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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.

user2497
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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.

Phil Frost - W8II
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    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. – user2497 Mar 25 '20 at 14:50
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    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. – Phil Frost - W8II Mar 25 '20 at 14:54
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    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? – user2497 Mar 25 '20 at 14:59
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    The probe would give you a 2nd way to measure power which you could use to check the accuracy of your frequency counter. – Phil Frost - W8II Mar 25 '20 at 15:06
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    Please see the added info. Does that look like I have to look for a better frequency counter? Your help is greatly appreciated. – user2497 Mar 25 '20 at 15:09
  • I went ahead and asked the question. – rclocher3 Mar 25 '20 at 16:01
  • @user2497 trying to calibrate a frequency counter by transmitting nearby isn't really productive since it's difficult to say what fraction of the transmitted power will be received by the frequency counter. – Phil Frost - W8II Mar 25 '20 at 21:45
  • @PhilFrost-W8II Yes, though I was using a shielded cable. I am not calibrating, just wondering why both my 150mW signal and my UV5R show 2.5dBm. I guess the signal strength is not measured correctly. – user2497 Mar 25 '20 at 22:12
  • @user2497 you connected a 2W transmitter directly into the frequency counter with a cable? Is the frequency counter rated to handle such power without damage? – Phil Frost - W8II Mar 25 '20 at 22:15
  • @PhilFrost-W8II No, my radio is low power. I dampened the 1-30mW signal with a -20dB attenuator, and connected it to my freqcounter with a cable. After a disappointing result (which was wrong, which I know because I did a range test, and the signal goes ~700meters in suburbia) I keyed my UV5R to see what the FC measured. Am I being unclear? – user2497 Mar 25 '20 at 22:22
  • @user2497 You just keyed the UV5R nearby? Again, hard to say what fraction of the 2W the FC will receive, so this is not a productive test. – Phil Frost - W8II Mar 25 '20 at 22:26
  • @PhilFrost-W8II Yes, but it's nonetheless indicative that my FC is measuring incorrectly. – user2497 Mar 25 '20 at 22:30
  • @user2497 for the 3rd time, the FC will not intercept 100% of the power, so if you transmit 2W nearby, it will not read 2W but something less. It doesn't indicate anything. – Phil Frost - W8II Mar 25 '20 at 22:38
  • @PhilFrost-W8II That's right, no one is contesting that. But the first measurement (low power, signal cable) was about equal to the second (~2W, air) at 2.4dBm. Both signals have a much higher effect. I am not calibrating anything, I am making do with what I have, until I get an RF power meter. – user2497 Mar 26 '20 at 01:45