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S11

VSWR

Reflection

(dB)

Loss (dB)

-6.0

3:1

1.2

-9.5

2:1

0.5

-15.0

1.4:1

0.15

-20.0

1.2:1

0.04

loss, S11, and VSWR are used interchangeably in this paper). These impedance parameters measure how much of the power supplied to the antenna reflects back from the antenna terminals. Ideally, but impossible to achieve, all of the power supplied to the antenna is radiated with no reflection. It is important to note that the return loss measured at the antenna terminals is relevant for both receive and transmit operation. In transmit mode, compromised S11 will reflect power back into the final output amplifier, while in receive mode the power is reflected back into the antenna.

Marginal

Acceptable (and typical Antenna Specification)

Good

Very Good

Table 2. Return Loss (S11) and VSWR Relationship

Comment

1 below shows the return loss of a typical dual-band embedded cell phone antenna. The horizontal scale is swept frequency from 700 MHz to 2 GHz; the vertical scale is S11 in dB (10 dB/div); and the markers identify the two operating bands.

The worst-case S11 is about -10.3 dB (a VSWR of 1.9:1). While the return loss data in 1 is acceptable, note that the response in the low band is just wide enough to cover the desired frequency range. In high-volume production, the frequency response will likely shift a small amount, resulting in degraded return loss at one of the band edges. The amount (and direction) of the shift depends on the type of

Antenna Fundamentals Technical Brief

10

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