900mHz Antenna Advice

I would like a recommendation about which antenna to use. I have a Canopy 900mHz SM on my roof. We are about 7 miles from the AP (non LOS). We started with the integrated antenna, but connection was poor and we experienced lots of drops.

We switched to the externalized antenna with a Pacific Wireless 36" 11dbi antenna. The connection quality is much better, but it drops the connection once in a while.

We then upgraded the yagi to the Pacific Wireless 57" 13dbi yagi, and the status are even better than the smaller yagi. However, it has dropped 9 times this afternoon.

I’ve tried mounting the antenna up to 10 feet above the roof line, and suprisingly the connection quality degrades. The connection seems to be best at about 4 - 6ft above the roofline.

Here are some stats:

Integrated: averaged -82
35" yagi: -79
57" yagi: -78

Integrated: ~1200
35" yagi: ~1400
57" yagi: ~1400

Integrated: 4 - 6
35" yagi: ~4
57" yagi: ~2

Downlink efficiency with the 57" yagi is 100%, and uplink is at about 71% for a 10 second test. Uplink is about 90% for a 2-second test. The uplink efficiency seemed about the same for the shorter yagi.

I’ve seen a 15dbi dish antenna on Pac Wireless’ site (http://www.pacwireless.com/products/GD9-15.shtml). Do you think it would be worth it to go for the dish?

We also have experienced antenna crashes, where we couldn’t get to the antenna’s IP address at all and our Internet connection was down. We would have to physically unplug the power supply and plug it back in to reset the antenna. The crashes seemed to occur under heavier load. The integrated crashed all of the time, and the 35" yagi crashed a couple of times. We haven’t seen the 57" yagi crash yet, but we’ve only had it for two days.

My ISP said that if your NAT table fills up, the antenna won’t allow any more traffic through, but it doesn’t totally crash. Have you ever heard of this and do you know what causes this?

Thanks for your help.

The signal strengths you are reporting are what I consider to be on the edge of operational levels. At that level, it would not take much of an interfeering signal to kick you off line.

Regarding the antenna, I have never installed a dish on a 900 MHz SM. There are a few general ideas you can apply though.

Higher gain will increase you signal from their tower, yes. But, it will increase all RF energy that the antenna is recieving, including any potiental interferance. A higher gain antenna should not be the default answer for a poor signal, and may not solve your problem.

Here’s a couple of questions to make sure your ISP is not making any silly mistakes:
What firmware version is your SM?
Is your SM running in hardware or software scheduling mode?

ahull wrote:
What firmware version is your SM?
Is your SM running in hardware or software scheduling mode?

Software Version: CANOPY 7.2.9 Jul 23 2005 01:49:03 SM-DES
Software Boot Version: CANOPYBOOT 3.0
FPGA Version: 070605 (Single, 40MHz ExtBus, DES, Sched )

We are using hardware scheduling.

Thanks for your reply.

On a side note, I just discovered our antenna lockup problem is documented in Motorola's knowledge based here: http://motorola.canopywireless.com/support/knowledge/index.php?page=index_v2&id=288&c=12 So disregard that question.

I agree I won’t leave an install with less then a -77. I would ask your ISP to upgrade the AP and the SM to 7.3.6. The day before that release came out I was setting up a list of borderline customers to roll a truck on that were having similar problems, About seven. I upgraded the AP’s and the SM’s the next morning from 7.2.9 to 7.3.6 and ended up only rolling a truck on one. I saw recieve levels go from a -78 to a -74 or better, and I saw re-reg counts disappear. I would start their first. Ahull has a vary valid point also. I use alot of 900 and see multipath quite often and you would be amazed at what just moving the antenna an inch one way or the other will do. When you say NLOS could you give a better description? Are their any large metel roof’s between the SM and the AP?

As Attitude suggested, I also think upgrading to 7.3.6 would be a good idea.

Depending on the tree cover at your location and terrain between you and the AP, the ultimate solution may just be more height.

I agree with the guys on all points except higher gain antennas. The software definately makes an improvement.

A higher gain antenna has a narrower pattern. This means that it’s ability to reject off-axis noise is better. True, a higher gain antenna will amplify the noise along with the signal, however in rejecting more off axis noise the ratio of signal to noise (S/N ratio) increased with antenna gain.

the directionality of antennas is generally as follows
- 8dB ~ 60 - 70 deg
- 11dB ~ 40 - 50 deg
- 14dB ~ 30 - 40 deg
- 18dB ~ 16 - 25 deg

A Canopy SM needs a continuous 3dB above the noise floor from the AP. Since the noise floor changes you want at least 10dB more signal than noise in order to make sure you can maintain a good link.

