So, with this cable you would have 2.45 dB of loss. $10^{-2.45/10} \approx 0.57$, so if you put 100 watts in you'll get 57 watts out.

the loss increases very quickly. 2.45 dB might not be much loss, but if your SWR were 10:1, then the loss might be 20 dB. That kind of loss is a real problem. (If you had a 100 W transmitter and 20 dB of loss, then you would be radiating only 1 W.)

There's a reason we use dB and not watts: watts is a completely unintuitive way to think about power in radio propagation. Converting decibels to fractions is also unintuitive, since fractions are equivalent to examples with a reference power of 1W.

If I have 3dB of loss I lose half my power! That's huge!

If I lose 3dB at 100W, that's 50W! But if I lose 3dB at 10W, that's only 5W so not as bad.

I lose more power with distance closer to the antenna than is lost at farther distances. So I guess if the distance between antennas is already large, adding more distance doesn't matter as much.

I'm using far more power than necessary. Let me turn it down from 100W to 75W. Ah...that's 25% better!

I can only get 50W from my new 100W transmitter on PSK31. I want half my money back!

When you write, "so if ... X dB, then that's Y watts", you imply:

We are talking about power, and power is measured in watts. We'd just use watts, but because this is radio we use decibels just to screw with you. So whenever you see some number in decibels, you'll need to have calculator on hand. We'd change it back to watts, but decibels is traditional so ¯\_(ツ)_/¯

Or maybe if the reader is a little more advanced:

We use decibels because it's easy to add and subtract the numbers together. Because calculating logarithms and powers is totally easier than multiplying numbers.

The reality is that we use decibels not because it's traditional or conventional or easier to add and subtract. It's because it makes sense for feedline losses, amplifiers, and path loss.The only time I'd want a power in watts is when calculating how hot my transmitter will get. So please don't explain decibels as if they are some 2nd class, oddball unit. If you are afraid the reader won't know what a decibel is, explain what decibels are, not how they can be converted to watts. Or just link to an explanation.

Let me buttress my example with some real examples. These are all from a search for "fan review but since debibel is logarithmic actually".

Going down 5dB is huge as the decibel scale is logarithmic!

The decibel meter scale is logarithmic, so 56 dB is considerably louder than 50

But when the unit was installed in the bathroom with a short 4” exhaust flexible duct according to instructions, it jumped to a loud 78 decibels. Since loudness is on a logarithmic scale, this is a big jump.

As a comparison, the Lenovo Y700 with the same CPU and GPU, and doing the same test, was closer to 44 dB(A) after the hour, and since decibels are logarithmic, that’s a big difference.

These are examples of the brain-damage that occurs from constantly equating decibels to powers. No, a 6dB jump isn't huge because decibels are logarithmic. It's a big jump because 6dB is a big difference psychoacoustically. It's not the decibels that are logarithmic, it's your hearing.

The decibels compensate for that logarithmic response so that the "loudness" expressed in decibels such that 50 to 60dBA, and 20 to 30dBA are perceived as equal changes in loudness despite that the change from 50 to 60dBA represents a much more huge increase in power.

This is why the message should be "response to power is logarithmic, and that's why we use decibels", and not "decibels are logarithms, so you have to do complicated math to get power."

  • $\begingroup$ Could you clarify how you feel about displaying the fraction (0.57 in the first example), independent of then also giving an example in watts? $\endgroup$ – Kevin Reid AG6YO Sep 10 '16 at 22:34
  • $\begingroup$ @KevinReidAG6YO edited. $\endgroup$ – Phil Frost - W8II Sep 11 '16 at 14:06
  • $\begingroup$ Thanks. (I'm still trying to figure out my considered position on the matter now that you've pointed it out. I agree that “I lost XX watts in my transmission!” is a terrible way to think about things but I feel like there is some value in the example anyway; need to think more.) $\endgroup$ – Kevin Reid AG6YO Sep 11 '16 at 14:09
  • 1
    $\begingroup$ I agree that for those of us that have mastered the idea of logarithms, decibels are almost always better. But I don't think that we can just assume that everyone here has. Linked explanations are often good, but when the point of a particular answer is to help the questioner understand the concept, then it seems to me that an explanation involving Watts can be very useful. $\endgroup$ – rclocher3 Sep 12 '16 at 16:29
  • 1
    $\begingroup$ @rclocher3 Try calculating EIRP in decibels. Now try it in watts. Which one requires more advanced math? $\endgroup$ – Phil Frost - W8II Sep 12 '16 at 18:28
  • $\begingroup$ ham.stackexchange.com/questions/6672/… $\endgroup$ – Phil Frost - W8II Sep 14 '16 at 15:51
  • $\begingroup$ I think I tend to lean to the comment from rclocher3, db's are good, but sometimes the concept need to be explained with an example. This saying I probably are guilty of using such examples to often, and would take this to heart to use them less often, only when needed. $\endgroup$ – Edwin van Mierlo Oct 4 '16 at 8:31

You must log in to answer this question.

Browse other questions tagged .