Electric utility meters are a common sight in most homes, offices and factories in the industrialised world. For security and antitamper purposes, utility companies need to ensure that any wireless AMR solution attached to the meter cannot be disabled or vandalised. It is therefore both convenient and appropriate to install the AMR modem and antenna solution under the glass of the target utility meter.
While it is possible to develop and integrate surface mount or embedded antenna technology on the modem board of the AMR solution, we know from experience with embedded antenna technology used in cell (mobile) phones that metal PIFA, loop or printed circuit antennas have lower efficiency and suffer from degraded radiation patterns. This is because of the parasitic effects of the components surrounding an embedded antenna, including the meter housing, modem EMI shields and so on. Additionally, electrically small embedded antennas have less surface area interacting with free space, and thus exhibit a higher radiation resistance, further contributing to lower antenna gains and efficiency compared to electrically larger antennas.
Therefore, one can conclude that a full sized dipole or PIFA antenna will radiate more effectively, compared to an embedded antenna, if it is positioned towards the outside periphery of a utility meter, where it is free to radiate through the glass, clear polycarbonate or opaque plastic meter housing. This means that mounting the antenna between the outer cylinder of the meter and the cover (called under the glass) is a convenient balance between providing a tamperproof location whilst optimising the potential RF performance.
IMPORTANCE OF ANTENNA PROXIMITY TO UNINTENTIONAL REFLECTORS
Anyone who has tried to integrate or develop an AMR antenna for under glass mount application will tell you that there are many metallic obstacles within and
around the meter, such as metallic foil ‘warning’ labels, current transformers
(CTs), power supply transformers and electrolytic capacitors (see Figure 1). These serve as reflecting mediums for the RF signal and radiation pattern created by the antenna. They sometimes have a beneficial effect on the antenna’s performance, but all too often they serve to de-tune the antenna VSWR and radiation pattern. Figures 2a and 2b show VSWR with and without the foil label effect. In Figure 2b the antenna that was passing the 2.5:1 VSWR specification in Figure 2a now fails at the 1850MHz point (>3.0:1).
It is thus critical to position the antenna away from unintentional sources of reflection, usually metallic components, to optimise the antenna’s performance. One way to verify that you have chosen an ideal location is to measure the VSWR or return loss of the antenna on a network analyser whilst positioning the antenna around the meter body.
Where possible, foil warning labels positioned near to the antenna should be replaced with non-permeable paper or plastic labels, or the foil label should be re-positioned away from the antenna. Paper labels should be avoided if in close proximity to the antenna, since in humid environments they can absorb water and also act as a source of reflection, or serve to absorb RF energy from the antenna.
OPTIMISING ANTENNA POLARISATION AND VERIFYING GAIN PATTERN
Because a good VSWR (approx 2.0:1 or lower) is no guarantee of optimal antenna radiation efficiency, it is also necessary to verify the antenna performance as installed on the meter by measuring the antenna gain pattern in an anechoic chamber or open antenna range (see Figure 3).
The antenna orientation inside the meter should be aligned for vertical polarisation to ensure that the transmit/receive signal path to the vertically polarised base station antenna suffers minimum loss. An antenna inadvertently placed in the cross-polar position will cause up to a 20dB loss and suffer cross polar isolation. For example, a printed dipole antenna should be positioned at 9pm or 3pm (assuming a clock face nomenclature) on the meter shown in Figure 2, to ensure a vertical polarisation (‘V’ position shown in arrow in
Figure 3). The resulting radiation pattern measurement is shown in Figure 4.
UNIVERSAL UNDER GLASS MOUNT (UGM) ANTENNA HOUSING
Since many different manufacturers of electric utility meters exist, with no doubt many unintentional reflectors scattered throughout them, a universal approach to mounting and optimising the position of an under glass mount antenna is desirable to ensure a standardised and consistent approach for mounting the antenna.
Figure 5 shows such a universal antenna housing approach, developed for application on utility electric meters. Other approaches such as double-sided sticky tape have been tried but are mostly unsatisfactory, since they do not provide a stand-off for the antenna from the metallic components within the meter. Over long periods of time installed in the field, the environment and climate are likely to cause the antenna to become detached from the meter, and consequently the system performance will be compromised.
AMR ANTENNA TECHNOLOGY SELECTION CRITERIA
Years of experience in the industrial and consumer wireless markets have shown that design engineers looking to integrate antennas with their radio systems consider the following key criteria in evaluating their antenna options:
• RF performance
• Physical size/space requirements
• Commercial availability
In residential meter applications AMR customers almost always place a higher rank order of importance on antenna price than RF performance. The main reason for this is that the system selling price is much lower for residential meters than their C&I counterparts.
Conversely, with C&I AMR meters the antenna budget is less of a concern than the performance itself, since these meters tend to cost much more and are more fully featured, demanding better performance. There is therefore less pressure on the design engineer to minimise the cost of the antenna solution.
In addition to the economic requirements for residential AMR antenna solutions, there is a need to minimise the size of the antenna to get it to fit inside the usually smaller meter housing. It often happens, therefore, that surface mount antenna solutions or printed antennas on the host modem are considered. These embedded antenna solutions range from LTCC ceramic SMD antennas (for the ISM 900/2,4GHz bands) to printed dipoles and simple loop structures. Metal PIFA structures, similar to those deployed on a mass scale in digital cell phones, also represent suitable low-cost technology candidates for residential WAN digital cellular AMR solutions.
Finally, where space permits, many AMR customers have found adequate performance can be achieved in WAN AMR applications using commonly available stubby monopoles or dipoles designed for the 850/900/1800 and/or 1900MHz carrier bands.