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

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:

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.

• Pedantically speaking, nothing has to be matched to anything else. And for perfect matching, everything has to be matched to everything else. Commented Apr 21, 2022 at 14:24
• What's the RF project you are planning to work on? Commented Apr 23, 2022 at 3:21
• @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. Commented Apr 23, 2022 at 11:28
• @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? Commented Apr 23, 2022 at 12:12
• @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. Commented Apr 23, 2022 at 16:28

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.

• Thank you for the answer. Does the same apply if the antenna is receiving? Commented Apr 21, 2022 at 15:41
• The receiver input impedance needs to be matched to the t-line. Commented Apr 21, 2022 at 18:05
• 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? Commented Apr 22, 2022 at 14:37
• 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. Commented Apr 22, 2022 at 17:18
• 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? 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.