting off the eyes of the deer.
 
jo2jo
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Mon Apr 23, 2007 5:55 pm

tgrand, if the issue is possibly the AP hearing the Client laptop (which almost always is the issue when serving laptops)

why would he want a lower DB omni? that would affect the APs RX too.


areyou using a laptop to test? maybe that needs a better card..or try a different laptop.
:beep :beep :beep
 
indianboy
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Mon Apr 23, 2007 6:18 pm

tgrand, if the issue is possibly the AP hearing the Client laptop (which almost always is the issue when serving laptops)

why would he want a lower DB omni? that would affect the APs RX too.


areyou using a laptop to test? maybe that needs a better card..or try a different laptop.
The lower the gain of the omni the less noise it creates. Laptop cards are very low powered and they don't like much noise. If you use a low gain omni then there is a chance that your laptop detect the signal but in high gain omni the laptop may not be able to even detect the signal due to lot of noise.

Before going for anything just check for 802.11 signals propagating in your area. if there is a crowd then better use a low gain omni or a sector.

BR
 
0ldman
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Mon Apr 23, 2007 7:11 pm

At roof level, 15dBi.
It has a tighter beam than the 9dBi, it has more rx capability than the 9dBi. I've done this.

I really don't know where these guys are coming from. The higher gain antenna the better. The only catch is as mentioned above, XR2 puts out too much power. You'd have to run it down to 21dB tx to stay legal, assuming no cable loss. Either way, I'd save some money and use an R52 or Cm9. 17dB tx + 15dBi antenna is still over 1W EIRP.

Plenty of power there. If you have line of site, you'll have more than a half mile range with just the laptop. I had a 10.5 dBi on a 130ft tower, 100-150mw output, don't remember now. I had access a half mile away, in my car, and my laptop card had issues. It would not transmit over 30mw.
 
indianboy
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Mon Apr 23, 2007 7:15 pm

At roof level, 15dBi.
It has a tighter beam than the 9dBi, it has more rx capability than the 9dBi. I've done this.

I really don't know where these guys are coming from. The higher gain antenna the better. The only catch is as mentioned above, XR2 puts out too much power. You'd have to run it down to 21dB tx to stay legal, assuming no cable loss. Either way, I'd save some money and use an R52 or Cm9. 17dB tx + 15dBi antenna is still over 1W EIRP.

Plenty of power there. If you have line of site, you'll have more than a half mile range with just the laptop. I had a 10.5 dBi on a 130ft tower, 100-150mw output, don't remember now. I had access a half mile away, in my car, and my laptop card had issues. It would not transmit over 30mw.
Would you like to comment on the Noise Figure in 9 dbi and 15 dbi Omni ?

Hope you understand that the noise figure in Omni increase with the increase in Gain.

The XR2 is a high power radio so you do not need high gain antenna and add more noise to it.

The higher the antenna gain the more the noise . Do you admit the theory ?
 
GotNet
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Mon Apr 23, 2007 7:46 pm

At roof level, 15dBi.
It has a tighter beam than the 9dBi, it has more rx capability than the 9dBi. I've done this.

I really don't know where these guys are coming from. The higher gain antenna the better. The only catch is as mentioned above, XR2 puts out too much power. You'd have to run it down to 21dB tx to stay legal, assuming no cable loss. Either way, I'd save some money and use an R52 or Cm9. 17dB tx + 15dBi antenna is still over 1W EIRP.

Plenty of power there. If you have line of site, you'll have more than a half mile range with just the laptop. I had a 10.5 dBi on a 130ft tower, 100-150mw output, don't remember now. I had access a half mile away, in my car, and my laptop card had issues. It would not transmit over 30mw.
Would you like to comment on the Noise Figure in 9 dbi and 15 dbi Omni ?

Hope you understand that the noise figure in Omni increase with the increase in Gain.

The XR2 is a high power radio so you do not need high gain antenna and add more noise to it.

The higher the antenna gain the more the noise . Do you admit the theory ?
Right! In fact we use a 50 ohm resistor (with a 0db noise figure) and cover three states here in the US with one AP.
 
indianboy
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Mon Apr 23, 2007 9:49 pm

As i already said before thinking of anything try to scan your are for wi-fi signal. If you do not have a crowdy spectrum then noise from 15dbi Omni can be acceptable as it will definitely boost your range.

