2008 Archives

Amateur Radio at Microwave Frequencies: Hands-on equipment and concepts demonstration of getting on the air at UHF and above.

"You will be surpised at how little it takes to get into microwaves. You can build a rig for less than $200."

Example components:

• discarded satellite TV dish
• mixer (~$50)
• stable frequency source module (~$50?)
• power supply for freq. source module (requires -20V @ 1.5 amp)
• Your existing VHF/UHF SSB transceiver for use as an IF section, or even a 10m rig could be used.

Concepts discussed and demonstrated:

• rectangular waveguides vs. coaxial cables or twin-lead t-lines
• polarization
• near-field and far-field regions in the vicinity of an antenna or aperture
• power units Watts, milliwatts, dBm, dB, orders of magnitude, signal levels
• standing waves
• reflective materials, lossy dielectric materials
• cut-off freq. in waveguides
• microwaves behaving like light rays
• parabolic dish antennas with very high gain and small beam-width
• interesting propagation effects/unexpected communication paths

Jeremy Haley, WG9T

"RF IMPEDANCE AND THE SMITH CHART"

• Resistance & Reactance (capacitance/inductance) >>> Impedance
• Reflection coefficient and SWR
• Relation between impedance and reflection coefficient
• Impedance mapped onto a polar graph
• Constant resistance circles, constant reactance arcs (circles)
• SWR circles, rotation along a 50-ohm transmission line
• Open-source, cross-platform RF/Microwave software for learning/experimenting: QUCS http://qucs.sourceforge.net
• Example impedance calculations at either end of the feedline
• Impedances of an approximate 40 m dipole at 20 m and 17 m
• Simple matching circuit calculation for the non-resonant 40 m dipole that could allow 17 m operation.