20M Antenna

20M Dipole Antenna Construct a 20M wire ½ wave dipole at 30’ Dipole Formula L = 468/FMhz Length = 32.96 @ 14.2 Mhz We will use 33’

This is a Smith Chart. The middle is 50 ohm This is a Smith Chart. The middle is 50 ohm. The middle line is resistance and above and below the line is reactance (capacitive or inductive). There are two circles – SWR = 2, SWR = 3 SWR = 2 SWR= 3 50 ohm Antenna Impedance

Characteristic Impedance of a dipole is 70 ohm SWR = 2.21 R = 73.3 -j 42.3 Characteristic Impedance of a dipole is 70 ohm

Do a SWR scan from 14.0 to 14.3 Mhz Notice the different values of SWR. Notice as the frequency goes higher the SWR comes down and all the indications are below the line. This indicates that the antenna is too short. (The higher freq. is getting closer to the correct length.) Increase Length to 34 feet Rule: The Higher the Frequency – the shorter the antenna!

Notice values are above and below the line. Why aren’t they towards the middle?? SWR < 2:1 for entire 20M Band Antenna changed to 34’

Notice 0dB are different?? (This is the value of the outside circle.) Elevation Azimuth Notice 0dB are different?? (This is the value of the outside circle.) This reference indicates the outside circle.

20M Dipole – 20 Feet Lower the above antenna to 20 feet. What happens to the SWR? What other changes can we see?

Grey Outline is the Antenna @ 30’ Outside Circle Value

Antenna @ 20’ SWR 2.9 (14.2 Mhz) Antenna @ 30’ SWR

Antenna @ 30’ Antenna @ 20’

20M Dipole – 40 Feet Raise the antenna to 40 feet. What happens to the SWR? What other changes can we see?

Grey Outline is the Antenna @ 30’

Closer to 50 ohms – very little reactance

Summary 30’ (.45 λ) 20’ (.30 λ) 40’ (.60 λ) Gain 7.19 dBi @ 33º SWR 1.6:1 80 + j1.2 20’ (.30 λ) Gain 6.19 dBi @ 40º SWR 2.9:1 100 + j61 40’ (.60 λ) Gain 8.46 dBi @ 24º SWR 1.2:1 60.6 + j.35