WLAN/WiFi
Bandpass and channel filters

Why filtering at all?

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.

So what? Does this interfere? They are on another frequency, aren't they?

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.

Ok, so what does such a filter help me?

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.

So what filter are there?

WiMo Offer two types of filters: Band filter and channel filter.

Band filter
A band filter has a pass range over the entire 2.4GHz WiFi band, from 2400 to 2485 MHz. Outsiude this range all signals are attenuated. So this filter can be used for all WiFi channels. A band filter should be used at locations where other services are co-located, services which do not operate in the 2.4GHz ISM band (e.g. GSM).
Channel filter
A WiFi channel filter has an even narrower passband, just for one channel. The neighbouring frequencies are strongly attenuated. A channel filter should be used when at the same location other WiFi users (or other services in the 2.4GHz band) are in operation. WiMo does offer a standard program of channel filters for ETSI channels 1, 7, and 13, other channels available on request.

Bandpass filter 2.4GHz

Bandpass filter for the entire 2.4GHz WiFi range, useable for all channels. Useful to reduce interference from close-by transmitters outside the 2.4GHz ISM band (2400-2485MHz).

Available are 4, 6 and 8 pole filters. The pole count of a filter signifies the quality of a filter, visible at the steepness of the skirts at the band edges. The higher the pole count, the steeper the filter skirts. All connectors are N female.

	BPF-FB-4P	BPF-FB-6P	BPF-FB-8P-W

Pole count	4	6	8
Center frequency	2441,5	2441,5	2441,5	MHz
Bandwidth	83	83	83	MHz
SWR	<1:1.3	<1:1.3	<1:1.3
Return loss	>18	>18	>18	dB
3dB Bandwidth	≈ ± 54	≈ ± 43	≈ ± 42	MHz
30dB Bandwidth	≈ +125/-145	≈ ± 70	≈ +57/-59	MHz
50dB Bandwidth	≈ +205/-270	≈ +95/-98	≈ +69/-74	MHz
Max. Power handling	200	200	200	Watt
Impedance	50	50	50	Ω
Temperature drift	≈ 40kHz/1°C	≈ 40kHz/1°C	≈ 50kHz/1°C
Size	63x82x57	70x120x57	165x146x63	mm
Weight	364	635	1900	g
Weatherproof	No	No	Yes
Connectors	N jack (female)	N jack (female)	N jack (female)
Order No.	18880.04	18880.06	18880.08W
Price	243.08 (236.00) EUR	389.34 (378.00) EUR	513.97 (499.00) EUR


SWR diagram
Passband diagram

The BPF-FB-8P-W (Art. 18880.08W) is suitable for outdoor use, in a massive aluminium case and includes a mast clamp for masts with 25-50mm diameter.

Channel filter 2.4GHz

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

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.??
Price	409.94 (398.00) EUR	409.94 (398.00) EUR
Filter for Channel 1
Filter for Channel 7
Filter for Channel 13/td>

SWR diagram
Passband diagram