Conventional wisdom based on the far-field (only) patterns calculated by M-o-M software such as NEC show that low-angle radiation from a horizontal dipole is very low, and zero in the horizontal plane. Is this belief valid?
$\begingroup$
$\endgroup$
Add a comment
|
2 Answers
$\begingroup$
$\endgroup$
4
The very small low-angle radiation noted above is the result of excluding the surface wave from the NEC calculations.
Also the commonly-used "far-field only" patterns calculated by NEC are based on an infinite distance from the radiator, and if a ground plane is included, it is considered to be flat. Both of these elements can lead to a misunderstanding of the real-world performance of antenna systems modeled in NEC.
Below is a comparison of NEC patterns including the surface wave. It shows the radiated field intensities at one wavelength in the horizontal plane for elevation angles from zero to about 17.2 degrees, with 100W of Z-matched r-f power at the antenna feedpoints.
Conclusions are left to readers to consider.
-
$\begingroup$ Compared to the "sky" wave, how useful is the "surface" wave for communication? Is the "surface" wave the mode relied upon by, e.g., medium wave local and regional AM broadcast stations? $\endgroup$ Commented Oct 19, 2018 at 13:01
-
$\begingroup$ A properly-defined NEC model including the surface wave shows that low-angle fields from about 5 degrees elevation and greater are space waves, which decay at a 1/r (inverse distance) rate. Under the right conditions they are capable of reaching and being reflected by the ionosphere to provide "DX" signals at receive locations hundreds/thousands of miles away from the transmit antenna. $\endgroup$ Commented Oct 19, 2018 at 13:56
-
$\begingroup$ Amazing, great simulation. You've plotted only Etheta, so this is a vertically polarised wave, from a horizontal dipole, right? I assume it's off the end of the dipole, perhaps specify this? Also, could you repeat the exercise at 1 km or 5 lambda? 1 lambda feels quite close to the antenna when it's so high up. $\endgroup$– tomnexusCommented Oct 19, 2018 at 13:59
-
$\begingroup$ E-field polarization: monopole = vertical, dipole = horizontal. The horizontal dipole is modeled at a constant elevation of 50 feet above the earth. E-fields at a horizontal distance of 1 wavelength from the dipole are shown at all elevation angles from zero to 17.2 degrees. The dipole fields were calculated for/at its peak directivity (+ and - 90° w.r.t. its longitudinal axis). $\endgroup$ Commented Oct 19, 2018 at 17:20
$\begingroup$
$\endgroup$
1
The reflection of an E field wave is well known to be 180 degrees out of phase with the incident wave, given a pure E field and a perfect reflector. Real world ground conditions, and the shift of the wave from pure E to a mix of E and H fields somewhat dilutes this effect, as shown by the chart.
If low angle signal strength is the objective, an interesting alternative is an H, or magnetic, wave generating antenna, as classical theory shows this energy is reflected in phase by the ground. Personal on-air 40 meter band amateur radio experience with such an antenna, a so-called magnetic loop, has borne this out. A 7 foot diameter loop 7 feet off the ground has consistently performed as well reaching DX as others' dipoles several times higher.
-
$\begingroup$ Just to note that ALL e-m waves are comprised of electric and magnetic fields in the same ratio, neither of which can exist without the other. No e-m radiator (antenna) can radiate only an electric field or only a magnetic field. $\endgroup$ Commented Oct 23, 2018 at 23:05