Since the 902 to 927 MHz ham band is full of many others signals it was desireable to filter the front end of the receiver as much as realistically possible. Paging transmitters, even though they are diminishing are still a big problem on over crowded hill tops; wireless transmitters are springing up everywhere and many manufacturers don't know how to spell the word filter.....900 is also the seconf harmonic of many 450 commercial transmitters. Let's face it, the reason we have the band at all is it's kind of a garbage dump for RF and as a result of that we need to accept some interference at times. There are many commercial spread spectrum transmitters as well.
The 4 pole combline band pass filter used in this application has proven itself out on many commercial mountain tops in recent years. It provides reasonable insertion loss and fairy good attenuation, 55 dbc at paging frequencies around 929 thru 931 and more at many commercial links from 931 and up to 960 MHz. One of the best reasons for using a combline band pass filter is the complete absence of individual coupling loop to loop responses normally found in "cavity" type filters.
Filter/preamplifier is mounted on a 2 rack unit panel
PHEMT preamplifier is mounted on the sub panel keeping coaxial cables short and saving rack space.
broad band width insertion loss plot from 1 MHz to 3000 MHz for this filter....nuttin gets through this filter except for in band signals.
Insertion loss plot (narrow)
Insertion loss plot thru duplexer and filter
The combination of >50 db attenuation of the filter and the notching of the duplexer (-110 db) provides in excess of -160 db of attenuation around the paging frequencies and other services at the top of the band.
The hi level hybrid PHEMT preamplifier was selected for it's very high output dynamic range. This preamplifier will deliver over one half watt of linear power compared to a standard small signal PHEMT which delivers about 20 to 30 milliwatts. Noise figure is under 1 db and has a 20 db input and output return loss making it cascade nicely with the filter assembly without affecting the tuning of the filter nor the duplexer. This makes its performance much more predictable in actual operation.
Insertion gain plot of entire receiver front end, duplexer, combline band pass filter and hi level PHEMT preamplifier.
Eventuallty the duplexer will be split in half and the entire receiver front end assembly will be mounted remotely near a master receive antenna.