Re: BPF too good? #filter


Ben Bangerter, K0IKR
 

Bob,

Coincidentally I constructed a QRP-Labs 20M BPF just last night.  When I saw your post today, I went to work characterizing my filter.  I don't have a VNA, but I do have a Siglent SA/TG here, and a home-made resistive return loss bridge.  I tuned the filter for minimum loss, and it peaked at 14.227 MHz with a loss of 2.54 dB.  The 3 dB bandwidth was 1.38 MHz, which seems reasonable (Hans aims for a BW of ~10% i think).  At 10 dB down the width was 2.72 MHz.  The loss is higher than I like, and may be due to the resonators being undercoupled.  I will mess with a gimmick capacitor to see if I can reduce the loss while keeping the filter shape nice.

Then I did return loss measurements.  My bridge is not very good, and shows only a 35 dB RL with an accurate 50 Ohm load.  I observed a 16.2 dB RL at 14.227 MHz, but the bottom of the RL curve was 14.32 MHz, with an RL of 17.2 dB.  When I touched up the tuning to put the minimum RL at the 14.227 MHz center frequency, the RL was 18.6 dB, corresponding to a VSWR of 1.22.  But the filter was degraded slightly.  The center frequency was unchanged, but the loss was 2.87 dB and the BW was 1.44 MHz.  The attached plots are from after the filter was tuned for maximum RL at 14.227 MHz.

73 Ben K0IKR


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