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
    * Ch 1, 6 & 11 are the only channels that don't overlap with 5 channel (25Mhz) spacing between them. These three channels are recommended in areas requiring up to 3 channels of operation.

Why would I need a band pass filter?

  • To improve radio reception by reducing interference
  • To increase data rates and coverage distance by improving reception

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

  • Protected against submersion in shallow water (fully weather proofed)
  • Enables tower top mounting
  • Full O-ring sealed top cap
  • Waterproof connectors

DCI Digital Communications Ltd.

Sales Inquires: Toll-Free:1-800-563-5351/ Local: (613) 258-1120/ Fax:(613) 258-7418

Post Sales Technical Support: Local: (514) 631-8837

E-Mail: dci@dci.ca