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Thread: Rule of thumb for caculating wattage to get signal a distance

  1. #1

    Default Rule of thumb for caculating wattage to get signal a distance


    I am a bit of a 'newbee' but noting in my training has told me how to estimate the power needed to get a signal a distance. I am not talking line of sight for which there are no doubt caculators. I want to get a reasionable signal to a friend in Finland about 2000km from me just outside london. Clearly this will rely on reflectiion and signal bounce (and of course atmospheric conditions etc), but how do I know whether to use 10wats 100 or 1000 (which I cannot because of my licence anyway). Is there a rule of thumb for this and where can I find it?


  2. #2


    Welcome to the hobby!

    RF propagation is a huge topic and there is no magic power level to ensure communication at any frequency or location. The pdf I attached is a propagation summary from the ARRL Handbook, that should give you a taste of the complexity of your question. That distance will almost certainly be an HF job. You can make VHF and UHF contacts that far, but weather and temperature inversions dominate DX at 2m and above. From the attached document, you will see that the choice of HF frequency will be dependent on the time of day.

    If you choose to skip the homework, for 2000km, id recommend a dipole as close to a half wavelength above the ground as you can manage. Run 40m during the day and 80m at night.

    As for power, this is where people will likely take offense, but oh well... You should NEVER need 1kw. Every time you double the power, you add 3dB signal strength (half an S-unit). Consider going from a 100w transmitter to a 1600w transmitter, that's doubling 100w 4 times for 12dB gain (2 S-units at the receiver). You can easily get that much gain out of a homemade antenna and save yourself $2k, so why spin the power meter? Sure, there will be pileups, and that's where more power is handy. However, when walking on other peoples transmissions with kilowatts of power, its polite to move your QSO off the busy calling frequency and find a quiet place where kilowatts are unnecessary anyhow. Long story short, you should have no trouble with 100w.

    Good Luck and 73!
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