If it was a Directional Antenna then the matter would have been totally different. The noise issue would have been negligible.

It is the matter of creating a noise free environment instead of adding a
100000 dbi Antenna to a Microwave Oven.
 
0ldman
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Mon Apr 23, 2007 11:30 pm

Would you like to comment on the Noise Figure in 9 dbi and 15 dbi Omni ?

Hope you understand that the noise figure in Omni increase with the increase in Gain.

The XR2 is a high power radio so you do not need high gain antenna and add more noise to it.

The higher the antenna gain the more the noise . Do you admit the theory ?
Bud, you're off in left field. A high output XR2 will do nothing for increasing the receive from a laptop.

A high gain antenna generally helps with noise as the beam is more focused, in this case, narrower vertical beam. A 15dBi omni on top of a tower would be a different discussion.

I have installed this, I'm not talking theory. Noise level does not increase much with the higher gain omni on top of a building.

Matter of fact, 10.5dBi on top of the 130ft tower, mentioned earlier, still only shows -98ish noise. I do live in a low noise area, however, a 15dBi omni on top of a press box at a ball field in town did not show much increase in noise, still around -96 with 3 ptp links shooting all around as well as every business on the street plus the hospital hotspot all within range.

I will not agree with your theory as I've done it in practice and proven you wrong.

antenna over tx power every time.
 
jo2jo
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Mon Apr 23, 2007 11:33 pm

gotnet what are you talking about? im assuming thats a joke/?

anyway, indianboy im pretty sure you are wrong.

having a higher gain antenna will NOT cause more noise...it WILL cause more noise to be received by the AP, but the assumption is your client will be some what more powerful than the noise and the card will be able to separate.

From the client end, having more powerful antennas at the AP increases the chances of your AP signal overcoming any noise (already in the air) with regards to your client's RX..

very often a HUGE issue we have seen alot is the fact that LAPTOPS have bad radios generally, and BAD BAD Rx sensitivity, so you really need a High power antenna..and a high power card for the laptop client (w its RX sens of -82 or -84 etc...) to hear the AP.

the weak TX power from the laptops (generally 28mw to 60mw radio and crap antenna , most ~35mw) is compensated for by a higher gain antenna at the AP and a the radio's good RX sens. of -92 to -95 etc...
:beep :beep :beep
 
0ldman
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Mon Apr 23, 2007 11:49 pm

Intel 2200 and above have pretty decent rx(-93ish), but they have a driver bug with tx power. Max I've ever gotten in Windows was 31.6mw.

I know I've gotten at least 50mw in Linux.

I'm not a fan of Broadcom's in laptops. Too many issues with too many AP's.
 
GotNet
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Tue Apr 24, 2007 2:54 am

jo2jo, yes sorry. Ya have to admit it is getting pretty deep in here. :)
 
kvan64
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Tue Apr 24, 2007 1:30 pm

Thanks guys,
It's interesting to see that there are different opinions and supporters on both sides. Obviously you all have good reasons to believe that a certain size of omni power is better for AP. Let me clarify my problem. I do get good connections within a few hundred metres but if I go any further, I can't even establish connections. I am just dissappointed because as specified, the 15dB Omni should extend the signal rang up to 5km. 500m is only 1/10th of the theoretical capability. In addition to the antenna, SR2 and XR2 also have high power. Simply dissappointed!
 
indianboy
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Tue Apr 24, 2007 2:18 pm

Thanks guys,
It's interesting to see that there are different opinions and supporters on both sides. Obviously you all have good reasons to believe that a certain size of omni power is better for AP. Let me clarify my problem. I do get good connections within a few hundred metres but if I go any further, I can't even establish connections. I am just dissappointed because as specified, the 15dB Omni should extend the signal rang up to 5km. 500m is only 1/10th of the theoretical capability. In addition to the antenna, SR2 and XR2 also have high power. Simply dissappointed!
Under that condition i assume that you will have to increase the height of the omni.

