th9xer wrote:I know things have moved on but..... I have been doing a bit more research and the correct way to handle dBm to dB mathematics is straight out addition or subtraction. So 18dBm - 5dB = 13dBm, my apologies Kilrah
Hmm, had forgotten about this thread, didn't even see your answer... but yes, dBs are logarithmic whether referenced or not. A value in dBm represents a power, referenced to 0dBm = 1mW. Then if you want to express a difference (gain) on top of that, you add a number of dB (dimensionless, just like any ratio of the same units).
th9xer wrote:The potential for big dB loss is very possible
Yep, it is. But as I said they the risk is more into the interconnection of the modules and switchers, with the connectors and how the switches are mounted. My point was that if you want to do things correctly you have to design a proper RF-spec PCB with constant impedance traces etc, which is probably out of the scope of most on here. Me included, in this field I know what's required but have no idea how to actually make it
th9xer wrote:As for the 5dB difference between modules. 18dBm = 63mW, 13dBm = 20mW. Rough rule is, all else being equal four times the power gets you double the range.
Yup. Which is why I'm a tad deceived by one of my modules giving me about half the range of the other in theory. The module I use with my 9X is the "middle" 15dBm one.
th9xer wrote:Using the default numbers, (which serve only for illustration as there are many variables involved) 18dBm gets 2.38Km and 13dBm gets 1.32Km. That's a good old difference.
With the default values those distances indeed don't mean much - however as adding 3dB means multiplying power by 2, and like you said doubling range requires multiplying power by 4, the 5dB difference between my best and worse modules equals pretty much a factor of 2.
th9xer wrote:(1dB seems to cost you around 260 meters)
This you can't say, because it depends "where that dB is". Let's say you have a 0dBm (1mW) transmitter and its receiver, and you can get 100m range with that. If you add 6dB you double range, and thus add 100m. Hovever say you have the same system but a 30dBm TX, leading you to a range of 3.2km, and add 6dB, those 6dB get you another 3.2km. Those 6dBs will also have cost 3 extra watts to reach, as opposed to the 3mW they represented in the first case - log calculations are fun.
th9xer wrote:I also wonder if the power meter is of the type you just hold near the object being tested do you have to tell it about antenna gain ?
I use an HP8593A spectrum analyser, and measure raw output (feeding the TX's output right into the SA). So antenna gain isn't included.
FrSky specify their TX modules at 60mW, they don't state whether this includes antenna gain or not. However, as they have a removeable antenna, the specified power would normally not include it. And 60mW (~18dBm) + 2dB gain from the antenna would equal the 20dBm legal TX power limit for spread spectrum devices in the 2.4GHz band in most of the world, so it would make sense.
Actually, I have more FrSky RXs than TX modules - but as the "receivers" are also transmitters (telemetry...), it would be interesting to measure those as well to see if the variation is similar...
