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I have a radio with a nominal output of 1-30mW (frequency dependent) and amplify this with a 21.8dB capable amplifier (frequency dependent, but should be ~150mW). The signal output then passes through a -20dB attenuator to my frequency counter. The frequency counter reads 121mV/-5.3dBm.

How can I calculate my amplified signal output, before -20dB attenuation? I understand -20dB is a reduction by a factor of 100. How do I calculate dBm/mW output?

Can someone provide clarity?

Note: This frequency counter reads 2.2dBm/287mV when I transmit with a UV5R (~2W) 5cm from it. I guess this could be a problem, since it sees only 1.7mW. This UV5R transmits >25km easily. The frequency counter is a Surecom SF-103.

Update: The Surecom SF-103 frequency counter cannot react quickly enough to changes in frequency (within a range of ~5KHz), and so it displays nothing. The solution was to output a stable signal, then it works fine. Signal strength is still not displayed correctly, even with the right shielded cable for UHF.

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If the frequency counter is measuring -5.3 dBm, then before 20 dB of attenuation the power was 20 dB more than that, so 14.7 dBm.

"dBm" means decibels relative to 1 milliwatt. 14.7 decibels can be converted to a ratio like so:

$$ 10^{14.7/10} = 29.5 $$

So 14.7 dBm is 29.5 milliwatts.

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    $\begingroup$ Yes, I got this result as well in an online calculator - but there is a significant improvement in signal strength relative to the unamplified output. I've even done a range test. It can't be right. Can I use a SWR meter and expect a reliable reading? It will be a few weeks before I have access to one, which is why I am using this cheesy chinese frequency counter. $\endgroup$ – user2497 Mar 25 at 14:50
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    $\begingroup$ If the SWR meter includes a forward power measurement (many do) it could work, though its accuracy is of course subject to manufacturing design and quality. If you want to have a 2nd way to check power, you can build an RF probe with a diode, capacitor, and resistor. Ask another question and I can provide a schematic. $\endgroup$ – Phil Frost - W8II Mar 25 at 14:54
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    $\begingroup$ Thanks for the offer, I may take you up on it. This radio transmits at 1-30mW before amplification (more at lower frequencies), but at 450MHz the signal shouldn't be so weak that my amplified output is only 30mW. What exactly can I discover with your RF probe? $\endgroup$ – user2497 Mar 25 at 14:59
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    $\begingroup$ The probe would give you a 2nd way to measure power which you could use to check the accuracy of your frequency counter. $\endgroup$ – Phil Frost - W8II Mar 25 at 15:06
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    $\begingroup$ Please see the added info. Does that look like I have to look for a better frequency counter? Your help is greatly appreciated. $\endgroup$ – user2497 Mar 25 at 15:09

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