I am still very new to RF and I am trying to understand Impedance matching, so my question could be stupid.

I have already learned the basics of it, I know how to use Smith charts, which components to use to match it etc. But one question has risen up:

Reading from most sources, matching must be made between the source and the load (antenna). However, reading on antenna-theory.com (which is the source I learned nearly all I know about Impedance matching) it says this: enter image description here

In all examples I have seen on the matching series on the website, the impedance used in the Normalized impedance formula was always 50 ohms (coaxial cable I suppose)

And that's the thing which is confusing me. I am going to try a RF project and I just can't get over this thing.

  • 3
    \$\begingroup\$ Pedantically speaking, nothing has to be matched to anything else. And for perfect matching, everything has to be matched to everything else. \$\endgroup\$ Commented Apr 21, 2022 at 14:24
  • \$\begingroup\$ What's the RF project you are planning to work on? \$\endgroup\$
    – vu2nan
    Commented Apr 23, 2022 at 3:21
  • \$\begingroup\$ @vu2nan I wanted to make a half wave dipole to catch one certain AM station. Though none of the radios I have at home go so high on the frequency band. So I decided to see how difficult of a circuit the receiver could be, thinking that I may just copy some circuit and use it without understanding (something I don't like to do.) But I came across a video series, where a guy was explaining such one, an it is very simple - just LC oscillator, diode for rectification, Capacitor to smooth the wave and a resistor (whose function I didn't quite understand). So now I am trying to understand the rest. \$\endgroup\$ Commented Apr 23, 2022 at 11:28
  • \$\begingroup\$ @TheMadTomato1209 - Hi! Thanks for the info. Is that AM station on medium wave or shortwave? What's the frequency? Are you trying to build a crystal radio or a powered one? \$\endgroup\$
    – vu2nan
    Commented Apr 23, 2022 at 12:12
  • \$\begingroup\$ @vu2nan The station is 126 Mhz. The design I was looking at didn't include any amplifiers, but I am not sure if one will be needed. Just so you know, although I marked the answer I will be grateful if you keep on helping me. \$\endgroup\$ Commented Apr 23, 2022 at 16:28

2 Answers 2


Does an antenna have to be impedance matched to its transmitter circuit or the transmission line from it?

If the t-line is short i.e. less than one-tenth of a wavelength of the carrier frequency, then match antenna to the transmitter.

Else, match the antenna to the t-line using an L-pad, T-pad or Pi network (or combinations) if the antenna is not particularly close to a resistive impedance.

Normally there's no great need to match the transmitter to the t-line although this is quite often done.

  • \$\begingroup\$ Thank you for the answer. Does the same apply if the antenna is receiving? \$\endgroup\$ Commented Apr 21, 2022 at 15:41
  • \$\begingroup\$ The receiver input impedance needs to be matched to the t-line. \$\endgroup\$
    – Andy aka
    Commented Apr 21, 2022 at 18:05
  • \$\begingroup\$ So if I take it right, if my antenna has, lets say 75 ohm impedance, the total impedance of the whole receiver circuit (after calculating it) should ideally be 75 ohm too? \$\endgroup\$ Commented Apr 22, 2022 at 14:37
  • \$\begingroup\$ Not quite. A 75 ohm receiving antenna can feed a t-line of 50 ohm but, the receiver needs to be nominally 50 ohm to avoid disruptive reflections. Reflections occur on things that transport signals a distance and they occur initially on the output of that signal transportation mechanism. It's not ideal to do this of course - the preference is to match everything AND, matching everything is usually so easy to do. \$\endgroup\$
    – Andy aka
    Commented Apr 22, 2022 at 17:18
  • \$\begingroup\$ I am starting to understand it well. I just have 2 more questions: 1. In the last example you gave, does the antenna need something to match to the t-line, or it can just be connected straight to it, the only "matching" being the one on the receiver side? 2. A stupid one but it bothers me - when matching, why is complex impedance (R + jX) always used, instead of the one, calculated with Pythagorean Theorem? \$\endgroup\$ Commented Apr 23, 2022 at 11:32

Antenna, transmission line, transmitter/receiver don't need to be matched. A better question would be 'what are the consequences of any degree of mismatch?'

Mismatch causes signal reflections, which create a loss of signal reaching the far end. For a transmitter, the loss reduces the available power to the antenna. For a receiver, the effect of the loss is to increase its noise figure.

This mismatch loss is a function of cable length and frequency, making it difficult to predict performance. In a properly matched system, varying the length of cable, or the frequency by a small amount, does not affect the loss significantly.

Mismatch causes signal reflections, which add to the forward signal to create antinodes of higher voltage and current. For a particularly fragile or powerful transmitter, the extra voltage can damage the transmitter components, or even cause sparks on the transmission line. Many transmitters are specified to operate into a match no worse than some limit. If a transmitter is safe to operate into a short or open, you can bet that will be prominent on the data sheet.


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