I have a 5W HT that only kicks out about 1W by the time it hits my antenna (which I understand is the norm). How do I go about calculating power loss in various equipment?
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$\begingroup$ Hmm? What equipment? Nothing should be causing a 6dB loss unless the HT is running through 50 feet of RG-58 or some such assembly before it gets to the antenna. $\endgroup$– Dan KD2EEOct 28, 2013 at 16:58
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$\begingroup$ It's a Baofeng handheld. $\endgroup$– DanOct 28, 2013 at 18:34
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$\begingroup$ How do you know that only 1W is hitting the antenna? Is this antenna mounted on the handheld, or through a feedline? What kind, and how long? $\endgroup$– Phil Frost - W8IIFeb 26, 2014 at 22:20
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$\begingroup$ @PhilFrost through an adapter and then through a 20 ft 10 ft feedline which is way too long. But I don't know the actual loss. I was hoping for a formula to calculate it, the current answer does a good job. $\endgroup$– DanFeb 26, 2014 at 22:35
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1 Answer
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There are a few possibilities.
First of all, in larger assemblies (mostly important in repeater design) each component will have a parameter called insertion loss. This is the loss, in dB, through that componenet - the loss caused by inserting that component in your feed line. Filters have insertion loss, as do duplexers, and even inline wattmeters and adapters, although the last two should be pretty low. Typically this is about 0.5 dB for each filter or other component, at least in the installations I've seen.
Then you need to add in loss due to feedline. Different types of feed line have different losses at different frequencies. For example, to get the loss you've described (5W to 1W), and assuming a UHF HT in the 440MHz band, and 50 feet of RG-58 coax (a common small diameter coax) equates to about 5.6dB. This is a parameter that you would look up in a table or a graph from the manufacturer of the coax you're using.
You may also have some loss at the antenna itself. Especially "rubber ducky" antennas, which are electrically shorter than a true quarter wave antenna and compensated with loading coils, will have a certain amount of loss, also computed in dB. This basically accounts for the reactive and resistive losses in the antenna, decreasing the actual emitted power.
Finally, to calculate your actual radiated power, you can simply add up all these values. Every 3dB equates to cutting your signal in half, and 10dB equates to one tenth of your original power getting out. You would compute the actual power by multiplying your transmitter's power by $10^{(-\mathrm{loss~}(\mathrm{in~dB})/10)}$