Oldman, It does'nt matter if you prove me wrong, but where did you check the RSI ? have you used a spectrum analyser ?If not then use a spectrum analyser and see the noise and interference in a 9 dbi omni and a 15 dbi omni.

15 dbi will increase the range but will also noise . It other thing is that the noise generated will cause no hard due to the increase power factor. I have already checked the same with spectrum analyser and if you want to see the result then i can even post the screenshots.

BR
 
indianboy
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Tue Apr 24, 2007 2:20 pm

Noise level does not increase much with the higher gain omni on top of a building.
So, you do accept that noise level increases with the increase of gain.

What an irony.. :roll:
 
0ldman
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Tue Apr 24, 2007 4:15 pm

A high gain antenna generally helps with noise as the beam is more focused, in this case, narrower vertical beam. A 15dBi omni on top of a tower would be a different discussion.
With altitude comes more interference, and the omni will hear more noise, as well as more signal. He's talking roof level. Not an issue unless he's smack in the middle of wireless central. Even still, a 15dBi omni is going to show signifigant gain in signal, it is not going to show a 15dB gain in noise.

I'm done with this, you can keep your eye rolling.

kvan64, theoretical loss is usually much less than real world. Also consider most of these calculations are working with hard power figures. OFDM and DSSS spread the power over 20(or 22)MHz. The calculations are not complex enough to allow for the variance in signal strength. At least none I have seen allow for it.
 
electrocomm
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Tue Apr 24, 2007 4:27 pm

I saw this sort of problem a lot years ago when the 15dB omni was much more common. Two problems is that high gain omni antennas have; very narrow vertical beamwidth and they actually tend to radiate their pattern upwards toward the sky. One of the common 15dB omnis in the business we had measured by a test lab. It only delivered 11.5dB of gain and at the upper 2.4 frequencies it radiated the main beam 7 degrees upward. So unless you mounted it upside down it would talk to airplanes better than folks on the ground. That's why the 9dB omni would probably work better. Less uptilt and wider vertical beam

Mike B
 
aaronm
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Tue Apr 24, 2007 11:21 pm

Not to add fuel to the fire or anything, but it seems pretty obvious that an antenna will amplify whatever signal is sent to it. Since no radio is perfect they all transmit a small bit of noise (some more than others lol) and in turn the antenna will amplify both the carrier signal (the part you want) right along with the noise from the radio. So yes, an antenna will increase noise, but the noise floor will stay at a proportionate level compared to the carrier signal.

Bottom line if you need a stonger signal get a more powerful antenna.
 
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jwcn
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Wed Apr 25, 2007 1:14 am

electrocomm is dead on.

The rest of you are a perfect example of why I don't use 802.11 for ptmp anymore.

Bunches of good network engineers who absolutely suck as RF engineers.

This is why noise exists to begin with.

Example A:

RB532a with SR2 connected to a 15dBi omni using a 5' Heliax jumper. 10 clients running at -71 to -83

Switched out to:

RB532a with CM9 and 6dBi omni using 5' Heliax jumper. Same 10 clients now running at -48 to -59

Antenna is roof top level.

Higher gain antennas are always better than higher gain radios EXCEPT when talking about Omni antennas- especially VPOL
 
0ldman
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Wed Apr 25, 2007 5:43 am

I've got one customer, odd shaped hotel, with WiFi I set up, WDS mesh, no ethernet access between buildings.

I have 10dBi omni's for 4 AP's and I had to go with a 15dBi on the last.

The 10dBi don't go much outside the building, which is good. The AP with the 15dBi, I can access from a quarter mile away on one side, which was the only way I got access to the other rooms penetrating the outside walls.

Knowing what I know now, I would have gone with a panel antenna for the last AP. That being said, the AP with the 15dBi has range the others with 10.5 dBi can't touch, and all of this is at ground level, 30mw output power.
 
indianboy
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Wed Apr 25, 2007 10:00 am

I've got one customer, odd shaped hotel, with WiFi I set up, WDS mesh, no ethernet access between buildings.

I have 10dBi omni's for 4 AP's and I had to go with a 15dBi on the last.

The 10dBi don't go much outside the building, which is good. The AP with the 15dBi, I can access from a quarter mile away on one side, which was the only way I got access to the other rooms penetrating the outside walls.

