WiFi installations often sit on top of tower of high-rise buildings where other services are also located in close proximity. Other services can be telephone (GSM), public radio services (police) or broadcast distribution systems.
Yes, other services can interfere. The reason for this lies in the design of the receiver, any receiver. A receiver (like it is used in the WiFi equipment) contains a control loop which reduces the amplification (i.e. the sensitivity) in the presence of strong signals. This serves to adjust the level to a range where the amplifier works in a linear mode. If this automatic gain control (AGC) wouldn't work, the signal might be amplified in the non-linear range of the amplifier, leading to distortions, ultimatively preventing decoding of the signal.
Further the input of such a receiver is designed with a rather wide filter, so signals far off the intended frequency reach the AGC circuit. This can only be prevented with some effort - just with better filters. And this is where the problem is: effective filters are complex and physically large. Of course the WiFi receiver does have some sort of filter in it's input circuitry, although a small and simple one. The problem is the construction and alignment of such filters, the size is given due to the physics and cannot be reduced. So the usual equipment does have only simple filter measurements. That's ok as long as there are no other strong signals in the vicinity of the receiver, a situation for most but not all WiFi installations.
A filter like the ones offered here strongly reduces all signals outside the intended band. The attenuation usually is around -50dB, farther off the intended frequency it can go down to -110dB!. With such a filter unwanted signals do not reach the AGC circuitry, so the amplifier gain is not reduced, the sensitivity remains at the maximum level. On the other hand such a filter introduces a small insertion loss of 0.2 to 0.5dB which has to be taken into account when calculation total path loss. But then again the total receiver sensitivity is raised due to the fact that the noise is reduced in general, the SNR gets better.
WiMo Offer two types of filters: Band filter and channel filter.
Channel filter limit the pass band to just one channel with 22MHz bandwidth. These filters are used at locations where other WiFi accesspoints or other transmitters in the 2.4GHz band are in use. WiMo offers a standard selection of filters for ETSI channel 1, 7 and 13. Other channels available upon request.
| BPF-CH-6P | BPF-CH-8P | ||
|---|---|---|---|
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| Pole count | 6 | 8 | |
| Center frequency | Channel center | Channel center | |
| Bandwidth | 22 | 22 | MHz |
| SWR | <1:1.3 | <1:1.3 | |
| Return loss | >18 | >18 | dB |
| 3dB Bandwidth | ≈ ± 11 | ≈ ± 11 | MHz |
| 30dB Bandwidth | ≈ ± 24 | ≈ ± 14 | MHz |
| 50dB Bandwidth | ≈ +34/-37 | ≈ ±16 | MHz |
| Channel isolation | 34 | 80 | dB |
| Max. Power handling | 200 | 200 | Watt |
| Impedance | 50 | 50 | Ω |
| Temperature drift | ≈ 40kHz/1°C | ≈ 50kHz/1°C | |
| Size | 70x120x57 | 140x95x63 | mm |
| Weight | 635 | 1000 | g |
| Weatherproof | No | No | |
| Connectors | N jack (female) | N jack (female) | |
| Order No. | 18882.06.?? | 18882.08.?? | |
| Filter for Channel 1 | --- | --- | |
| Filter for Channel 7 | 278.10 (270.00) EUR Buy now | --- | |
| Filter for Channel 13 | 278.10 (270.00) EUR Buy now | 299.73 (291.00) EUR Buy now | |
| SWR diagram | |
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| Passband diagram | |
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