The previous article, Degrees of Antenna Occupied by a Loading Coil, showed how The Hamwaves Inductance Calculator can be used to determine the parameters of an 80m mobile loading coil. Note that the accuracy of the inductance calculator is thought to be ± 10%. The specifications for the 80m mobile loading coil are:
In the previous article, we converted the Axial Propagation Factor to degrees/inch by multiplying by 1.4553 and then multiplied by the 10 inch length of the coil to obtain the number of electrical degrees occupied by the loading coil at 3.5 MHz. We also calculated the Velocity Factor of the coil at 4% of the speed of light in free space. Knowing the VF of the 10 inch coil allowed us to calculate the RF propagation time through the coil.
One wavelength at 3.5 MHz is 281 feet so we can estimate that the whip occupies 8.83/281 = 0.0314 wavelength. We can calculate that the loading coil occupies 26.37/360 = 0.0733 wavelength.
The coil occupies 0.0733 wavelengths (26.4 degrees). The whip occupies 0.0314 wavelength (11.3 degrees). That adds up to 0.1047 wavelengths = 37.7 degrees. We know that the resonant mobile antenna is 0.25 wavelength (90 degrees) long. Where are the missing 52.3 degrees?
Knowing the impedance at the coil/whip junction allows us to calculate the characteristic impedance (Z0) of the whip. The normalized value from the Smith Chart on the whip end is approximately -j5.0 which we know from the above calculation is -j2374 ohms. Therefore, the Z0 of the whip at the coil/whip junction is 2374/5 = ~475 ohms which is a reasonable value.
We can now conclude that the "missing" 52.3 degrees is caused by the impedance discontinuity at the coil/whip junction, i.e. where the Z0 changes from 4747 ohms to 475 ohms. The same thing happens when two transmission lines are connected together if they have different characteristic impedances, i.e. different Z0s.
In a base-loaded mobile antenna, there are three components that contribute to the 90 electrical degrees required for resonance.
1. The coil contributes a certain number of degrees, 26.4 degrees in the above example.
2. The impedance discontinuity at the coil to whip junction point contributes a certain number of degrees, 52.3 degrees in the above example.
3. The whip contributes a certain number of degrees, 11.3 degrees in the above example.
Thus a base-loaded mobile antenna can be analyzed in the same manner that Dual-Z0 Shortened Stubs can be analyzed. The following dual-Z0 1/4WL stub analysis is virtually identical to the above base-loaded mobile antenna analysis. Note that the phase shift at the impedance discontinuity is proportional to the ratio Z0H/Z0L and 4747/475 = 500/50.
A secondary reason is that having a bottom section under the loading coil (center-loading) complicates the analysis by introducing a small additional negative phase shift at the bottom section to coil impedance discontinuity where the Z0 changes from the low Z0 value of the bottom section to the high Z0 value of the coil. Because the phase shift at the bottom of a center loading coil is negative, the inductance of the coil must be increased to add to the degrees occupied by the coil, in order to maintain the same resonant frequency as the base loaded configuration.