NanoVNA for 137MHz Satellite Weather BPF+LNA Testing

Introduction

In a previous post I tested a 433MHz BPF + LNA for its frequency response or S21 (Ref.1). In this post I will test a 137MHz BPF + LNA for the satellite weather band. Recently the NOAA15/18/19 satellites were decommissioned, so I wanted to know if my existing BPF/LNA could accommodate other weather satellites.

Measurement Setup

Fig.1 Block Diagram Measurement Setup
Fig.2 NanoVNA Connected to 30dB pad
Fig.3 NanoVNA Connected to 137MHz BPF/LNA

Figure 1 shows the Block Diagram of the Measurement Setup. Figure 2 shows the NanoVNA connected to a 30dB pad (10dB + 20dB). Figure 3 shows the NanoVNA connected to the 137MHz BPF/LNA. Port 1 feeds the unit through a 10dB pad. The unit output feeds a 20dB pad on Port2. The pads are chosen so that we don’t overdrive the amp and don’t overdrive Port2. The NanoVNA will measure the S21 characteristic or forward voltage gain of a network.

Measurement Results

Fig.4 137MHz BPF/LNA Response to White Noise Gain=30dB (nom)
Fig.5 S11 & S21 of 30dB Pad (10dB + 20dB)
Fig.6 137MHz BPF/LNA Gain = 30dB (nom)

I did a quick check first using my spectrum analyzer. I connected a whip antenna to the BPF/LNA and just examined the response to white noise. From the peak to average noise level the gain is about 30dB. Next I measured a pad of 30dB (10dB + 20dB) which will have a flat response (-32dB). Then we consider the filter/amplifier to be a filter with response of Gain(Filter-Amp) – (-32dB) = -2dB – (-32dB) or approx 30dB at the band pass frequency.

Fig.7 YouTube Video NanoVNA for 137MHz Satellite Weather BPF + LNA Testing

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References

#1. – “NanoVNA for 433MHz/70cm BPF + LNA Testing”
https://jeremyclark.ca/wp/telecom/nanovna-for-433mhz-70cm-bpflna-testing/

By Jeremy Clark

Jeremy Clark is a Senior Telecommunications Engineer and Advanced Amateur Radio Operator VE3PKC. He is the author of E-Books on Telecommunications, Navigation & Electronics.