User's Manual

Atmel AVR2092
21
8427A-AVR-10/11
The second step is to calibrate the network analyzer (NWA) to the 50 connector as
usual. After normal calibration, the reference point for the NWA is at the cable SMA
connection. To determine the tuning elements, the reference point has to be moved to
the antenna feed point using the extended length parameter inside the NWA. To
determine this parameter, a hard short is required at the antenna feed line end.
Remove the solder resist on the left and right sides of the feed line end and short the
line end to ground with two solder bumps (see Figure 5-12). Do
not use any wires to
create a ground connection. The calibration procedure will only work when the short
is exactly at the line end and has a minimum of parasitic inductance. Now the NWA
extended length parameter can be adjusted until the NWA’s Smith chart displays a
nice short for the desired frequency range.
In the third step, the antenna behavior can be measured without any tuning elements.
To see the real antenna behavior, the board must be placed in a position similar that
of the final application. If the final application has a housing installed, then all these
measurements must be done with the housing attached. Any piece of metal or plastic
can tune the antenna to a different frequency.
In the case of small boards with an edge length of less than 10cm, the connected RF
cable is often a source of measurement errors. The outer conductor of the coaxial
cable could interact with the field radiated by the antenna and, therefore, create an
additional counterpart ground for the antenna. To avoid this effect, the coaxial cable
can be fed through several ferrite beads. The ferrite beads need to be placed close to
the test board.
The initial measurement shows that the antenna is already working nicely in the
desired frequency band. The feed resistance is a bit low, and the antenna has an
inductive behavior.