In the UK the way your power is represented in the bandplans and in your licence is in dBW - Decibel Watts. This actually means decibels of gain with respect to 1 watt of power. Most hams will remember this from their RAE exam, but equally many will either have never been told or long since forgotten. I thought maybe someone out there may benefit from a easy way of converting power values from Watts to dBW.
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A couple of things to remember about Decibels, dB. Decibels are a way of representing an increase in a figure from a given value. 0dB is zero increase, 3dB is a doubling in power. 6dB is doubled again, so 4-times original value. 9dB doubles again so 8-times original value etc etc. Working the other way, -3dB halves your power. The table on the left gives you a quick reference for most popular power levels which you can use to record your power levels in your logbook. Where dBW comes in really useful is when working out your Effective Radiated Power (ERP). Assuming your power output for your transmitter is 100W (20dBw from the table), your feeder losses are 6dB and your antenna has a gain of 3dBd (3dB with respect to a dipole). Your ERP is calculated as follows:
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For example, UK Amateurs are permitted a maximum ERP on the bottom portion of the 70cms band of 16dBw ERP. Assuming your feeder losses are 4dB (10m of RG58 at 430MHz) and your antenna has 6dBd gain (or 8.2dBi - to convert dBd to dBi just add 2.2) then the maximum power you can put into the feeder so as to give you 16dBW ERP is:
| Permitted ERP | = | 16dBW | |
| Feeder Losses | = | + | 4dB |
| Antenna Gain | = | - | 6dBd |
| Total into feeder to give 16dBw ERP |
= | 14dBW |