End Fed Half Wave Antenna Calculations

Resonant Halfwave End Fed Antenna

Last month, we learned about dipole antennas and how to calculate wire length. This month we will look at an End Fed Half Wave (EFHW) antenna. A little refresher, a dipole antenna is typically designed to be a half-wavelength long at its resonant frequency. Two quarter wave lengths, one on each side of feed point or center insulator to make a half wavelength.

An EFHW antenna will be a wire one half wavelength with a connection to the transmitter on one end of the wire. We calculate the length the same way we did for the dipole antenna using the speed of light (c) divided by the frequency (f): λ = c / f.

Last month’s technical corner determined that if we are using a wire for an antenna, we can use 468 for c in formula to calculate length in feet, and 143 for c in formula to calculate length in meters.

For example, for 14.2 MHz, the calculation would be 468 ÷ 14.2 = 32.95 feet is the length of the EFHW antenna

This same example for 14.2 MHz, the calculation in meters is 143 ÷ 14.2 = 10.07 meters is the length of the EFHW antenna

Now due to the EFHW antenna having a much higher impedance, approximately 2450 ohms, we need to use a 49:1 ratio balun. This allows the impedance to be reduced from 2450 ohms by 49 times to equal approximately 50 ohms.

If the antenna has an impedance of 3000 ohms this will still be close with 49:1 ratio of 61 ohms. This would be a Standing Wave Ratio (SWR) of approximately 1:1.25 ratio

Remember a dipole is close to 72 ohms normally and can work with most radios with an acceptable Standing Wave Ratio (SWR) this would be approximately 1:1.5 ratio

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