Effect of Handset Orientation and a Human Head on Cordless Range
VTech Innovations, dba
Advanced American Telephones
246 Industrial Way West
Eatontown, NJ 07724
Phone 732 544 3145
Fax 732 544 2956
This contribution documents two concerns about the emerging cordless phone range standard. The first concern is that the emerging standard does not consider the orientation of the handset with respect to the base. The second concern is that the antenna loading effect of having a human head near the handset antenna is not considered. An experiment is described with a 900 MHz cordless phone that confirms the worst case handset orientation sensitivity is ~ 4.6 dB less than the best case orientation, for that model. The combined worst case effect of the handset orientation and having a human head near the handset antenna can degrade sensitivity up to 11.4 dB.
This contribution addresses a concern that the emerging range standard does not consider the orientation of the handset with respect to the base. An additional concern is the loading effect of a human head on the handset antenna. To obtain insight into whether these effects are significant the following experiment was conducted.
An experiment was conducted in a Lindgren RF Enclosure, model 03X07, to measure the radiated sensitivity of a 900 MHz analog cordless handset. This enclosure has ferrite absorbers and cone absorbers on the walls, ceiling and the end near the cordless handset to absorb RF energy up to 19 GHz.
The handset was mounted vertically on a turntable about 1 ¼ m above the floor. The experimental setup is illustrated in Figure 1 below.
F IGURE 1
There were no electrical connections to the handset which operated on it’s normal battery power. A length of Tygon tubing with an ear piece adapter captured the acoustic energy from the handset ear piece and directed the energy to a microphone and pre amplifier on the floor which then generated an electrical signal for SINAD measurements. The handset receiver was turned on to receive a radiated signal at 925.568 MHz from the source antenna located 3 meters away. The level of the signal driving the source antenna was varied to achieve a 12 dB SINAD signal at the handset ear piece.
Data was first taken in the four orientations of the handset labeled A, B, C & D as defined in Table 1 below. The handset orientations in Table 1 refer to Figure 1 above. The handset was carefully positioned so that the antenna was at the center of the axis of rotation of the turntable to minimize the effect of any reflected energy.
From the symmetry of the situation, we expected the SINAD values for positions A & C to be similar, and for positions B & D to be similar. The pattern of data among the different orientations gave us confidence that the experiment was working correctly. The difference in the average SINAD between orientations A or C and B or D is 4.6 dB for this handset. I suspect the improved sensitivity in orientation A or C (compared to B or D) is due to the ground plane on the pwb in the handset which is more favorable for the antenna to receive signals in front of or behind the ground plane. Documenting this difference, rather than understanding it, was the goal of this part of the experiment
The second part of the experiment was to gain insight into the loading effect of a human head on a cordless handset antenna. The experiment consisted of having a human stand on the turntable and hold their right or left ear close to (but not touching) the handset ear piece. The results are shown in Table 2. In each case, the column labeled Delta in 12 dB SINAD is the difference between the signal level for a 12 dB SINAD with and without a human head present.
Referring to Figure 1, in setup 1 the human is standing such that the view of the handset in Figure 1 would be blocked and then placing their left ear at the back of the ear piece. Setup 2 is similar except they are placing their right ear near the front of the ear piece. In setup 2, the human head obstructs the RF path to the handset antenna in addition to the loading effect of having a head close to the antenna. In setup 3, the human is standing to the right of the handset and table with their right ear at the back of the handset. In setup 4, the human is standing between the source antenna and the handset (left side of the table in Figure 1) with their left ear at the back of the handset.
HS between source antenna and human
(R ear on back of HS)
Ant loading + RF attenuation
~ 6.8 dB
B Dial facing into
Human between source antenna and HS. (L ear on back of HS)
Ant loading + RF attenuation
~ 6.8 dB
The absolute value of the SINAD numbers in Table 1 is not significant. From the results in Table 1, we can assume the receiver sensitivity for setup 3 and 4 in Table 2 is roughly 4.6 dB lower than setup 1 and 2 in Table 2. The reminder of this discussion refers to Table 2. The 3 dB in setup 1 is largely the effect of antenna loading from the proximity of the human head. In this setup, the human head is not directly attenuating the RF signal between the source antenna and the HS. Setup 2 includes the 3 dB loading effect of setup 1 plus the attenuation of the RF signal due to the human head being between the source antenna and the HS antenna. I think of the 7.5 dB in setup 2 as the 3 dB loading effect of setup 1 plus an attenuation effect of 4.5 dB for a total of 7.5 dB. Setup 3 and 4 include the 3 dB of antenna loading from setup 1 plus 3.8 dB of RF attenuation from the proximity of the human head. In setup 3 and 4, there was a line of sight available from the source antenna to the HS antenna but it was not the unrestricted view available in setup 1.
This data documents a variation of ~ 4.6 dB in the measured SINAD between the best and worst case orientation of the handset with respect to the source antenna, without the additional effect of a human head being present. And additional degradation of ~ 3 dB is caused by the loading effect of a human head near the handset antenna. Setup 3 and 4 confirm that RF attenuation of 3.8 dB is caused by the presence of a human head. This data predicts that the worst case orientation of the handset and the presence of a human head can degrade range performance by ~ 11.4 dB (4.6 + 3.0 + 3.8). Considering that cordless RF signals degrade about 10 db per octave of range, this difference could degrade range performance by more than 50 percent.
These conclusions are for the 900 MHz analog set used in this experiment. Antenna orientation and loading effects may vary for other handset designs or frequency bands.
This contribution has been prepared to assist the TIA standards development process. It is offered as a basis for discussion and is not a binding proposal on Advanced American Telephones. The contents are subject to change in form, numerical value, or both after further study. Advanced American Telephones specifically reserves the right to add to, revise, or otherwise amend the information contained herein.
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