Bredhurst Receiving and Transmitting Society

Advanced Radio Amateur Examination


Field strength 

In this section we come across the formula for field strength given the ERP and distance from the antenna.

Example: If your transmitter has a power out of 100W to an antenna with a 2 times gain what will be your Field strength at 2m in front of the yagi ?
For this example we are using the CASIO fx-83GT PLUS

For this we have to introduce another key on the calculator the square root key and note the white dot has been added to show which side of the button to press as it is multi functional.

7 2 100 2

after pressing the equals sign you will see displayed the first answer so then press you will then have displayed the second screen above showing the result ( rounded to 3 decimal places) Result 49.497 V/m

NOTE: you have to use the brackets around the square root calculation else it take the square root of 2 and not 200 !!!

also you have to use the new to stop the square root sign. You will see that the square sign extends as you enter the calculation and would continue over a second bracket if the new button is not used. This would cause an error result to be displayed

Example: What ERP would be required to have a FS of 1.5V/m at 25m ? The re-arranged equation is


Result 28.69Watts (ERP)

So that completes the calculations for the maths in the Advanced Amateur Radio Syllabus. It is hoped that the benefit of the use of the calculator is now clear to you !!

Now enjoy a coffee / tea break !!! BUT do practise the use of the calculator !!

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