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Every noIM₃ article on RF Engineering, newest first. Written by engineers, for engineers.
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RF Engineering What Is NVIS? Near Vertical Incidence Skywave for Regional and Emergency Radio Near vertical incidence skywave, or NVIS, is a high frequency technique that sends the signal almost straight up so it reflects off the ionosphere and returns over a wide area beneath the antenna, with no skip zone and no need for line of sight. It is how you hold a reliable radio net across rugged or remote country, which makes it central to regional and emergency communication in Australia. This guide explains how NVIS works, the band it uses, why the frequency has to follow the ionosphere up by day and down by night, a worked frequency choice, why the antenna is mounted low, and where it fits for emergency services, mining and pastoral operations. 
RF Engineering What Is Free Space Path Loss (FSPL)? Formula, Examples and the 6 dB Rule Free space path loss, or FSPL, is the signal a radio link loses simply because the wave spreads out as it travels, before any obstruction, rain or terrain is added. It is the floor of every link budget and the first number a planner reaches for. This guide explains what FSPL is, the formula in every useful form, why it depends on frequency even though empty space absorbs nothing, a worked 5 GHz example, the 6 dB rule for doubling distance or frequency, and where FSPL stops and real world path loss begins. 
RF Engineering What Is Passive Intermodulation (PIM) and How to Prevent It? Passive intermodulation, or PIM, is interference a radio system makes for itself when two or more strong transmit signals mix in a passive part that is not perfectly linear, such as a connector, cable joint or antenna. The new signals it creates can fall straight into the receive band and quietly desensitise the receiver. This guide explains what PIM is, the formula for where its products land, a worked cellular example, how it is measured in dBc, what causes it including the rusty bolt effect, and the practical steps that keep it off a site. 
RF Engineering What Is Fade Margin and How Much Do You Need? Fade margin is the number of decibels by which the received signal sits above the receiver's usable threshold, the reserve a radio link holds to ride out multipath, rain and atmospheric fading without dropping out. This guide gives the formula, a worked example, the link between fade margin and availability, why each extra nine of uptime costs about 10 dB in the multipath regime, how rain rewrites the rules above 10 GHz, how much margin Australian paths actually need, and the practical ways to buy more of it. 
RF Engineering What Is EIRP? EIRP vs ERP and How to Calculate It EIRP is the power a perfect isotropic antenna would have to radiate to match what your real antenna sends along its main beam. You get it by adding the transmitter power and the antenna gain, then subtracting the feeder loss. Here is the formula, how EIRP differs from ERP, a worked example, and the licence limits and safety distances that all depend on it. 
RF Engineering What Is the Noise Floor? Thermal Noise, SNR and Receiver Sensitivity Explained The noise floor is the faint thermal noise present in every receiver, the level a wanted signal has to rise above to be decoded. At room temperature it starts at −174 dBm per hertz, then climbs with bandwidth and the receiver noise figure. Add the signal to noise ratio your modulation needs and you have the receiver sensitivity. Here is where −174 dBm/Hz comes from, the noise floor formula with a reference table, how noise figure and SNR fit in, and a worked example that turns into a sensitivity figure.