28-09-2016, 07:19 PM
(This post was last modified: 28-09-2016, 07:26 PM by Mike Watterson.)
For coax, it's the ratio of inner diameter of braid and the outer diameter of the core that mainly defines the characteristic impedance, though the dielectric constant of the insulation is a term too. The dielectric (and its RF losses) and absolute diameter and length affects the loss and maximum working voltage. Thus very low loss coax may be a tube for inner core and as fat as a D cell. At RF the current flows mainly on the surfaces facing each other, the depth depending on frequency. Effectively skinny cable has higher capacitance and higher losses due to higher resistance too. One trick for Ham Radio on VHF/UHF transmitters is to put the power amp at the top of the mast as losing over half the power on input driving it doesn't mater compared with losing half the power on the output.
A RF mismatch can double the voltage on the output devices, open circuit or short, depending on length of cable to fault. So "tuning" (really matching) an HF aerial should always be done at minimum power, apart from it being annoying to listeners.
Resistive losses are significant, hence copper is better than gold and silver is good. At higher RF, the core can be hollow or copper plated steel as little or no current flows in it.
Aluminium phone wires were used for a while to save cost. They are rather poor for ADSL, which is using approximately 120 Ohm Cat 3 twisted pair designed for 3KHz max to over 1MHz, or much more for VDSL. The 1Gbps ethernet uses Cat5e and must be fitted to patch panels and connectors carefully and is "terminated" on the network card as it uses echo cancellation and transmits and receives simultaneously, like audio on phone lines, unlike 10Mbps that uses separate pairs. It uses all four pairs in parallel so to have 1/4 the clock rate.
A RF mismatch can double the voltage on the output devices, open circuit or short, depending on length of cable to fault. So "tuning" (really matching) an HF aerial should always be done at minimum power, apart from it being annoying to listeners.
Resistive losses are significant, hence copper is better than gold and silver is good. At higher RF, the core can be hollow or copper plated steel as little or no current flows in it.
Aluminium phone wires were used for a while to save cost. They are rather poor for ADSL, which is using approximately 120 Ohm Cat 3 twisted pair designed for 3KHz max to over 1MHz, or much more for VDSL. The 1Gbps ethernet uses Cat5e and must be fitted to patch panels and connectors carefully and is "terminated" on the network card as it uses echo cancellation and transmits and receives simultaneously, like audio on phone lines, unlike 10Mbps that uses separate pairs. It uses all four pairs in parallel so to have 1/4 the clock rate.
