What is the big metal box?, i see that y ou use a AMP on youre feed. i tryied that on the PIT feed but the powered amp makes it worse, but just the round tube makes it sound better?? idk??
The big metal box is a custom bandpass filter designed for the aviation band. It is what we call a helical bandpass filter because the filter elements are made out of helical coils. Its frequency response looks something like:http://dcidig.sasktelwebhosting.com/html_commercial/graph_118-18-8_wo322.htm
The amp is used primarily to provide a small gain stage ahead of the bandpass filter, which does have a little bit of signal loss. The power divider on the output side of the filter is an active power divider which has its own small gain stages to overcome splitting losses. SO the whole system is pretty much lossless, except for the small loss in the 50-60' of Belden 9913 coaxial cable.
If your amplifier works better without being powered, that's a new one on me. Without power it is, at best, a lossy passthrough. At worst, it's an attenuator. Perhaps you have some powerful transmitters very close somewhere in the FM radio broadcast band or close to it. Those preamps are broadband in nature (i.e., they cover a wide frequency range) and a strong signal will saturate it, causing some of the problem you mentioned. I do have an additional filter right after the tubular amplifier (which I got from Radio Shack) - an FM trap filter to attenuate FM radio stations. Then the big airband filter knocks down any signal that is not in the 118-136 MHz band.