DCI Digital Communications

WiFi Filtering Frequently Asked Questions

General Wi-Fi Information

802.11b standard defines 14 frequency channels in the 2.4 GHz range (known as the ISM band) but only 11 are allowed for unlicensed use in Canada and the United States
Channels 1 - 13 are usable in Europe
22 MHz wide channels
5 MHz channel spacing centered around the following frequencies:

Channel	Center Frequency (MHz)	Bottom Edge (MHz)	Top Edge (MHz)
1*	2412	2401	2423
2	2417	2406	2428
3	2422	2411	2433
4	2427	2416	2438
5	2432	2421	2443
6*	2437	2426	2448
7	2442	2431	2453
8	2447	2436	2458
9	2452	2441	2463
10	2457	2446	2468
11*	2462	2451	2473
12	2467	2456	2478
13	2472	2461	2483

Why would I need a band pass filter?

What is interference and why is it bad?

Interference is unwanted transmission from sources within the ISM band or other transmission sources near the band
Interference can be identified by frequency and signal strength
Interference can confuse the receiver or cover up the signal you're trying to receive

How do band pass filters work?

They pass a band of frequencies (channels) and reduce signals (interference) outside of the filter's pass band
They will not reduce interference from other signals or users on your channel

How can I try to limit my interference problems before I try a filter, or when a filter won't work?

Move to another channel away from the interference
Try channels 1, 6 and 11 to see which works best
Use directional antennas to limit the direction from which you can pick up interference
Use a more powerful signal, to overpower the interference
Change the polarity of the antennas by rotating them both by 90 degrees

What is full band filtering?

Passes the whole ISM band from Channel 1 - 13 (2401 to 2483 MHz)
Allows frequency hops within the whole band (required by some equipment)
Allows you to use any channel within the entire band
Will reject interfering signals such as PCS on 1900 MHz and other signals below 2350 MHz and above 2520 MHz
Will not protect you from interfering signals from other users and sources of interference in the ISM band, only from interference coming from outside of the band

What is channel filtering?

Limits equipment operation to only one channel
Pass only one channel and reject all other non-overlapping channels in the ISM band: A channel 1 filter (2402 - 2422 MHz) will fully protect you from other Wi-Fi signals on channels 6 to 13 (2427 - 2483 MHz)
Reduces broadband noise and crosstalk between channels 1, 6 and 11
Used when interference is inside the ISM band, but on a different channel from the one that you have selected to operate on
Useful for isolation between channels when installing up to 3 different channels at the same location
Also protects against signals outside of the band such as PCS cellular

How does the number of poles affect a filter?

Each pole is a resonant filtering circuit; with additional poles there will be additional filtering strength, but more signal loss
6 pole filters will work well to filter out weaker interfering signals
8 pole filters provide additional filtering of interfering signals and are recommended in high wifi traffic zones or in areas of other strong RF activity

Why use a bi-directional amplifier?

Amplifies signals in both the transmit (Tx) and receive (Rx) directions
- 16dB Tx gain
- 20dB Rx gain
Eliminates the effect of coax losses between the radio and antenna if placed near the antenna
Allows you to mount both filter and amplifier at the tower top near the antenna, which gives benefits for both receive and transmit
- In Receive (Rx) you amplify the signal while it is at its best quality right off the antenna, before the strength and clarity are reduced in the coax cable running between the equipment and the antenna
- In Transmit (Tx) you amplify the signal just before it is injected into the antenna to maximize the transmitted power rather than have it lost in the length of coax cable

Why get a weather resistant filter?

weather-resistant filter cross-section