The electrical type or design of the mobile antenna is commonly referred to in terms of its wavelength: 1/4 wave, 1/2 wave, 5/8 wave, etc. Each electrical type has a specific radiating pattern to be considered when selecting a mobile antenna. For example, the signal radiating from a 1/4 wave antenna is directed more vertically, thus making it ideal in urban environments where buildings might obstruct the signal. A 5 dB collinear mobile antenna is designed to direct the signal more towards the horizon. This type of antenna is ideal for geographically flat regions where signal coverage is sparse.
Ground plane availability is another critical factor in mobile antenna performance which must be considered when determining the location and type of the antenna. Ground plane requirements vary given the type of mobile antenna and the frequency of operation. A typical 5/8 wave antenna at 150 MHz requires a ground plane at least 42” in diameter. At 450 MHz a 15” diameter ground plane is required, At 800 MHz a minimum of 8” is considered sufficient.
In terms of mounting mobile antennas on a vehicle, there are five general locations: roof, front fender, rear fender, trunk and rear window glass (although other glass mount locations may be used). Of these, the center of an automobile roof is considered the best for mobile antenna placement, followed by the center of the trunk lid, the fenders, then on-glass mounting. This ranking is determined by the amount of ground plane provided by the positioning and clearance from obstruction (i.e., the roof line).
The center of the roof is considered the ideal mounting location, provided the roof is metal.
The diagram below illustrates the effective loss (at 800MHz) due to insufficient symmetrical ground plane.
| 01. Permanent Mount - Center of the roof: 0.0 dB | 07. Magnetic Mount - Trunk Corner: -3.4 dB |
| 02. Magnetic Mount - Center of the roof: -0.02 dB | 08. Magnetic Mount - Trunk Center: -2.1 dB |
| 03. Magnetic Mount - Corner of the roof: -0.02 dB | 09. Permanent Mount - Trunk Center: -2.8 dB |
| 04. On Glass Mount - Upper Center: -0.5 dB | 10. Trunk Lip Mount - Trunk Center: -2.8 dB |
| 05. On Glass Mount - Middle Center: -1.2 dB | 11. Magnetic Mount - Corner of the hood: -2.4 dB |
| 06. On Glass Mount - Lower Center: -3.0 dB |
A Successful Installation
A successful installation means that all product instructions are read, the right tools are available, and best practices applied. Below are a few tips to ensure your antenna performs as specified.
NMO Mounts: One way of ensuring proper grounding of your antenna system is to clear the paint on the underside of the mounting surface, such as the roof or trunk, with a medium grit sandpaper or other by other means. The NMO mounting hardware is designed to “cut” into the paint, however, paint thickness and total applied mounting torque may make conditions where grounding is insufficient. The VSWR of the mounted antenna is the primary indicator that the ground is poor.
Glass Mounts: Glass mounts have very specific instructions for preparing the glass for an on-glass antenna and should be strictly followed. The most important instructions are to clean the glass avoiding ammonia-based cleaners, glass temperature must be close to room temperature (70 degrees), and after install, the whip should remain off and the mount dry for 24-72 hours. By preparing and installing correctly, you can rest assured knowing that your antenna is not going anywhere.
Tuning: Many users trust that the cutting charts provided with an antenna are absolute. It should be reinforced that cutting charts are guidelines. Ground plane size (a car versus a van), antenna mounting locations (the roof versus the trunk), and even mounting types (permanent roof mount versus magnetic mount versus trunk lip mounts) all have an impact in the tuning of an antenna. Using the cut chart with an economic SWR meter will help ensure the antenna is tuned correctly.
No comments:
Post a Comment