User Manual

APPENDIX A: Calculating Power

This appendix explains how to calculate the input power to the amplifier for your configuration.

1. Using Table A, convert the output power of the radio modem from Watts (or milliWatts) to dBm.

The Model Amp 2440 amplifier works with input powers between 3.2 mW, +5 dBm, and 100 mW

(+20 dBm).

2. Calculate the cable attenuation for your installation.

First, determine the attenuation for the length of your cable at 2.4 GHz. Use the cable

manufacturer’s specifications, or for convenience you may refer to Table B for typical values. (For

example, Table B shows that typical attenuation for LMR-400 is about 6.9 dB per 100 foot at 2.4

GHz.) Then add 0.6 dB to that figure for connector, adaptor cable, and DC injector losses.

3. Calculate the maximum power that can be expected at the amp on the pole:

Radio output (dB) - Cable loss (dB) - Connector Loss =

Signal level at the amp’s input (dBm)

For example, a radio with 40mW (+16 dBm) output and 75 feet of LMR400 (about 3.4 dB of loss)

would have the following input level to the amplifier:

+16 dBm – 3.4 dB – 0.6dB = +10 dBm

Using a standard amplifier with 14 dB of linear transmit power gain, the output power is calculated

as follows:

+10 dBm + 14 dB gain = +24 dBm output power (250 mW)

If the input to the amplifier will exceed +20 dB (100 mW) by your calculations, an attenuator pad will

be necessary between the modem and the DC injector. Or you can order a special version of the

amplifier from YDI that will accept higher transmit input power.

Note: Never put attenuator pads between in the cable the DC injector and the amplifier since there

is a +12V DC voltage on the cable. Doing this would prevent DC power from reaching the

amp and will also damage the attenuator. However, a longer cable or one with higher loss

could be used.

Effective Radiated Power (ERP)

ERP is defined as the sum of the power feeding an antenna and the gain (in dBi) of that antenna.

For example, with 250 mW (+24 dBm) of power into a 24 dBi gain grid dish antenna (like the YDI

PT2424), the ERP would be:

+24 dBm + 24 dB = 48 dBm or 64 watts EIRP