1 AP, 2 Omni-directional antennas?

I have an 9000APC (hardware scheduling, v7.3.6) connected to a Pacific Wireless OD9-11D1 atop a 60’ tower. This is an VPOL, omni-directional, 11dBi mast, which I used instead of an HPOL omni primarily for aesthetic reasons - I’m in an area where “ugly towers” are severly frowned upon.

Things have been working well but I have a problem with users who are closer than 1700’ to the tower, because the OD9-11D1’s vertical 3dB beamwidth is only 7 degrees so it doesn’t come to ground for 1700’.

The tower is on top of a 120’ (above sea level) hill and I’m trying to reach folks who are at the bottom of the hill, so the actual vertical height of the antenna is more like 150’ above these users.

I’d like to mount a smaller omni-directional antenna on the same tower; one with a greater beamwidth that will hit users who are within that 1700’ radius.

What’s the best way to do this? There will only be 5-6 users in this category so I would love not to spend a lot of money doing this. Can I split the output from the AP into two antennas? I presume that I could use a 2nd AP that is synch’d to the first by cable and keep them separated by 8MHz, but that’s probably more expensive.

Appreciate your suggestions.

Put down the low gain omni, and step away from the water tower!!! :lol:

One AP cannot drive two antennas in the configuration you are considering. You will have self interference in a monumental way, and 4dB less power going into each antenna.

The better solution is to put more gain at the customer side by using a connectorized SM and the appropriate antenna. What are you using for client side radio/antenna?

Ah, Jerry, I loved your response. I am using a connectorized SM (Canopy 9000SMC) at the client site, with an MRI 10dBi 12"x12" panel antenna.

If I use the “Omnidirectional Antenna Beamwidth Analysis” at http://cgi.gbppr.org/omni.main.cgi, using values: antennas verical beamwidth of 7; antenna’s downtilt of 1-degree and height of 120’, the program tells me that the “lower 3dB lobe intersection” is 1525’. One of the guys at Pacific Wireless led me to this calculator and asked if I could use the two antennas.

The client is 958’ away from the tower on the downslope of the hill on which the tower is mounted. The tower is 167’ ASL while the client location is 48’ ASL, giving the net height of (about) 120’, as used in the calculation.

The Pacific Wireless fellow was suggesting that this meant that the AP signal was just shooting over the client, giving me a connection that is on-and-off, on-and-off.

Another client who is 1100’ feet away and at an ASL of 55’, was a real problem last summer. Seemingly the same problem.

Hope this gives you what you were asking for. If the AP signal is, indeed, shooting over the client site, what can be done to resolve the issue? That’s why I was trying to bring the signal lower.

I could replace the OD9-11D1 with a 9dBi omni that has a 10-degree beamwidth. That brings the lower lobe intersection closer, but halves my AP output.

The current AP is outputting at 915MHz. Would co-locating a 2nd AP at 906MHz and synching the two not help? What about using a 2nd AP at 906MHz with an HPOL omni (know of any that have good beamwidth and aren’t terribly expensive?)?


The -3dB point is not a brick wall. There is still signal below that point, it just rolls off quickly. If the -3dB point is -4.5deg down (1/2 of 7deg + 1deg tilt) from 0 axis, then the -10dB point is probably somewhere around -8deg down from the 0 axis. You can confirm this looking at the polars of the antenna.

You could certainly add another AP and antenna, but if a customer is connecting intermittently with a 10dB panel, then a 13dB yagi should do the trick. That is a much better solution than another $2k investment for 5-6 customers.

Thanks, Jerry. I have several 13dBi yagi antennas, so I’ll give them a try. I also have some 17" 12dBi panel antennas. Would this suffice or do I want the more focused signal of the yagi?

I’d try them both at a site and spend some time testing to see which works better. My money is on the Yagi (assuming equivalent build quality).

Hi Jerry,

The 13dBi yagi has been installed now for 6 days and 2.5 hours. When I first put it in, the RSSI ran around 1800 and the jitter kept popping around from 1 to 2 to 6 to 1. I have the refresh set to 2 seconds and the jitter changes on every refresh. Here are the stats for this SM:

Software Version : CANOPY 7.2.9 Jul 23 2005 01:49:03
Software Boot Version : CANOPYBOOT 3.0
FPGA Version : 070605 (DES Sched) P9
Session Timeout: 0, AirDelay 9 (approximately 0.25
miles (1323 feet))
Session Count: 102, Reg Count 102, Re-Reg Count 37
RSSI (Avg/Last): 1276/1282 Jitter (Avg/Last): 4/4
Power Level (Avg/Last): -79/-79
DnRate(A): 3500 DnLimit(A): 500000 UpRate(A): 3500
UpLimit(A): 500000 (kbit)
LoUpCIR(D): 0 LoDnCIR(D): 0 (kbps)
Rate : VC 24 Rate 1X/1X

The power level shown here is -79 but when I just looked it a minute ago, it was -59!

Naturally, it would be great if the re-registration number was zero, but it seems to be working Ok - at least it’s better than it was.

I’m still wondering if I should put a 2nd AP on the tower and sync it to the first. If I did that and had them transmitting 8MHz apart, both using omnis, would that be Ok? The 2nd omni would have a great beamwidth so that it would get these close in customers.

Appreciate your help.

At least you are on the right track, you needed more gain. Based in your wildly varying power and jitter, I suspect there is interference. Either trees, or another RF source.

As far as the 2nd AP/low gain omni, my first thought is that the lower gain will negate the increased vertical axis and you will gain nothing.

You could always try it as a single 900 omni could always be used as a filler somewhere else.

You may be looking at needing to go with 3ea AP’s and 3ea 13.5dB 120deg sectors so that you can get some down-tilt.


I’ve modified several SM installations as you suggested and they’ve been running for pretty successfully for almost three weeks. The lowest site has re-registered 27 times in the last 18 days; one that is about 20’ higher and a little farther away from the tower has only re-registered once in that period - and this one was re-registering a lot when configured with a 9dBi 12"x12" panel antenna. Thanks!