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400 / 5 = 80 (another easy one).

Log 400 - Log 5 = Log 80 (subtract log 5)

The value of Log 400 is 2.6 (just as before)

The value of Log 5 is 0.7 (the figure 0 says there are no zeros)

The value of Log 80 is 1.9 (2.6 minus 0.7).

dBW // decibel watts

Decibel watts are calculated just like logarithms except that there is a "deci-"prefix which you must take account of when converting from watts into dBW. Whenconverting from Watts to dBW you must multiply by ten. So here are some valuesto remember:

Watts dBW32 15

40 16100 20160 22400 261000 30Convert Watts to dBW

Look up the log of 32 (use log tables or calculator): 1.5Now we must multiply by 10 to make it dBW so we get 15

Look up the log of 400 (use log tables or calculator): 2.6Now we must multiply by 10 to make it dBW so we get 26

Convert dBW to Watts

First we must divide by 10; so 30dBW/10 = 3.0Now we look up the anti-log and get 1000 (there are three zeros)

First we must divide by 10, 26dBW/10 = 2.6Now we look up the anti-log and get 400 (there are two zeros)

Back to logs

Calculations

Once we have our power values in dBW we can add and subtract them. The schedulesays that I must only use 15dBW between 1.85 and 2.00MHz (this is the bottom endof "top band"). My rig can generate 50w power using telephony on this band. I have 20m of feeder between the rig and the antenna. The loss in this feeder is 0.1dBW per metre of cable. The total loss in the feeder will therefore be 2dBW. 50w is 17dBW subtract 2dBW (loss in the feeder) to find the power delivered to theantenna is 15dBW which is what I am allowed. When I replace my feeder with a shorter one or one with lower losses, I must reduce power or I will be in breach of the terms and conditions of my licence.

Here is a simplification from the "schedule":

Watts dBW Bands MHz

32 15 1.85 - 2.0040 16 430.0 - 432.0100 20 51.00 - 52.00

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160 22 70.00 - 70.05400 26 All Others400 watts +26dBW is the maximum permitted power in the UK. The values in the table above only refer to attended operation; if you use automatic methods you should look at the terms and conditions booklet more carefully.

You should also bear in mind that it is not necessary to use maximum power. Even

without a linear amplifier I have made contacts with the USA and the Middle East, that is with just 50w. You do NOT need 50w let alone (400w) to contact your neighbour! Some amateurs take great delight in using QRP (see the Q codes page),they make interesting contacts using less than 1w.