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The 100 Watt FM / 200 Watt SSB Amplifier Myth. Talk by George G4RNI

THE 100W FM / 200W SSB AMPLIFIER MYTH

George G4RNIWe often see amplifiers for sale, claiming to offer an FM power output and double that amount in SSB.

The “double it in SSB claim” is often a lie!

If the output devices in the amplifier can develop say 100 Watts then they cannot suddenly develop 200 Watts just because you ask them to handle a different type of signal.

A device rated at 100W output can give you “100 Watts at the crest of the RF waveform during one radio frequency cycle”.

Sounds technical but look at it this way:

Imagine a gallon bucket: If the bucket is sat on the floor and never moved you can put a gallon into it. That’s a bit like an FM signal of 100 watts. It never varies.

Start moving the bucket about, say sat on the floor of a moving van or stirring the contents and those contents will splash up the sides. Put too much in and some of the contents will splash out. You can probably only 25% to 30% fill the bucket here. The “mean” contents of the bucket will be say one third of a gallon whereas the “peak” will easily splash up to the full mark.

So if amplifier manufacturers are being honest they could tell you that their 100 Watt amplifier will give you 100 Watts of FM and 100 Watts peak SSB. To get 200W of SSB they’d have to fit 200W transistors.

WHY DOUBLING YOUR POWER DOESN’T GET YOUR SIGNAL TO GO TWICE THE DISTANCE

It’s a popular misconception that the M0 running 100 Watts will be able to work twice as many stations as the 2E running 50 Watts because he/she has double the power.

Here’s why:

If our 50 Watts of power was a single drip of paint we could spread it out to cover a small circle, maybe 5cm in diameter.

How many drips of paint would we need to paint a 10cm circle?
The answer is down to the AREA we need to cover.

The area of a circle is found by Pi (3.142) times the radius squared. (The radius times the radius.)

In the case of the 5cm circle, its radius is 2.5cm.

Its area is 2.5 x 2.5 x 3.142 = 19.6375 square centimetres.

In the case of the 10cm diameter circle its radius is 5cm.
5 x 5 x 3.142 = 78.55 square centimetres – FOUR times the area of the circle that has half of the diameter. This shows we need FOUR times the power to double our signal strength.

This is also why, as you travel away from a line of sight signal if you double the distance the average “field strength” doesn’t half but drops to a quarter.

That is called the “Inverse Square Law”. It also explains why broadcasters don’t normally use just one huge transmitter to cover a country. The power needed to reach all the edges of the country with sufficient strength would be impractically immense so instead they use a number of lower powered transmitters throughout the land.

SPEECH PROCESSORS

When correctly designed and used they can increase your average power in SSB, adding punch to your signal. There are two main types: AUDIO and RADIO frequency units. The best processing is done at radio frequencies.

Audio frequency processing adds more distortion and wastes power because of unwanted audio harmonics and “intermodulation distortion products” (IMD).

“Intelligence” information in the human voice is in the range 300Hz to 2.5 kHz (approx). Lower and higher frequencies add character but contain little useful or “intelligence” information.

With audio frequency processing you also amplify the 2nd, 3rd & higher harmonics to a large degree. So a 500Hz voice component will also produce 1kHz, 1.5kHz, 2kHz etc which can’t be filtered out as they’re within the usable audio range. They only waste precious power. 700Hz gives 1.4kHz and 2.1kHz.
Additionally, these harmonics “beat” with others and produce “IMDs”. (1kHz will beat with 1.4kHz, producing 400Hz and 2.4kHz. 400Hz beats with 500Hz giving 100, 600 and more! All adding distortion and wasting power

RF processors do so at radio frequency. Harmonics are outside of its bandwidth. Less distortion and less wasted power.

Many radios use AF processing. Better designs use RF processing. Your handbook should tell you which you have.

Many external processors are simple audio units. Better ones generate a very low power RF signal. They “transmit” it within their own circuitry, process that signal then “receive” it and return it to audio to feed to your radio mic socket.

DOCUMENTS  

Amplifier Myths

 

Power

 

Speech Processing

 
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