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I'm building an antenna array that operates at high frequencies (around 2 GHz).

I want to use a SDR dongle that has a 75 ohm input impedance as receiver, I've also bought the coax cable, a 4 way adder and a line amplifier that have 75 Ohms as well as the Antenna.

The problem is I need to connect the antenna to a Low Noise Amplifier (LNA) and the LNA to the adder and eventually to the SDR. but they only sell 50 Ohm input/output impedance LNAs.

My question will this have a huge impact on my signal? what is the effect of this mismatch? is reflection going to be really significant?

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  • \$\begingroup\$ What SDR are you using with 75 ohms? \$\endgroup\$
    – user94729
    Commented Oct 23, 2018 at 6:04
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    \$\begingroup\$ @Mikey im using RTL-SDR \$\endgroup\$
    – Marco Castro
    Commented Oct 23, 2018 at 6:10

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The mismatch attenuation will only be 0.177 dB. Not much.

However, a 50 ohm LNA is designed to operate in a 50 ohm system. It will provide the specified noise figure for a 50 ohm input only! With 75 ohms input, the noise figure will be something else (worse) than stated in the datasheet. This leads to loss of sensitivity, but with the data you have provided it is impossible to say how big of a loss.

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  • \$\begingroup\$ Since I'm building the antenna, I can reduce the antenna impedance to 50 ohm so the input matches the LNA but then the output of the LNA is going to be 50 ohm and the input of the other components is 75, could that possibly work? \$\endgroup\$
    – Marco Castro
    Commented Oct 23, 2018 at 6:12
  • \$\begingroup\$ If you could tell me what data do I need to provide I'd be glad to look for it \$\endgroup\$
    – Marco Castro
    Commented Oct 23, 2018 at 6:26
  • \$\begingroup\$ Personally, I think you should definitely use a 50 Ohm antenna and LNA and probably even coax to the receiver. The input impedance at the receiver, as seen by the LNA, can be converted to 50 Ohms by putting a 150 Ohm resistor in parallel with the receiver. There will be signal loss, but probably not SNR degradation (SNR is usually set by the first amplifier). An alternative to the resistor is to use an impedance matching transformer with an appropriate bandwidth (google the term "impedance matching transformer.") \$\endgroup\$
    – user57037
    Commented Oct 23, 2018 at 6:51
  • \$\begingroup\$ @mkeith brings a good point \$\endgroup\$
    – user94729
    Commented Oct 25, 2018 at 4:10

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