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I have the following setup to receive NOAA APT satellite images at 137 MHz. It is a DIY v dipole antenna and band-pass filter.

enter image description here

I measured the impedance of the antenna with a network analyzer. It is far from 50 Ω. Where do I have to place an impedance matching circuit, at P1, P2, or both?

This is the circuit of the band-pass filter. It expects 50 Ω on each end as far as I have understood it:

enter image description here

Source: DIY 137 MHz WX sat BP filter

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  • \$\begingroup\$ You need an impedance matching network between the antenna's impedance and your bandpass filter's input impedance (50 Ohm). No impedance matching should be required between the bandpass filter and the SDR since the SDR probably also has an input impedance of 50 ohms. But you have to check this. \$\endgroup\$
    – Charly
    Aug 5, 2022 at 12:50
  • \$\begingroup\$ Test the antenna impedance by itself, and test the filter network with a 50 ohm dummy load. \$\endgroup\$
    – Bryan
    Aug 5, 2022 at 14:38
  • \$\begingroup\$ How is the antenna cabled to the filter? What length of cable? \$\endgroup\$
    – Andy aka
    Aug 5, 2022 at 14:43

1 Answer 1

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My question is where do I have to place an impedance matching circuit? P1, P2 or both?

Your attention should gravitate toward the input side (P1). As @Charly says, assume that output port (P2) is well-matched, and appears as a resistive 50 ohm load. This assumption may be dangerous in the case of cheap SDR receivers, since their designers are far less rigorous than designers of spectrum analyzers in achieving a well-defined input impedance.

Adding components to achieve a match at this critical low-noise point (where losses can impact signal-to-noise ratio) should be done reluctantly, so a designer might modify the filter's input side - you should be able to achieve a match by adding only one extra part. If you add lumped-component matching outside this filter, two extra parts are needed.

One might regard capacitance as more-easily variable than inductance: both C1 and C2 might be made variable. If L1 is a hand-wound copper air-core coil, it might be regarded as variable too, instead of making C2 variable.

This network can accommodate an input resistance up to about 1200 ohms. A pure 1200 ohm resistance allows one to merge C1 and C2 together:

schematic

simulate this circuit – Schematic created using CircuitLab


For input resistances lower than 1200 ohms, the input side of this network could be modified by making both C1 and C2 variable:

schematic

simulate this circuit


One has a problem if input reactance cannot be accommodated by the tuning range of variables C1, C2. In this case, a fixed coil or capacitor would be added to tune out the excess reactance.
If input impedance falls below (roughly) 40 ohms, try using a 10pf variable capacitor for C1...it can accommodate an input resistance as low as 10 ohms.


BTW: the filter center frequency as shown is a bit low for weather satellite - its centre is 134.5 MHz. Both L1 and L2 should be reduced a bit.

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  • \$\begingroup\$ Thank you very much for your detailed answer! I will have some time next week to try out your suggestions. \$\endgroup\$
    – Darellon
    Jan 25, 2023 at 17:50

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