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Abstract-- The Wheeler cap method is used to measure the

efficiency of antennas for use in on-body communication

systems. A monopole and a planar inverted F antenna on small

ground planes were mounted in contact with body phantom

material and their efficiency measured using the Wheeler cap

and by a conventional anechoic chamber method. Results from

the two methods show close agreement and also are close to

results of simulations using commercial software.

Index Terms--Efficiency, Antenna, Wheeler-Cap, Phantom

I. INTRODUCTION

he use of wireless communications on the body is a topic

of increasing importance. Various forms of wearable

(a)

(b)

computers that use body area networks are now being

Fig. 1. Monopole antenna with body phantom, (a) in free space, (b) in Wheeler

developed,  [1].  Recent  studies,  [2],  of  on-body

cap

communications links have shown that antennas that couple

to the surface wave give reduced link loss. Such antennas

should be vertically polarized with respect to the body surface

and have a radiation pattern that is maximized along the body

surface. In many on body links and for many body postures,

the quarter wavelength monopole on a small ground plane

has been shown to be better than patches and loops. Whilst

this antenna is omni-directional in the plane of the surface

and hence suitable when the link geometry is not known, the

planar inverted F antenna (PIFA) was shown to give reduced

link loss compared to the monopole, when oriented so that its

direction of maximum radiation was oriented in the link

direction, due to its higher directivity. It is clear that the

design of such antennas is different from that for mobile

phone antennas, where the need is to reduce the interaction

with the body both to improve the communications link and

to reduce SAR. To get a deeper understanding of the way

such antennas operate and to help in link design, it is

important to evaluate the efficiency of these antennas when

they are mounted close to the body.