144 mhz Cavity preamp


Ray Johnson:
I am looking for plans for a 144 mhz cavity preamplifier need some more rx gain and want to knock down some pager imd. any help appreciated! Thanks!!!

Steve Katz:
There were plans for such an item in the ARRL VHF-UHF Manual in the early 1970's editions; however, I'd not recommend a "cavity preamp" in that the return on investment isn't very good due to the inherant limitations of the design.

If you need to reduce/eliminate pager interference causing receiver IMD, the PAR Electronics filters do a better job, at far lower cost and smaller size, than any kind of cavity because they notch the entire VHF paging band while passing 2m signals with literally zero loss.  And it's more effective to notch than to "bandpass."  A single cavity having an unloaded Q of 600 at 146 MHz will only have about 30 dB rejection in the 152 MHz range, where pagers populate, while having about 1dB passband loss at 146 MHz.  The PAR filter has >40 dB rejection in the pager range while providing <0.2dB loss at 146 MHz.  And the cavity occupies considerable space, has substantial weight, and if not built very precisely of premier materials, will drift with temperature.  The notch filter can be held in one's palm, weighs a couple of ounces, and doesn't drift.

73 de Steve, WB2WIK/6

Ray Johnson:
Many thanks for your input Steve!!


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