Basic Antenna Construction (and a bit of how they work) Douglas County Amateur Radio Club Sept 12, 2018

Topics The Dipole Antenna Impedance & VSWR Multi-Banding the Dipole (TRAPS) Balun Construction Project

The ½ Wave Dipole Antenna ½ wavelength fed in the center, often used horizontal Length calculation: L (feet) = 490 / F(MHz) Narrow Bandwidth ~5% Impedance is 73 ohms + 43 ohms inductive at center frequency and is ballanced. Heigth above ground effects impedance Horizontal pattern is bi-directional, vertical is omnidirectional

The ¼ Wave Dipole Antenna ¼ wavelength mounted over ground plane Usually used in vertical polarization Ground plane acts as a mirror, reflecting the ¼ wave element to perform like a ½ wave antenna Ground plane should be ¼ wave radius or larger Impedance is 73+43 ohms but unballanced Common application is VHF mobile antenna

Impedance & VSWR Will it work ? - my coax and radio are 50 ohms VSWR = antenna impedance / system impedance For transmitting applications, 2:1 is good. That is 25 to 100 ohms. So the dipole at center frequency the 75 ohms will be about 1.5:1 vswr What about other frequencies ? About 1% is reasonable, beyond that an antenna tuner is used to fix the VSWR and extend the range

Impedance & VSWR I want to use the antenna horizontal but feed it with a coax. How do I convert the ballanced ½ wave impedance to unballanced coax ? A “BALUN” is used (Ballanced to Unballanced) This can be a physical RF transformer type or transmission line transformer. The transmission line transformer can be made by using large ferrite beads over the coax just before it connects to the ½ wave dipole

Impedance & VSWR What if I don't use a balun ? Without the balun, some of the RF current will flow down the outside of the coax instead of flowing into the antenna This affects the antenna pattern and reduces power delivered to the antenna A possible hazzard is the RF current ending up on the radio chassis, causing equipment issues and possible shock.

Impedance & VSWR What about Grounding ? Grounding is important for lightning protection Difficult to get a good ground at radio frequencies – if the ground wire is ½ wavelength the “ground” becomes an open. Ground wires greater than ~1/10 wavelength have little benefit.

Multi-Band Dipole Require cheating mother nature Use a switch on each side to change length “Traps” are common solution Traps work by becoming a short or open at their resonant frequency. A parallel LC is an open circuit at resonance and low impedance away from resonance A series LC is a short circuit at resonance

Aproximate Dipole Lengths 80 M 3.75 MHz 62'- 8” (each side) 40 M 7.175 MHz 32'- 10” 20 M 14.2 MHz 16'- 10” 15 M 22.25 MHz 11'-5” 10 M 29.2 MHz 8'- 5 ½” 6 M 52 MHz 4'- 8 1/2” 2 M 146 MHz 19 1/8”

Balun Construction Project