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IMPROVED ANTENNA DESIGN THROUGH USE OF A FLIP

IP.com Disclosure Number: IPCOM000008054D
Original Publication Date: 1997-Mar-01
Included in the Prior Art Database: 2002-May-15
Document File: 4 page(s) / 173K

Publishing Venue

Motorola

Related People

Leigh M. Chink: AUTHOR

Abstract

The standard omnidirectional antenna used in land mobile radio applications has the problem that part of the energy radiated by the antenna is neces- sarily directed towards the user of the radio. That energy is largely absorbed by the user in the form of heat and is, therefore, not available for communica- tions purposes. The degree to which the energy is absorbed is dependent on a number of factors, including the shape of the absorbing structure, its composition, and the separation between the absorbing structure and the antenna. Therefore, we would expect much mote absorption from a radio worn on the belt, with the antenna pressed against the user's side or back, than we would expect from a radio held in the interconnect position. The effect that is at work here is not simply the user subtend- ing part of the solid angle into which the antenna is radiating energy. When the user is close to the antenna, effects due to the near electromagnetic fields dominate. These effects drop off rapidly with distance (d-S according to Andersen'), and, as Jensen and Rahmat Samii2 note, peak SAR (specific absorption rate) decreases in a nearly exponential fashion with handset-body separation. But, because this is not a simple geometrical problem, we have seen in simulations that a radiating antenna placed close to a dielectric half plane can have more than 80% of its energy absorbed by the plane.

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MOTOROLA Technical Developments

IMPROVED ANTENNA DESIGN THROUGH USE OF A FLIP

by Leigh M. Chink

BACKGROUND

  The standard omnidirectional antenna used in land mobile radio applications has the problem that part of the energy radiated by the antenna is neces- sarily directed towards the user of the radio. That energy is largely absorbed by the user in the form of heat and is, therefore, not available for communica- tions purposes. The degree to which the energy is absorbed is dependent on a number of factors, including the shape of the absorbing structure, its composition, and the separation between the absorbing structure and the antenna. Therefore, we would expect much mote absorption from a radio worn on the belt, with the antenna pressed against the user's side or back, than we would expect from a radio held in the interconnect position. The effect that is at work here is not simply the user subtend- ing part of the solid angle into which the antenna is radiating energy. When the user is close to the antenna, effects due to the near electromagnetic fields dominate. These effects drop off rapidly with distance (d-S according to Andersen'), and, as Jensen and Rahmat Samii2 note, peak SAR (specific absorption rate) decreases in a nearly exponential fashion with handset-body separation. But, because this is not a simple geometrical problem, we have seen in simulations that a radiating antenna placed close to a dielectric half plane can have more than 80% of its energy absorbed by the plane.

SOLUTION

  Our concept for improving the performance of the antenna has two main components. The first is to build a near field directive antenna. Our model shows that, in a situation in which the user can expect to absorb 50% of the energy from a typical omnidirectional antenna, the use of a well designed shield can reduce that number to about 5%, or about

1OdB less energy absorbed by the user. This being the case, if the communications efficiency of the antenna system is defined as the ratio of the energy radiated into the far field to the energy input to the antenna, we can expect an improvement of almost 3dB.

  Our plan to implement this is to design the antenna into a flip structure which could have a metallic component to provide the directivity. An example of a radio with a flip antenna design is shown in figure 1.

Fig. 1 Illustration of a Flip Antenna

0 Motorola. 1°C. 1997

I22 March I997

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MOTOROLA Technical Developments

  Figure 2 shows the computer simulation results na shield included. The radio, antenna, and antenna of the far field radiation patterns for an antenna shield are modeled as wire grids. The deviation mounted on a radio box with a well designed anten- from the typical omnidirectional pattern is evident.

Fig. 2 Far field radiation patterns for an shield concept

  The second component of our solution takes the user and the antenna equivalent to twice the adv...