RF Practice:  better to know

create @2014/6/25

something important to know when you design/debug/thing about a RF circuit, like , the SRF of a inductor is?  inductance of a component lead? capacitance of a 1/4 watt resistor?

• pure resistance in parallel with a reactance, the apparent resistance vary with frequency. If the dominant parallel element is capacitive, the effective resistance will fall with frequency. If the dominant parallel element is inductive (as it is  a wire or low-resistance conductor)  the effective resistance will increase with frequency.

• *skin-depth in a conductor is proportional to the square-root of the resistivity (ρ)

• since the reactance of an inductor (XL=2πfL) increases with frequency, and the RF resistance increases with frequency(Proximity effect), the Q of the coil (being the reactance divided by the RF resistance) varies relatively slowly as the frequency is changed <<G3YNH From Transmitter to Antenna>>

Selecting components

*skin-depth in copper (for example) being only 48 μm (0.048 mm) at 1.9 MHz. <<G3YNH From Transmitter to Antenna>>

*a tubular conductor is just as good as a solid wire, and may offer a considerable saving in weight and cost when a large diameter is required. <<G3YNH From Transmitter to Antenna>>

*silver-plated copper wire or tubing is the best commonly available conductor for radio applications, with plain copper (bare or enamel coated) coming a close second. if the silver is allowed to tarnish, the RF resistance will eventually exceed that which can be achieved using plain-copper that has been subjected to the same environmental conditions . increase in losses for not using silver can often be offset simply by increasing the wire diameter. <<G3YNH From Transmitter to Antenna>>

•  Maximum enclosed area: Maximum stray inductance, minimum stray capacitance.

•  Minimum enclosed area: Minimum stray inductance, maximum stray capacitance.

• The #24 AWG wire typically used for component leads has an inductance on the order of 20 nH per inch., 18 awg had 25nh per inch. <<arrl hand book 2011 Chapter 5>>( what is the inductance of a piece of wire is almost tell you nothing, cause wire inductance domain by external current loop. <G3YNH From Transmitter to Antenna> )

•  ½Watt metal-film resistor. body-length of 9 mm, a body diameter of 2.5 mm, and lead diameter of 0.8 mm, given about 13nH inductance , @ several Mhz. <G3YNH From Transmitter to Antenna>

•  A wire-ended ½Watt carbon-film resistor will typically have a shunt capacitance in the range 0.2 - 0.5 pF, and a metal film resistor typically 0.2 - 0.6 pF [20].

• 100 Ω resistor is very nearly a perfect component over the entire HF spectrum, and a 1 kΩ resistor is not too bad. Our cheap 10 kΩ resistor however, has an impedance of about 5000-j5000 Ω at 32 MHz, and resistors of higher value than that are basically just poor-quality capacitors.      <G3YNH From Transmitter to Antenna>                                                         (this does not mean that high value resistors are of no use at radio frequencies; a 1 MΩ resistor providing DC bias to the gate of a JFET, for example, will still do its job, but from a radio-frequency point of view, it simply adds about 0.5 pF of stray capacitance to the amplifier input impedance.  only the 100 Ω resistor (in this set of examples) is capable of displaying a positive reactance when the inductive reactance is added. In all other instances, the impedance is vastly dominated by the capacitance, and the circuit inductance is effectively immaterial.)                                        [NOTE YOUR SWR RESISTOR DIVIDER/ resistors in parallel ]