DLW Associates

AM Broadcast Band (BCB) Reject Brick-Wall High Pass Filter

FL1718BW Photo

FL1718 - AM Broadcast Band Reject Brick-Wall High Pass Filter -

This high pass filter is intended to be placed between a HF transceiver output and an amplifier or antenna tuner. It is optimized for a 50 ohm input and output impedance. The filter is a 9th order elliptic design that has a very sharp slope occurring between 1.8MHz and 1.7 MHz. The FL1718 effectively attenuates all AM broadcast band signals by over 40 dB. The filter is packaged in a cast aluminum (o-ring seal) case with SO239 flanged teflon dielectric coaxial fittings. The components consist of high-Q high voltage NPO ceramic or mica capacitors and high Q toroidal coils mounted on a two sided printed circuit board. The PCB and coils are conformal coated. All mounting hardware is stainless.

We offer modifications to this basic filter model that can enhance performance for difficult user applications. These modifications are provided at no additional cost to the user as long as housing and PCB remain common to the design. Changes usually consist of moving a stop band or pass band endpoint and placing a filter null over an interfering frequency. Call or email for a free consultation on your particular interference problem.

Filter Specifications

Filter Performance

The model and spectrum plots of the filter showing the signal frequency Vs the amplitude response of the filter when connected to a sweep generator and an actual antenna are presented below.

model graphic Filter Model

This is a graph of the insertion and return loss calculated by the filter design model generated by the filter design tool.

The design is a 9th order elliptic filter that has the ability to provide a very sharp transition from pass band to loss band.

Each null in the graph represents a high Q series resonant response from each of the four coil/capacitor sets.

These nulls can also be seen in the filter swept response depicted below.

 

 

 

Filter Insertion Loss (Shown on HP3577A Vector Network Analyzer)

HP3577A S(12)

Broadcast band Spectrum - With No Filter Installed (Shown on SDR-14 Spectrum Analyzer)

BCB Spectrum No Filter

The station at 1120KHz is running 50,000 Watts. The close-in 1460KHz station is only running 5000 Watts but uses a highly directional antenna to increase it's effective power toward my location (within 5 miles) and is showing a reading near 0 dBm.

Broadcast band Spectrum - With Filter Installed (Shown on SDR-14 Spectrum Analyzer)

BCB Spectrum with Filter

With the filter installed the station at 1460KHz is down 47 dB and the one at 1120KHz is down 50 dB.

 

 

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