FMF-01 Broadcast-FM Filter Pilot

What insertion-loss and broadcast-FM rejection profile does FMF-01 show in its labeled direction and in reverse, and are the controls stable enough to support a later receive-only A-B-A comparison?

FMF-01 is the Nooelec Distill:FM Barebones package with exposed PCB silk reading Flamingo FM. A NanoVNA-H4 swept S21 from 50 to 150 MHz at 401 points with 1 kHz displayed bandwidth. Two-port calibration state C3 placed the reference plane at the fixed cable reference planes.

The control path was Port 1 → cable C1 → calibration through → cable C2 → Port 2. The labeled-forward path placed RF Input toward Port 1 and RF Output toward Port 2; the reverse path swapped those connections.

• Only S21 LOGMAG values are used in this draft.
• The filter remained a passive cabled accessory; no antenna, SDR or intentional radiator was connected.

The THRU control displayed -0.00 dB at 100 MHz at the start and 0.00 dB at 100 MHz after the final search check. No calibration-drift correction was required.

At 98 MHz, disconnect-and-reconnect repeatability was 0.03 dB forward and 0.00 dB reverse. These controls make the bounded result intelligible without converting it into a whole-band specification.

The table below is a set of discrete marker readings, not a continuous measured trace. Loss values are absolute magnitudes of the displayed negative S21 LOGMAG readings.

After the S21 trace was correctly selected, the focused minimum searches found -35.52 dB at 94.500 MHz forward and -34.45 dB at 95.000 MHz reverse. The earlier S11-search photographs are excluded from the minimum claim.

FMF-01 functions as a passive broadcast-FM bandstop filter in this bounded test. The 98 MHz rejection is 33.17 dB forward and 32.97 dB reverse, while the sparse transition readings show material loss in the lower VHF airband. Small forward/reverse differences remain measurement uncertainty, not evidence of nonreciprocity or anomaly.

The pilot does not establish the current product-page statement of greater than 40 dB typical attenuation. The older Revision 1 datasheet provides context closer to the measured result; this is recorded as a specification ambiguity, not a defect, fraud or definitive product dispute.

Sources: [1] [2]

The filter is accepted as a characterized receive-only comparison accessory for a labeled-forward A-B-A trial. The receiver bias-T must remain disabled, and any newly visible signal in that later trial should first be interpreted as reduced receiver overload or intermodulation rather than an anomaly.

• Use antenna side → RF Input and receiver side → RF Output.
• Treat the 98 MHz marker and focused minima as measured values in this bounded sweep, not guaranteed whole-band specifications.
• Acquire a Touchstone export and a denser calibrated numerical sweep before publication-grade flatness or continuous-response claims.

No Touchstone export was collected. The table is sparse, the search minima are photographic instrument readings, and S11/Smith-chart values are excluded because the SOL reference plane was not established at the DUT connector planes.

The transition affects the lower VHF airband. Broader passband, whole-band attenuation and product-specification claims require a broader calibrated sweep and raw trace export. No environmental, emitter or unexplained-anomaly claim follows from this pilot.