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I am building a broadcast FM receiver (88MHz to 108MHz).

My antenna is a 1/4 monopole with 4 quarter wave radials and has an impedance (Zsource) of 50 ohms.

I am connecting it to a 75 ohm rg6 coax cable that will go to an L network for impedance matching 75 ohm coax to 25 ohm circuitry.

Antenna (Z=50ohm) -> RG6 Coax (Z=75 Ohm) -> L-network -> radio amplifier (Z=25 Ohm)

I have read on many HAM blogs that the mismatch between 50 ohm antenna/75 ohm coax is negligible;

I know that transmission lines act as impedance transformers when source impedance differs from characteristic impedance, ie 50 ohm source to 75 ohm coax.

My question: would I have to buy a coax cable that is 1/2 length of a 98 MHz (middle of my frequency band) to best negate impedance transformation effects of mismatched transmission line for my frequency band?

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  • \$\begingroup\$ Why not use 50 ohm coax for your 50 ohm antenna ,and then length does not matter. \$\endgroup\$
    – Autistic
    Commented Dec 7, 2022 at 4:09
  • \$\begingroup\$ A valid point! 50 ohm coax is pretty expensive; I guess the real question is whether the price is worth avoiding the headache of a mismatch. \$\endgroup\$ Commented Dec 7, 2022 at 4:39
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    \$\begingroup\$ If you are avoiding 50 ohm coax as it's expensive, but seem to be OK with 75 Ohm stuff, how about use 1/4 wave of two 75 ohm in parallel (=37.5 ohm) to get between your 50 ohm antenna and your 25 ohm receiver? \$\endgroup\$
    – Neil_UK
    Commented Dec 7, 2022 at 6:14
  • \$\begingroup\$ I did some surfing and was able to find 10' rg58 and five so239 connectors for $20 on Amazon! Problem seems solved, thanks for your ideas. \$\endgroup\$ Commented Dec 7, 2022 at 7:11

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Use a matching network, with this network you can match 50 to 75 ohm coax.

enter image description here

Source: http://ham-radio.com/k6sti/match.htm

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  • \$\begingroup\$ Thanks for that, great link! Question: Is there benefits for a matching network over an impedance transformer in the general sense? I have a feeling that transformers have more bandwidth but I am not sure on that. \$\endgroup\$ Commented Dec 7, 2022 at 7:13
  • \$\begingroup\$ It's adjustable, a coax transformer isn't very adjustable \$\endgroup\$
    – Voltage Spike
    Commented Dec 7, 2022 at 15:19

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