At 3/9/2010 20:12, you wrote:
>OK, question... > >If you put a cable which is 1/4-wavelength at VHF between the T and the >UHF cavity, it's 3/4-wavelength at UHF. Since any odd multiple of a >quarter wavelength will invert the impedance, what will this really >accomplish on the UHF cavity side? Doesn't matter at UHF, since the cavity "looks" like (hopefully something close to) 50 + j0 ohms @ UHF, so the cable length has no effect (other than plain ol' cable loss) @ UHF. At VHF, the short at the UHF cavity connector (I'll take Gary's word that it looks like a short off-resonance, though to be sure you'd want to put the can on a VNA to get the actual phase angle at the connector) needs to be transformed to an open at the T so it has no effect & VHF. The short-to-open transformation @ VHF is accomplished with a 1/4 wavelength of coax @ VHF. > The dual-band diplexers are usually high-pass/low-pass arrangements, and > lose something like 0.2 dB while providing 40 dB or more isolation. > Assuming you get a real one, and not something made with PIM-prne > materials, would this not be a safer bet? It's true you wouldn't need to mess with cable lengths if a cross-band diplexer were used, but OTOH it would be another piece of hardware in the system that really isn't necessary, since the cavities are already there. Plus if you're really worried about PIM, you'd probably have to move up to something like a cross-band coupler from TX-RX, which IIRC runs over $300. Bob NO6B
