The RF front-end filter plays a critical role in airband reception, where strong out-of-band signals such as FM broadcast (88–108 MHz, 76-100MHz in Japan) and image frequencies can significantly degrade receiver performance.

Initial Approach

At the beginning of this project, a 5-pole Chebyshev band-pass filter was considered in order to achieve high selectivity and strong out-of-band rejection. However, practical constraints quickly became apparent.

A higher-order filter requires:

Larger PCB area
Careful control of inter-stage coupling
Complex tuning and adjustment

In a compact DIY implementation, these factors make the design difficult to realize reliably. As a result, a simpler and more practical 3-pole configuration was selected.

3-Pole Filter Implementation

The implemented filter is a compact 3-pole band-pass design using air-core inductors and discrete capacitors. This approach prioritizes:

Simplicity
Small PCB footprint
Ease of construction

Air-core inductors were chosen for the resonant elements due to their stability and suitability for small inductance values in the VHF range.

Measurement Results

The filter response was measured using a nanoVNA. The results are summarized as follows:

Insertion loss (passband around 120 MHz): approximately -9 dB
FM broadcast band (~100 MHz): approximately -30 dB attenuation
Image frequency region (~160 MHz): approximately -27 dB attenuation

These results confirm that the filter provides effective suppression of strong out-of-band signals, particularly in the FM broadcast band, which is critical for practical airband reception.

Observations and Limitations

While the out-of-band rejection is satisfactory, the insertion loss in the passband is higher than expected.

This led to an important realization:

In VHF filter design, practical factors such as component Q and PCB layout can dominate over the theoretical filter response.

In this implementation:

Small inductance values (e.g., single-turn coils) limit achievable Q
Parasitic effects from PCB layout and component leads contribute to loss
Strong coupling between stages can reduce filter sharpness

As a result, increasing filter order alone does not guarantee better performance in a real-world design.

Design Trade-offs

This filter represents a deliberate trade-off:

Pros
Good suppression of strong FM interference
Compact and simple construction
Stable and reproducible behavior
Cons
Higher insertion loss than ideal
Moderate selectivity compared to higher-order filters

Rather than pursuing a more complex topology, the design prioritizes practical usability and robustness.

Next Steps

Based on these observations, the next step is to improve the filter performance by focusing on component quality and implementation details rather than increasing filter order.

Conclusion

Although a higher-order filter was initially considered, a simpler 3-pole design proved to be more practical for this project. Measurement results highlight that, in VHF designs, real-world implementation factors often outweigh theoretical advantages.

This reinforces an important lesson:

Improving component quality and layout can be more effective than increasing filter complexity.