RF Power, Frequency & Antenna Calculator
The conversions an RF engineer does twenty times a day: dBm to watts, frequency to wavelength, dish diameter to gain — plus an ordered gain and loss chain that shows the level after every component.
Power
Frequency and wavelength
The velocity factor is 1 in free space. Inside coax it is typically 0.66 to 0.85 and in fibre about 0.67, which is why a physical quarter-wave stub is shorter than the free-space figure.
Parabolic antenna
Gain and loss chain
The three anchors worth memorising
0 dBm is 1 mW. That is the definition — dBm is decibels referenced to a milliwatt.
30 dBm is 1 W. Because 30 dB is a factor of 1000, and a thousand milliwatts is a watt.
0 dBW is 1 W. dBW is referenced to a watt instead, so dBm and dBW always differ by exactly 30.
From those three, and the fact that 3 dB is roughly double and 10 dB is exactly ten times, most everyday RF arithmetic can be done in your head. 43 dBm is 20 W: 40 dBm is 10 W, plus 3 dB doubles it.
Why decibels at all
Because a link budget spans an absurd range. A transmitter puts out a few watts; the receiver works with something in the region of a hundred femtowatts. Written in watts, a budget runs across fourteen orders of magnitude and nobody can check it by eye.
In decibels the same budget is a column of two-digit numbers that add up, and addition is something people get right. That is the whole reason for the unit — not compression, but turning multiplication into addition so a chain of components can be verified by reading down a page.
Antenna gain is not a property of the antenna alone
A parabolic dish concentrates energy into a beam, and how much gain that produces depends on the wavelength as well as the diameter. The same dish gives roughly 6 dB more gain at twice the frequency, because the aperture is twice as many wavelengths across.
Aperture efficiency is the fraction of the geometric aperture that is doing useful work. Typically 0.5 to 0.7 — it is never 1, because the feed does not illuminate the dish perfectly, some energy spills past the edge, and the feed itself blocks part of the aperture. It is an input here because it belongs to the specific antenna.
The figure this produces is an estimate from aperture theory, useful for planning. When it matters, the manufacturer's measured gain at the frequency in question is the number to use.