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Isotron 6
 
PRACTICAL USE OF DECIBEL AND SWR VALUES.1
 
The purpose of this article is to help explain how complex measurements or terms can be used and understood in a practical way. Therefore, many of the statements are not an exact science, but what can happen in real applications.
 
Standing Wave Ratios (SWR) and Decibels (dB), are terms used regularly among the Amateur Radio Community. If you had not had in depth technical training of what this stuff is on paper, it can appear a bit hazy. Actually it can be hazy to those of us that were supposed to learn it. DECIBEL: A term used commonly representing a loss or gain ratio of Radio Frequency power, voltage or current.

Two related values are needed to calculate dB.

For example, power applied to an amplifier, compared to the power coming out of an amplifier. We express this in dB. Why?

Incredibly, our ears are on a logarithmic response. My, how did that happen? Oops! I did not intend to get religious.

The formula for dB power is not too bad. dB = 10 x log P2/P1.

P1 = power in, P2 is power out.

As an illustration, if a person estimates that the signal is "twice as loud" when the transmitter power is increased from 10 watts to 40 watts, he will also estimate that a 400 watt signal is twice as loud as a 100 watt signal. The human ear has a logarithmic response.

This fact is the basis for the use of the relative-power unit called the decibel (dB).

Our receiver is a sophisticated Field Strength meter. It takes a power value from the air and puts a number to it on our S-Meters. How can this compare to dB?

There is still not a set standard among radio manufactures of how many dB it takes to move 1 S unit. However, most will provide this rating for there own radios.
 
For example, 1 S-unit is equivalent to a 6dB ratio. That would indicate you would hear the change in signal 6 times from one S-unit to the next, whether you are going up or down in signal strength.
 
When you operate your stations, keep this in mind as you watch your S-Meter in respect to comparing antennas, directive measurements and power levels. This will give you a better understanding of what changes in actual measurements are relevant, or significant, and which are not.
 
Boy, I'm at the end of the page and I hardly got started. Well, we have something for next month.
 
73,
Ralph WD0EJA
May 2022
 
04-15
BILAL COMPANY
137 MANCHESTER DR.
FLORISSANT, CO. 80816 U.S.A
PH/FX: 719/687-0650
 
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