Knowing what I know now, I would have gone with a panel antenna for the last AP. That being said, the AP with the 15dBi has range the others with 10.5 dBi can't touch, and all of this is at ground level, 30mw output power.
It is obvious that you will get extended range with a 15dBi Omni. But the topic in here is the noise . Why are you pulling the topic to the range.

I aleady said that there will be increase in range with a high gain omni but there will also be an increase in noise. It doesnt matter how significant is the noise figure but it will be amplified.
 
0ldman
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Wed Apr 25, 2007 1:15 pm

It is obvious that you will get extended range with a 15dBi Omni. But the topic in here is the noise . Why are you pulling the topic to the range.

I aleady said that there will be increase in range with a high gain omni but there will also be an increase in noise. It doesnt matter how significant is the noise figure but it will be amplified.
maybe because it is part of the question which is the reason for this thread, hmm? It does kind of matter how signifigant the noise figure is, there will always be noise. ALWAYS. Whether it causes problems with your link is the issue.
Thanks guys,
It's interesting to see that there are different opinions and supporters on both sides. Obviously you all have good reasons to believe that a certain size of omni power is better for AP. Let me clarify my problem. I do get good connections within a few hundred metres but if I go any further, I can't even establish connections. I am just dissappointed because as specified, the 15dB Omni should extend the signal rang up to 5km. 500m is only 1/10th of the theoretical capability. In addition to the antenna, SR2 and XR2 also have high power. Simply dissappointed!
 
indianboy
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Wed Apr 25, 2007 2:25 pm

Thanks guys,
It's interesting to see that there are different opinions and supporters on both sides. Obviously you all have good reasons to believe that a certain size of omni power is better for AP. Let me clarify my problem. I do get good connections within a few hundred metres but if I go any further, I can't even establish connections. I am just dissappointed because as specified, the 15dB Omni should extend the signal rang up to 5km. 500m is only 1/10th of the theoretical capability. In addition to the antenna, SR2 and XR2 also have high power. Simply dissappointed!
No One seems to sort his problem.

To Kvan64

1. What is the output of your card ?
2. What is the height of your Antenna ?
3. What cable are you using to connect to antenna ?
4. What is the height of the obstruction in your ares i.e Trees , Building etc
5. What do you see if you increase the height of your Antenna ?

Answers to the above can help in solving your link problem

BR
 
kvan64
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Wed Apr 25, 2007 3:31 pm

I set TX power default to both of my SR2 and XR2. As Chipi suggested, I have connected the XR2 to the 15dB Omni. The Omni is about 1.6m and is mounted on my roof which is roughly 8m above the ground. There are very few trees arround and the antenna is just above the surounding houses. There are also some open spaces/grasslands on one side which leads to a small parkland. I went through the parkland and still had good connection up to 500m (still see the antenna from my roof through some leaves). Above that distance, connection could not establish.
The cables that I use are super low lost CFD400.
I will try to increase the height of the antenna and see. Just have to find a proper pole to extend the mounting base.

Ta
 
0ldman
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Wed Apr 25, 2007 8:49 pm

What card is in the laptop?
 
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jwcn
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Thu Apr 26, 2007 8:59 am

Sounds like the more present issue here is you don't have line of site.

Plus I'm sure the output on an XR2 combined with the 15dbi omni makes what your doing illegal almost anywhere.
 
kvan64
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Thu Apr 26, 2007 2:05 pm

Oldman, I have an Artheros Mini PCI 54M b/g in the laptop
 
kvan64
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Thu Apr 26, 2007 3:19 pm

Sounds like the more present issue here is you don't have line of site.

Plus I'm sure the output on an XR2 combined with the 15dbi omni makes what your doing illegal almost anywhere.
yes I know. We have Max legal output power (EIRP) to be 4W / 36 dBm here in Australia. However, with my current conditions and loss in long cables, I don't think I have reached the limit.
 
0ldman
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Thu Apr 26, 2007 8:20 pm

Unless you're running some really cheap cable, you're still illegal.

LMR 400 or equal would take around 75ft to lose 5dB at 2.4GHz.

Another thing, the AP may be drowning out the laptop's weak signal. Try dropping your tx power.

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