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There seems to be some debate on emphasizing the importance of impedance matching for maximum power transfer vs maximizing signal to noise ratio with receiving antennas. Some vote for one, some state that maximum power transfer from an impedance matched termination will maximize SNR.

  1. Would connecting a low impedance antenna (75 ohms or less) directly to an emitter-follower (no transmission line) with a high input impedance (600 ohms+) be a valid design approach? It seems that one can ignore such a mismatch as the unloaded antenna signals will be still replicated at the emitter follower output.

  2. The output impedance of an emitter-follower is proportional to emitter current, which can be quite a mismatch from 50 ohms (for example). Could a series 50 ohm resistor be placed between the 1nf caps and primary phase dots on "B" to force a "50 ohm output impedance" regardless of the emitter-follower design? What drawbacks would this approach have?

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    \$\begingroup\$ I believe I know the answer 1: if there is a mismatch at the Emitter Follower, adding a TL won't help because it'd mostly amount to "kicking the can down the road" - instead of having a mismatch with the antenna, you'd have the same mismatch with the TL. So might as well connect the antenna directly. \$\endgroup\$
    – Daniel
    Commented Apr 10 at 8:28
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    \$\begingroup\$ For 2, looks like the RF already sees 6.8k in parallel with 3.3k in parallel with 220 in parallel with the transformer. So it's probably already pretty matched. It's just that 2N3904 are not ideal at 100 MHz. Sometimes it makes it more stable to put a resistor before the Base of the Emitter Follower (even as it might skew the impedance match a bit). \$\endgroup\$
    – Daniel
    Commented Apr 10 at 8:33

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Would connecting a low impedance antenna (75 ohms or less) directly to an emitter-follower (no transmission line) with a high input impedance (600 ohms+) be a valid design approach?

No, you are not maximizing the power of a very weak signal (in an ocean of noise) and hence you degrade the signal and then degrade it more by using a voltage follower. Better to impedance match the antenna to the feedline using a lossless impedance transformer. And, you are also missing the benefit of using a lossless impedance transformer that will also remove out-of-band signals whilst not degrading the power of the signal from the antenna.

Your question 2 is irrelevant because question 1 is a negative.

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  • \$\begingroup\$ I understand the lack of an impedance transformer causing signal degradation. How would a voltage follower cause additional degradation, amplifying stronger out of band currents? In an attempt to simplify the design I removed the buffer all together. Random wire now directly connects to mixer and it seems to work alright still. I have seen block diagrams do that but haven't seen enough schematics to judge. \$\endgroup\$
    – Yousif Alniemi
    Commented Apr 10 at 21:37
  • \$\begingroup\$ You seem to be answering your question in the first line of your comment. \$\endgroup\$
    – Andy aka
    Commented Apr 10 at 23:43

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