I recently acquired a Digital Communications Inc. DCI-146-4H band-pass filter. This is a 4-pole cavity filter for the 2 meter Amateur radio band, with a pass band of 144 to 148 MHz. Physically it's about 12" long, 6" high, and 3" deep, and is very light. The connectors are UHF SO-239.
I purchased this band-pass filter for operating Summits On The Air (SOTA) radio events. Operating 2 meters FM simplex on the top of mountains is a lot of fun, but many SOTA mountains also have high-powered broadcast transmitters, public safety repeater systems, etc. These strong transmitters cause desensitization in receivers, and I can't hear other stations unless they are very strong. This type of band-pass filter removes all out-of-band signals, but won't remove any in-band transmitters like amateur radio repeaters.
Close-In Performance
The datasheet (local pdf) shows the insertion loss at around 0.4 dB. I measured this at about 0.7 dB, and the extra loss was probably the short RG-58 jumpers and RF adapters I used.
Zooming out a bit, rejection at 151 MHz is approximately 20 dB, which is 2 dB less than what is specified in the datasheet. Close enough!
The lower end of this plot is at 126 MHz, where the rejection is around 80 dB. Since the line gets a bit ratty at the low end, we can probably assume we're hitting the dynamic range limit of the Agilent N9912A FieldFox network analyzer. So the rejection down at the FM broadcast band is probably much better than 80 dB, which is excellent.
Far-Out Performance
All datasheets fudge the numbers a bit to make the product seem better than actual life. Some datasheets even outright lie about specifications, and it's up to you to figure out what is misleading in the datasheet. In this case, there is no mention of harmonic resonances in the cavity filter. The manufacturer knows those resonances are present, you know they exist, but the datasheet only shows a 50 MHz span centered at 146 MHz. So how bad are the harmonics?
The third harmonic (at three times the fundamental frequency) is around 435 MHz, and the insertion loss at this frequency is only 5 dB or so. Higher harmonics have better loss, more than 15 dB.
However, does this very low insertion loss at the third harmonic actually pose a problem? Probably not, for several reasons. The radio receiver probably has a low-pass filter on the input. This low-pass filter isn't really good close in (close to the 2 meter band), which is why there is a lot of front-end overload (the problem we are trying to solve). But the radio's low-pass filter should be good up at 435 MHz, so we're protected there.
Additionally, 435 MHz is in the center of the 70 cm amateur radio band. While there are repeaters near 435 MHz, these aren't megawatt-class broadcast FM radio or television transmitters. So
All in all, this band pass filter works really well. If you are receiving on a mountain top with powerful broadcast transmitters nearby, this filter will protect your receiver against nearby