Half Wave Dipole Antenna

by Nicolas Medalla on March 27, 2020 0 Comments

Speed of Light = 299,792,458 m/s = 300 = s

Frequency = Operating Frequency in MHz = fo

Velocity Factor (see text*) = constant for 50 ohms cable = 0.95

Results is in meter…..

Meter -> Inch = 1 meter = 39.3701 inches

Formula:

λ = s / fo

Vertical Monopole Element = (λ*0.5)*vf

Actual wavelength approximate rounding off to = 81.3990 or 81.4 inches

Substitute from the formula for 145.000 MHz

λ = s / fo (meters)
λ = 299,792,458 /145,000,000 = 2.0675 meters

λ = = 2.0675 meters

Converting to inches by multiplying wavelength results to 39.3701

λ = 2.0675 x 39.3701 = 81.3990 inches

Hence:

λ = 81.4 inches = L

vf = 0.95

Solving for one-half wavelength:

Radiating Element = (λ*0.5)*vf = E

= (81.4 x 0.5) x 0.95 = 38.6645 inches

Divided by 2 elements for upper and lower elements

= 38.6645/2 = 19.3 inches

Gap between Radiating Element = λ / 200 = 0.407 inch = G

L = 81.4“

E = 19.3”

G = 0.407”

Note: important
Adjust To the lowest Standing Wave Ratio (SWR)

Add 5% to radiating element -> = 20.265 inches
Check SWR, using SWR/Power Meter, tune to Operating Frequency (e.g. 145.00MHz)
Connect cable. Antenna -> SWR/Power Meter -> Radio
Set to SWR / Power to test
Hence: if SWR = 1-1.2, Power = Radio Power Output Set, if you set your radio power to 5 watts it should be 5 watts meter reading in the SWR/Power meter.
If the measurements of the cut elements are correct for the Operating Frequency, fixed and finalized antenna, else if the results reading is/are not meet the ideal reading, cut both elements.

Cutting:

Operating Frequency = 145.000 MHz
If the SWR is high and Power is low
Adjust radio frequency to either higher or lower frequency to read to the lowest SWR, take note the Frequency with the lowest SWR
Now, example the frequency with the lowest SWR is 146.000 MHz
New formula New Frequency divided by old Frequency multiple by original radiating element results.

= (146 / 145) x 20.265

= 20.125 inches

BY: Nicolas C. Medalla, DW7NCM

HRC – Tribu Amihan

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