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The unit dBm (decibel-milliwatt) is used to express multiple parameters of amateur radio devices. Level 0 dBm corresponds to 1 mW, or a voltage of 0.225V on a 50 ohm load. What does it mean? We will show you in the following examples:
1, Increase in signal strength at the opposite station by 1S
Jeden stupeň S zodpovedá šiestim decibelom. Vezmime si príklad: máme TCVR s výkonom 10W, ktoré privádzame do dipólovej antény so ziskom 0 dB. Ak chceme, aby náš signál u protistanice zosilnel práve o jedno S, môžeme buď použiť anténu so ziskom 6 dB, alebo zvýšiť výkon vysielača o 6 dB. Desať wattov je +40 dBm, z tabuľky zistíme, že +46 dBm je 40W. (Pozn.použitie iného druhu antény znamená v skutočnosti väčší rozdiel, pretože do hry vstupujú aj ďalšie faktory)
2, By how much will the receiver sensitivity increase by turning on a preamplifier with a gain of 10 dB?
Again, let's show it with an example: our transceiver has a specified sensitivity of 0.25 uV for an S/N ratio of 10 dB with a bandwidth of 2.4 kHz. By how much will the receiver sensitivity increase by turning on a preamplifier with a gain of 10 dB? Looking at the table, we find that the voltage of 0.25 uV corresponds to level -119 dBm. Level -129 dBm is 0.08 uV on a 50 ohm load.
3, At what power level do harmonic products appear at the transmitter output?
Let's take a standard 100W transceiver. In the manual, we read that the suppression of harmonic products from the transmitter is 50 dB. But what is their power at the transmitter output? According to the table, one hundred watts is +50 dBm. If we subtract 50 dB from +50 dBm, we get 0 dBm, which is 1mW.
4, What voltage does the diode mixer UZ07 from the oscillator need?
UZ07 belongs to the category of mixers requiring an oscillator signal level of +7 dBm. A power meter is not found in a hamshack, but a common tool is a VF millivoltmeter. To set the correct level for the mixer, we convert +7 dBm to 500 mV/50 ohm according to the table. We will then set the oscillator signal to this voltage on the load (on a 50 ohm load, or directly on the mixer output).