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

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

3. 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

4. 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 ohms but unballanced
Common application is VHF mobile antenna

5. 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

6. 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

7. 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.

8. 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.

9. 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

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

11. Balun Construction Project