Not really.

True, using 10 MHz-wide channels instead of 22 MHz-wide channels should help protect the adjacent-channel receivers from overloading and the resulting throughput reduction.

The downside is that a narrower 10 MHz-wide channel won't deliver as much throughput as a wider 22 MHz channel - everything else (over-the-air protocol, interference levels, etc.) being equal. Because the aggregate throughput of the 10 MHz-wide channel site will be lower than a 22 MHz-wide site, the site won't be able to serve as many customers.

Rather than build a lower throughput system, I'd rather build a higher throughput system and just design in enough receiver overload protection by using the methods that we've already discussed.


George Rogato wrote:
If one was to use 3 10MHz channels with 10MHz seperation between channels, they would see the same or better performance than 3 full size channels side by side. So thats my advice, but it won't be wifi. Would be nice if there was a 5 gig option per sector so that you could do 2 2 gig channels at 1 and 11 and a 5 gig sector.


Jack Unger wrote:

Here's what I see that's good about the Pac Wireless antenna:

1. The 3-in-1 design may be the only solution where only one antenna-mounting space exists on a tower.

2. The inclusion of electrical downtilt will minimize overshooting customers.

3. The inclusion of null fill will minimize connectivity problems with customers located very close to the tower.

4. The 15 dBi gain is in the appropriate and usable range.

5. The weight (14 lbs) will make the antenna easy to raise and mount.

6. The price range ($700) is moderate considering that you're really getting three sector antennas in one.

Here's what I see that will need special consideration and planning.

1. The front-to-back ratio is only 15 dB. This is low compared to the 30 dB that I usually recommend for high-quality antennas.

2. The sector-to-sector isolation is 35 dB. When compared to the 83 dB of isolation that 10 feet of vertical separation would provide, 35 dB is a lot less.

In summary, this appears to be a good antenna system with the one limitation that the somewhat low f/b ratio and sector-to-sector isolation will combine to place the burden for good AP-to-AP isolation on the quality of the co-located access point receivers and the quality of the overall AP/site design. Receivers with poor or moderate selectivity (in other words, receivers on Wi-Fi cards) will be overloaded by the other AP transmitters with the result being a throughput reduction that begins when traffic levels increase. The more traffic, the more missed incoming packets, the more retransmissions and the more throughput reduction. The result will be that the site reaches saturation sooner and won't handle as much traffic as a site where the AP receivers are not being overloaded.

The solution (other than to use really expensive equipment which has good receiver filtering and selectivity built in) is to again use single-channel bandpass filters on each AP. These will reduce the level of signal from the co-located adjacent-channel AP transmitters and permit the site to handle more traffic (reach saturation later) than a site where the receivers ARE being overloaded.

Finally, in addition to the above, it's important not to "defeat" the antenna isolation by letting RF energy "leak" directly from AP to AP either on the ground or from PC-card to PC card. Proper shielding and grounding will help to maintain that hard-won receiver-to-transmitter isolation resulting in a high-performance, high-traffic handling, reliable, profitable site.


Rick Harnish wrote:

While we are on this topic, look at the .pdf file and give me your comments
on this all in one sector from Pac Wireless?


Rick Harnish
OnlyInternet Broadband & Wireless, Inc.
Founding Member of WISPA

-----Original Message-----
Behalf Of Jack Unger
Sent: Friday, May 18, 2007 4:18 PM
To: WISPA General List
Subject: Re: [WISPA] 180* sector choices


Thanks for replying to my concerns and suggestions.

Have you tried using 5 to 10 feet of vertical separation between the antennas rather than mounting them all at the same height? 10 feet of vertical separation should be enough to prevent one AP transmitter from overloading another AP receiver.

Secondly, single-channel bandpass filters are available which will provide additional receiver protection from nearby transmitters operating on non-overlapping frequencies.

Finally, higher-priced antennas normally have a higher front-to-back ratio which increases isolation (minimizes interaction) between the antennas. A minimum f/b ratio of 30 dBm is recommended.

Keep in mind that without dealing with these antenna isolation issues, you will likely have the same problems using two 180* sector antennas as you have had using three 120* sector antennas.


Mac Dearman wrote:
See in line please

Behalf of Jack Unger


Before you discard the idea of using three 120* sectors are you
absolutely certain that the "separation" issue is really due to the
antennas not being far enough apart? Have you thoroughly looked at other
reasons why there could be interference between three APs like:
[Mac says:]
Well Jack - I'd like to think that I have exhausted the other
:-) but I am always willing to listen.

1. Using three cards on a single motherboard in one enclosure without
sufficient shielding between them.

[Mac says:]
All of our sector arrays utilize 3 different enclosures with one radio to the enclosure and then I try to separate the enclosures (and antennas) as
far as possible.

2. Using three APs that are configured on frequencies that are too close
together and are interfering with each other?
   [Mac says:]
 Unless we deploy 4 90* sectors we use non overlapping channels (1, 6 &

I guess what I'm asking is (before you go spend money on antennas that
may or may not solve the problem) what equipment are you currently
using, how is it configured, and how far apart can you actually get
three sector antennas on the 25G tower?

[Mac says:]

I have come to the conclusion that it is interference from the backplane
the antennas due to not having adequate separation from each other. These small towers (although 180') like Rohn 25G aren't but 12" across - so you
wind up with the 3 sectors only 1' apart at their bases.
I wish you had some more ideas :-), but thanks for the thought and the

Jack Unger ([EMAIL PROTECTED]) - President, Ask-Wi.Com, Inc.
FCC License # PG-12-25133
Serving the Broadband Wireless Industry Since 1993
Author of the WISP Handbook - "Deploying License-Free Wireless WANs"
True Vendor-Neutral Wireless Consulting-Training-Troubleshooting
FCC Part 15 Certification for Manufacturers and Service Providers
Phone (VoIP Over Broadband Wireless) 818-227-4220

WISPA Wireless List:



Reply via email to