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What type of interface circuit is used between the final amplifier output and the antenna? I see a lot of circuits use a group of inductors and capacitors after the amplifier output to the antenna. For example what are these capacitors and inductors after the coupling capacitor up to the antenna

1- Are these just filters?

2- I also want to know about impedance matching. Do I need impedance matching for my simple wire antenna, at frequency of 433 MHz?

3- How I determine the values of these capacitors and inductors ? enter image description here

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  • \$\begingroup\$ Haven't seen a final amplifier without filters between the amplifying device and antenna. An example circuit would help clarify the context: some filters also include impedance-matching. \$\endgroup\$
    – glen_geek
    Commented Jul 20, 2017 at 12:59
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    \$\begingroup\$ the inductors and capacitors also act as your impedance matching network \$\endgroup\$
    – circuitpatrol
    Commented Jul 20, 2017 at 13:45
  • \$\begingroup\$ Agreeing with @glen_geek: without you actually showing the circuit you're considering, how should we answer this? \$\endgroup\$
    – Marcus Müller
    Commented Jul 20, 2017 at 23:03

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Most often you'll see a PI network, either CLC or LCL. The LCL provides DC blocking. But the CLC only needs one inductor. You can add a 4rth component to implement DC_blocking on either end of the PI; use a "large" cap.

The PI performs by resonating at your frequency of interest, or approximately, based on your antenna mag/phase properties.

If you caps are 10pF at PowerAmp and 30pF at Antenna, the voltage at Antenna will be 10/30 = 1/3 of the PowerAmp output RF voltage. In this CLC loop, the same current flows through each capacitor.

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  • \$\begingroup\$ Mmmm interesting so this is the name of this circuit after the amplifier to antenna (pi circuit) , this is the answer that I am looking for . \$\endgroup\$
    – user156489
    Commented Jul 21, 2017 at 4:09
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That's probably a matchbox, an impedance matching device.

enter image description here

A: Do I need it? Q: It depends on the antenna impedance/type you have it and the amplifier output type/impedance. If you have a 50 ohm output, 50 ohm coax cable and quarter wavelength antenna 50 ohm, then you don't need the matchbox. Especially if you don't how how to tune it, you'd better use direct connection, because a small mismatch is better than total mismatch.

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