A combination of a band pass + a notch filter
A cable notch is less deeper than a helix notch. But, a cable notch will make notches close and symmetrical to 145 MHz. A cable notch therefore could enhance the flank-steepness of a helix band pass filter. After this idea the combination: "Helix BPF + Coax cable notch", also was measured. This filter combination combines low loss with high selectivity and good match. In the measured combination simple RG58CU cable was used.
At 150 MHz the attenuation will be already over 20dB!
Do not use PFTE cable for the "notch cable", performance will be less due to
higher cable losses.
If possible, use low loss cable like Aircell7 or Aircom.
Aircell7 or RG58CU coax could be formed to a small roll, also fitting into a paint can. The 3 cans could be soldered together, to form one compact unit.
Insertion loss: 1.1 dB.
-1 dB bandwidth: 2.7 MHz.
-3 dB bandwidth: 4.5 MHz.
"IN": 22 dB,
"OUT": 12 dB.
|Fig. 1: Schematic combi filter.||Fig. 2: Frequency characteristic.|