The receive sensitivity of the SM is -90dB. As long as the noise floor is below the receive sensitivity of the SM, you could get away with -80dB at the SM. However this is rarely the case. More often the background noise is closer to -80dB, and so you would need a minimum signal at the SM of -70dB. If you have trees in the way, this compounds the issue as you can lose a substantial amount of power trying to get through trees.

So, for SM’s that are right on the edge, the difference between a 13dB Yagi with a 35 degree pattern and an 18dB parabolic with a 16dB pattern can make the difference. We happily provide antenna upgrades to our customers, they have to pay for it.

We charge 199 for a 13dB yagi. The reason we charge 199 is that I probably have already burned alot of time on trying to get the connection up, and it’s probably the second time I have been out there. Besides that, DirectWay is 599.00 so we are still a deal.

When you say NLOS could you give a better description? Are their any large metel roof's between the SM and the AP?

Here's a picture of what our antenna is pointing at: http://www.netwurx.net/~laurap/antennaview.jpg. The terrain between us and the AP includes mostly rolling farm fields and a couple of small lakes. In the picture, the back line of trees is where the ground starts to slope towards the AP. The AP is mounted on a watertower (photo: http://www.netwurx.net/~laurap/watertower.jpg). The AP is about 795ft above sea level (map: http://maps.google.com/maps?f=q&hl=en&q=origen+%26+reynolds,+burlington,+wi&om=1). Our house is about 7 miles NNW of the AP and 887ft above sea level. Both of these elevations are at ground level. I got these estimates from Google Earth. I am not aware of any large metal roofs between the SM and AP.

Here's a photo of our antenna installation: http://www.netwurx.net/~laurap/antennainstall.jpg. This is really on the roof, we just have a step ladder on the roof. We can slide the antenna up and down, but find that this height has the best reception.

We have experienced times during the day when the antenna cannot stay connected. For example, last night the antenna started dropping at 9:30 PM and was able to register again at about 10:10 PM. The integrated would be down for many hours or days at a time. With each antenna upgrade this length of time shortens. In addition, the ethernet port lockup problem hasn't happened yet with the longest yagi. Our power level today has ranged from -74 to -76. It seems to fluctuate on a daily basis.

I will definitely give my ISP a call regarding the AP firmware upgrade. Thanks for your reponses!

Signal levels in the mid -70s are acceptable as long as the background noise is low enough, as Jerry said.

Do you know what the noise floor is?

Is that a cat on top of your ladder? Awesome!

7 miles with no LOS is going to be sketchy. There is a lot of ground clutter that is disrupting the path. Try raising the antenna all the way tot he top of the mast. When aiming make sure you are not touching the antenna forward of the connector.

More gain should blow through that, but of course the only way to know is to order the grid and try it.

Again, thanks for your replies. I just spoke with my ISP. They didn’t want to do the firmware upgrade (sounds like a kind of a pain). They did reboot my AP while on the phone with me and added a slot on the AP. Sorry I can’t remember exactly what the guy said, but it sounded like you can assign slots for the SM’s to connect to.

Anyway, before the reboot and config change, I was getting about 76% uplink effiency for a 10-sec test, and now I’m getting about 93%. We’ll see if it stays that way and will be watching for drops.

If this doesn’t seem to improve performance over the next couple of days, we’ll go for the grid.

Oh, yes, that’s a cat on the ladder. She loves to climb on the roof with us. :slight_smile:

http://motorola.canopywireless.com/supp ... php?t=1995

Your problems seem to be the same issues I'm having. Some days are good, others are awful. Today I'm running great, but tomorrow I may get drops all day long. I'm 1 mile from the AP trying to go through about 600ft of trees. My ISP say they will be out this week or early next week to try to relocate the radio and try a higher gain Antenna.

If I find out anything I'll be sure to pass it along.
Do you know what the noise floor is?

How do you determine noise floor? Can I use the spectrum analyzer to figure it out?

Yes, if you are on the customer side of the SM you can enable the Spectrum analyzer and look at the RF levels within the spectrum you are using.

You are primarily interested in the noise level at the frequency your AP is operating at, but look at the others as well.

You need to look at different times of the day as the noise floor changes. Certainly you want to look when you are having issues and see if the noise is higher during the times you are having issues.