A microVolt (uV) is a measure of voltage, usually across receiver terminals. Assuming a 50-Ohm impedance, it translates to a particular input power to the receiver, usually in dBm (dB above one milliWatt). Usually what the manufacturer's specs for a receiver give you is the input voltage or power. The units could be different (for example, dBW for the input power expressed in dB above one Watt, etc.). This is usually what you are looking for when you are doing a field strength study: Where are all the locations where I have enough input power (or voltage) to meet the manufacturer's specs for this hardware?

A microVolt per meter (uV/m) is a measure of field strength. Think of it as expressing how much voltage (in uV) would be induced by the signal into a one-meter long antenna. In TAP, the value is usually expressed in dBu (which is actually an abbreviation for "dBu/m", or dB above one microVolt per meter). This gives you a measure of how much field strength is present at the location, but doesn't say anything about whether or not that is adequate for a particular receiver hardware configuration.

When you are running a field strength study, that becomes an important and useful distinction. TAP will compute the field strength at the locations you specify in your coverage study (a radial or tile study, or a single point field or link budget). Once the study has completed, then TAP enables you to apply that value to any number of receiver configurations without having to run the study again.

For example, you might have a handheld that requires 0.5uV at the input terminals and has a 0dB gain antenna, and you might have a vehicle receiver that requires 1.0uV at the input and has a 3dB gain antenna and 0.5 dB loss for the coax cable. (These are just example values. You should find the actual values for the mobile hardware you use.)

You can use the "Compute Required Field" utility in TAP to find the equivalent field strength (in dBu) for each receiver.

If you are curious, the conversion equations are also in an FAQ.

For example, using the two sample situations (the handheld and the vehicle) at 455MHz, the handheld would need about 15.2dBu for the minimum specs, and the vehicle unit would need about 18.7dBu.

You could use those values in TAP to assign a color on the coverage map, say everything above 15.2dBu as green and everything above 18.7dBu as blue. Then the map would show the areas where the different radios would work.

But in real world situations you want to consider other factors as well. For example, with a hand held, suppose the user has the radio clipped to his belt, or stuck in a pocket. Since the human body is a pretty good absorber of rf energy, that will affect the calculations. For example, if you add 30dB of loss for the handheld in the pocket, now the handheld needs about 45.2 dB for the hardware to work. You can then add another color, say red for locations where the field strength is at least 45.2dBu so the map would show the "worst case" scenario, where the handheld would work even under those conditions. Maps like this can be useful not only in the design process, but as guidelines to the users: "If you carry your radio in you pocket, you may only be able to receive a signal when you are in the red areas."

You can use this same approach to add different values of losses for different configurations, such as handhelds inside buildings, or vehicles with the roof-mounted antenna right next to a light bar on emergency vehicles, etc.

There are a couple of FAQs about loss values at:

• Building and Body Loss Values
• Building and Body Losses Measured at the 4th Annual TAP Seminar

You can see that you can determine any number of different field strength levels that would be pertinent to your situation and create coverage maps showing how your coverage calculation from the single study relates to all those different mobile hardware configurations. (Note this assumes that the mobile units - the handheld and the vehicle in this example - have comparable antenna heights, since that affects the calculations of line of sight and path losses to each location.)

An additional FAQ that discusses the "required field" values and relating field strength to input power or voltage is at:

• Required Field Value

That's a long answer to a short question, but it is an important concept for creating meaningful field strength study results.

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