I understand return loss is the ratio of how much signal is reflected in a transmission line. But for wireless devices there is no cable. So how is this measured?

So is the impedance mismatch for wireless devices between air and the antenna?

  • \$\begingroup\$ at the antenna port \$\endgroup\$ Aug 30, 2018 at 7:20

1 Answer 1


A transmission line can be a cable.

But a transmission line can also be air (or vacuum).

To get a signal (from a cable) into the air we would use an antenna. The reverse is also true, to get a signal out of the air into a cable also an antenna is used.

An antenna is basically the device that "connects" the cable to the air.

If this would all go perfectly (no or little losses) then all the signal from the cable ends up in the air (or vise-versa).

But if there is a mismatch than (part of) the signal will reflect, this is what your question is about.

This mismatch can be measured from the "cable side" of the antenna. Instead of connecting it to a transmitter or receiver, we connect it so a network analyzer or a vector impedance analyzer.

This is a device capable of making a plot of impedance over frequency, reflection coefficients (S-parameters, for your question it is S11). There are also network analyzers that can make a plot of the characteristic impedance as a function of the distance the signal has traveled. That can be used to find where a cable is broken but also where the mismatch is in a cable to antenna connection.

Here's an example of a plot of the characteristic impedance of as a function of the frequency. We can see that the system reflects the least at 871 MHz. At 871 MHz the of the 0 dBm input signal only -24.29 dBm reflects back. Obviously this system is to be used at 871 MHz!

enter image description here

  • \$\begingroup\$ There is already a lot of arguing about things like waveguides being transmission lines or not (and vice-versa). Calling free-space a transmission line does not seem correct? \$\endgroup\$
    – Joren Vaes
    Aug 30, 2018 at 7:29
  • \$\begingroup\$ OK, free space is different since the wave is not contained (as opposed to in cable or waveguide). Still the wave travels through free space like it does in a cable. If there is a better word to use for the "free space transmission line" I'd be happy to hear it :-) \$\endgroup\$ Aug 30, 2018 at 7:32
  • \$\begingroup\$ Thanks. So is the impedance mismatch for wireless devices between air and the antenna? \$\endgroup\$
    – Bobbie
    Aug 30, 2018 at 8:04
  • \$\begingroup\$ That is one part of it, first the cable has to match to the electrical connection of the antenna. If both are say 50 ohms there's no problem. But if the antenna is 80 ohms a matching network must be used (usually just a couple of caps and inductors). Secondly, the antenna must of course be able to be efficient at interfacing (transmit/receive) the signal to the air. If you use a 1 GHz antenna at 800 MHz, this will never be optimum and also a source for reflections. \$\endgroup\$ Aug 30, 2018 at 8:08
  • \$\begingroup\$ What are some tests to do so the antenna is transmitting/receiving the correct signals \$\endgroup\$
    – Bobbie
    Aug 30, 2018 at 8:39

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