Increasing transmitter output power – What effect does it really have?

Power is always an emotive subject so I thought it would be interesting to review transmitter power levels and see what difference increasing power makes to your signal.

To start with we need to look at how signals strength is measured. In 1981 the IARU Technical Recommendation R.1 was that S9 on the HF bands is a receiver input power of -73 dBm which equates to 50 uV at the antenna input assuming the input impedance is 50 ohms. It also suggests that there should be a diffence of 6dB per S point.

Let’s assume you’re running 10 Watts output power (the limit for UK Foundation licence holders) and the station you’re working gives you an S5 report.

If you increase your power to 20 Watts, your signal strength will now be S5.5 because each time you double your output power, you’re increasing it by 3dB – As already said above, one S point is equal to 6dB difference in signal.

Double your power again to 40 Watts and your signal strength will now be S6.

If you then increase power from 40 Watts to 50 Watts (the limit for UK Intermediate licence holders), that’s just another 0.9dB gain which is less than one third of one half of an S point. In reality, it probably won’t even be noticed.

Let’s start this at 50 Watts. Again, your 50W transmitted signal is being reported by the other station at S5.

You double your power to 100W which is a 3dB increase and will give you an S5.5 report.

Fortunately you’ve got a Kenwood TS-990 or an old FT-1000 with 200W output so you turn the power up to full which is another 3dB gain to give you a S6 report.

It’s time to get serious. You spend a load of cash, buy an amplifier, plug it in and and double your output power to 400W (another 3dB) which takes your received signal up to…  wait for it…  S6.5.

If you want to raise your signal to S7, you’d need to increase your power by another 3dB up to 800W which of course you can’t do in the UK.

I’ll give a third scenario.

If you start with 10W and a received signal of S5 and increase your power to 400W, that’s a gain of 16dB which will take your signal up to just over S7.5.

Finally, if you’re running 100W and the station you’re working gives you an S9 report, you’d need to increase power to 1kW to increase that report to S9+10dB or up to 10kW for the report to go up to S9+20dB!

To summarise
Foundation (10W) to Intermediate (50W) power gain = increase of just over one S point
Intermediate (50W) to Full (400W) power gain = increase of one and a half S points
Foundation (10W) to Full (400W) power gain = increase of just over two and a half S points

As you can see increasing power in order to improve your transmitted signal strength really is a case of diminishing returns – You have to double your power each time in order to benefit from just half an S point in signal. It’s far better off to make improvements to your aerial system than to increase power because then not only will the station you’re working benefit from a stronger signal from you but you’ll also benefit from stronger received signals.

In practise, this is what the difference will look like on your s-meter.

Difference in signals between 10W and 400W

Difference in signals between 10W and 400W

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7 Responses to Increasing transmitter output power – What effect does it really have?

  1. David Harvey says:

    Higher power costs many dollars yet provides far less gain than most new amateurs understand. Invest your time to improve your antenna system as much as you can and the results will be far better. Your reception is very likely going to improve as well.
    73 de vk2dmh

  2. Ray walters says:

    OK yes I agree with your explanation regarding increase of S points, BUT the thing is; it’s not the increase in S points so much at the receiver that makes the difference. It’s the very noticeable increase in resolved audio that makes a the biggest difference.
    Ray Walters, VK3ACT. Let the arguments begin.

  3. Eduardo Hari Erlemann says:

    I agree with Ray VK3ACT since this S system adopted by hams does not mean much, what matters in real life is the signal to noise ratio SNR. 73 de Ed PY2RN.

  4. mar victor says:

    well i use 5w during my holdays with ft817. If your correspondant has a good reception without noise it work.

  5. Bill, G4GHB. says:

    This is why QRP works. My h/b rig is 6 Watts max. and FT817ND 5 Watts, one valve xtal c.w. tx. at 2 Watts. I had an FT101E then started running it lightly at 25 Watts, sold it and went QRP. QRP now over 30 years. This week a G station asked me to repeat my power level on the 817 on 40m using ssb. He had given me 5.8. near London from here near Manchester. I think he was amazed. Inverted Vee 25 feet maximum at centre.
    73, Bill, G4GHB.

    • John, G0RGU says:

      So this is where you’ve been hiding Bill ? On a site that proves well that power is one of the least effective ways of improving a signal. A worthy use of time.

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