Micrel, Inc.

MICRF008

August 2008

10

M9999-080108

Input Connector/Antenna

Starting at J2, a SMA connector to connect to a RF

signal generator. Levels above –20dBm should be

avoided. An antenna pad is also provided. Solder a

piece of solid wire for the frequency of interest. See

Table 2 for 1/4 wavelength antennas. It should be noted

that 1/4 wavelength antennas work best ONLY with

ground planes of an equivalent size of the antenna. This

is not the case here since the board size is very small.

Freq. (MHz)

ANT1 (mm)

ANT1 (inches)

315

231

9.1

390

185

7.3

418

173

6.8

433.92

168

6.6

Table 2. 1/4 Wave Antenna Lengths

The size of the antenna is calculated by the following

equation:

[]

mm

4

f

1000

0.97

10

3

Ant

4

1

8

×

×

×

×

=

λ

[]

inches

6

.

101

f

1000

0.97

10

3

Ant

4

1

8

×

×

×

×

=

λ

where,

•

per seconds.

•

0.97 is a correction factor for the difference

between electrical length and physical length

and varies depending on ground plane, and

objects next to the antenna.

•

f is the frequency of interest in Hertz.

1/4 wavelength antennas are good for initial tests, but

should never be used in a final product, unless the PCB

area has a large ground plane. Small ground planes

makes 1/4 wavelength monopole antennas unstable,

and any object close to the antenna changes its

impedance. It is completely useless if the antenna is not

straight up or if any object touches it. Other antennas,

such as half-wave dipole, or 1/8 wavelength monopole

are better solutions for small ground planes. A 1/8

wavelength antenna has approximately half the size of a

1/4 wavelength antenna, which makes it very attractive

for small designs. The 1/8 wavelength antenna is not as

sensitive to proximity effects as the 1/4 wavelength

antenna. However, the 1/8 wavelength antenna has a

reactive element in its impedance that needs to be taken

into account in the matching network. The halfwave

dipole has a size approximately double the 1/4

wavelength and does not require a large ground plane.

One side of the antenna is connected to the RF input

and the other is connected to ground as seen in Figure

8.

one side connected to the RF input

Halfwave-Dipole Antenna

one side connected to ground

Figure 8. Halfwave-Dipole Antenna

Matching Network

Z1 to Z5 are used for the matching network. Not all parts

are used at the same time. Several types of matching

network can be implemented: T,

π, or a L network. Here

a matching network has two purposes. The main

purpose is to match the antenna to the antenna pin. By

matching it is meant to reduce the loss there is between

two circuits; here the antenna being used or signal

generator and the antenna pin. The secondary purpose

is to filter undesired signals, which will reduce the

receiver performance. Since it is a secondary function,

more filtering may be required, depending on the amount

of noise or adjacent signal interference present in the

antenna. A T-Network was chosen assuming that an 1/4

wavelength antenna

is

being

used,

which has

impedance close to 50Ω in free air or the receiver is

being connected to a RF signal generator that has

50Ω output impedance. By using the proper matching

elements, almost 9dB better sensitivity can be obtained.

There are several techniques used for impedance

matching. One frequently used is to plot impedance

values in the Smith Chart. However, one needs to know

the antenna impedance and the antenna pin impedance

in the MICRF008YM to perform the matching. Table 3

below shows the antenna pin input impedance versus

frequency and the matching elements needed to match

50Ω to the antenna pin.

Freq. (MHz)

ANT1 (mm)

ANT1 (inches)

315

231

9.1

390

185

7.3

418

173

6.8

433.92

168

6.6

Note 1.

All inductors used are Coilcraft 0603 size, and the capacitors

are Vishay COG 5%. Z3 and Z5 are not used.

Table 3. Antenna Input, Frequency and Matching Values