JUST A CONCEPT
A 1/4 wave monopole whip with horizontal groundplane has a curve like a bandpass filter. See below a curve for such a monopole for 400 Mhz.
If two such monopoles are used, enclosed in metallic container, and one is connected to antenna and other to receiver, the signal picked by antenna will be transmitted by first monopole, and received by second monopole. There will be double the filtering effect of one monopole.
Can this be a workable filter?
How much is insertion loss?
How much is bandwidth of the pass-band?
A VNA is required to determine all these, and I dont have a VNA 
https://farm1.staticflickr.com/382/19056047500_e70e64498b.jpg
https://farm1.staticflickr.com/431/19056080338_b0ec120bf9.jpg
