After just getting through turning a set of Aerial Facilities Limited SC-220-2N Band Pass cans into a Band Pass Band Reject (tm) duplexer here is what I am going to share from my experiences.
The Science Behind It: Please refere to US Patent #4080601 My Experiences: Please refere to: http://blog.myspace.com/index.cfm?fuseaction=blog.view&friendID=85474861&blogID=252104937 http://blog.myspace.com/index.cfm?fuseaction=blog.view&friendID=85474861&blogID=286228333 http://blog.myspace.com/index.cfm?fuseaction=blog.view&friendID=85474861&blogID=289106710 Inner cavity coax length is not critical when the cavity is in Band Pass mode. Meaning when the can has two inductive coupling loops that work out to be j0+50 ohms. Length for inner cavity and the antenna tee becomes critical in BpBr or reject mode. This will add 6dB of additional isolation if the cable is sucsessfuly cut to be a electrical 1/4 wave or any odd multiple of 1/4 lambda, (3/4 etc.) Velocity factor changes between the connectors and coax must be compensated for. A 8.875" calculated 66% velocity factor cable becomes 9.125" when connectors are on it. Coupling loop design can be optimized when the loop is out of the can. Adjust tuning cap to mid range and sweep loop with spectrum analyzer, tracking generator and tee until notch appears at target frequency. My dimensions were 1" by 1.125" square. Using 0.062" X 0.25" brass stock. Loops work acceptably when not silver plated. If you have rotatable loops, Rotate the coupling loop for optimum coupling before final tuning. When viewed on spectrum analyzer this will be when the distance between the notch and the pass is becomes maximum. Then procede with fine tuning. Cavities seem to exhibit approximently .25 to .3dB less loss each when connected to the phasing harness. A Single cavity scanned at 1.1dB loss, 2 cavities measuered 1.43dB. Johanson caps work fine for 220 and above. Check function of capacitor before installing, I had one that had a broken ring and tuning was all over the place and I could only get like 15dB of notch in some spots. Something with better Q should be used for 2m and below, such as 1/4" hardline or a variable di-electric capacitor such as the Wacom 'trombone'. Now if I can get the rest of the repeater up and running we will be golden.
