July 2003

9

MICRF008

MICRF008

Micrel

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 equa-

tion,

1

4

Ant =

3 10

0.97 1000

f4

mm

1

4

Ant =

3 10

0.97 1000

f 101.6

inches

8

8

λ

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 wave-

length 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 sensi-

tive 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 approxi-

mately 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 50

Ω in

free air or the receiver is being connected to a RF signal

generator that has 50

Ω output impedance. By using the

proper matching elements, almost 9dB better sensitivity can

be obtained. There are several techniques used for imped-

ance 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

MICRF008BM to perform the matching. Table 3 below shows

the antenna pin input impedance versus frequency and the

matching elements needed to match 50

Ω to the antenna pin.

Antenna Pin Input

Frequency

Impedance

(MHz)

(ohms)

23 - j207

315

8.2pF, 18nH, 82nH

14 - j166

390

3.9pF, 16nH, 43nH

12 - j159

418

4.7pF, 12nH, 39nH

10 - j151

433.92

4.7pF, 11nH, 39nH

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