Is a matching network needed if the antenna feed line is very short?

Question

When designing PCBs with RF functionality it is considered good practice to place pads for a matching network immediately before the antenna (or antenna connector). This allows you to compensate for any impedance mismatch arising from design and/or manufacturing issues in the PCB:

A different rule of thumb is that you don't have to worry about RF propagation complexities if your trace is shorter than 10% of the highest frequency component wavelength.

Does this imply that I can omit the matching network if the feed line is very short, electrically speaking?

For example, would the following layout be OK? This is for 915 MHz, with 2mm between the antenna and the edge of the ground plane. The IC is a filter/balun from Johanson Technology. (The 2mm distance was dictated by the antenna datasheet).

(The design isn't completed; I still need to add stitching vias, pull the ground back from the differential microstrip, etc.)

Answer 1

Do not assume that you do not need any matching network if the line is short.

Your circuit which is connected to antenna, probably is designed for certain impedance. If antenna itself doesn't have it, your circuit probably doesn't work optimally. Radio transmitters even can get overheated or generate overvoltages with wrong antenna impedance. Radio receiving can be too noisy or have wrong filtering if the antenna is mismatched.

Your 10% rule makes the transmission line matching not so important, but it doesn't remove the need to match the antenna with the rest of the circuitry.

Is that need existent - the designer of the circuit knows.

Answer 2

The antenna you linked is nominally 50 ohm at 915 MHz and, with the VSWR limit indictated in the data sheet, this might mean the antenna looks like 100 ohms or 25 ohms. So, all you need to ensure is that your driver/balun is capable of working effectively with this range of load impedances.