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## Re: [TowerTalk] elevated short vertical dipole or quarterwavemonopole?

 To: "Al Williams" ,"Towertalk" Re: [TowerTalk] elevated short vertical dipole or quarterwavemonopole? "Jim Lux" Sat, 4 Dec 2004 12:16:07 -0800
 ```----- Original Message ----- From: "Al Williams" To: "Towertalk" Sent: Saturday, December 04, 2004 9:43 AM Subject: Re: [TowerTalk] elevated short vertical dipole or quarterwavemonopole? > > ----- Original Message ----- > >. Did you ever contemplate where the difference was coming from? > > Ground losses? > > I have difficulty in understanding or accepting the notion of "ground > losses--...wasted power heating the earth, etc..." > I have no problem with using a dc, ac, or rf resistance meter to measure > between two earth points, but radio waves striking the earth, I dunno! RF propagating through a lossy medium heats it up. Think of 2.5 GHz radio waves hitting that cup of coffee in the microwave. > What follows is what is puzzling; maybe someone can help answer them. > > 1. Radio waves from a horizontal antenna striking the ground are > reflected (exactly what is going on by reflection?) and add signal > strength to the skyward wave. However radio waves from a 1/4 wave > vertical striking the ground are absorbed (all? partially ? and as a > result do not provide the ~2 db gain of a dipole and the ~ 4 db gain > from ground "reflection". Not quite as you've described it. EM waves (whether light or RF) striking the ground are partially reflected and partially transmitted into the soil depending on the relative index of refraction and the polarization. A good example is looking into a body of water (river or lake). At some angles, the light reflects completely, at others the light penetrates. And, it depends on the polarization (why polarized sunglasses reduce glare). The reflection process changes both the amplitude and phase of the reflected wave. The horizontally polarized waves reflect fairly well, regardless of incidence angle. Vertically polarized waves only reflect at angles sharper than a certain value, otherwise they penetrate (and are absorbed). So, a horizontal dipole (which has the same gain at all elevation angles, at least broadside) picks up gain in some elevations where the reflected and direct wave are in phase. A significant case is where the dipole is 1/2 wavelength above the reflector. The reflected wave is exactly in phase with the ray heading straight away from the reflector (towards the zenith in the case of ground), so this antenna has most of its gain straight up. 1/4 wavelength high actually produces a null straightoverhead (the reflected wave is 180 out of phase with the direct wave). For vertically polarized antennas the situation is a bit different. First, they already have a null straight up. Second, the reflection is phased differently, and the "apparent source" isn't as obvious in the vertical plane. > > 2. The horizontal dipole radiates because each half of the dipole > alternately charges and discharges oppositely and equally. But what > goes on when the feedpoint is moved off center so that metal to charge > on each side is unequal? Any antenna radiates because charge is accelerating and decelerating. A dipole is a special case which is symmetrical. An asymmetrical antenna still moves charge around, so it still radiates. To a first order, the effect is an impedance transformation. The basic pattern (given that the antenna is the same length and you assume lossless) will remain pretty much the same. > > 3. The 1/4 vertical alternately charges the vertical portion against the > ground (or ground wires, salt water). Therefore is the problem not that > the ground is lossy but rather that the vertical part of the antenna > cannot be charged as well it is over metal wires or salt water? Has > anyone measured the power going into the vertical part when over ground > vs when over wires or salt water? Lots of people have done this measurement. It's how programs like NEC are validated. And, the ground IS lossy. In a vertical monopole there are two sources of loss: the ground return (the current flowing in and out of the antenna) and the eddy and dielectric losses from having the field penetrate the earth. You could put up a radial field above the ground and practically eliminate the ground return losses (think about an inverted V dipole turned on it's side...) , but you'd still have the losses due to the electric and magnetic fields interacting with a lossy substance in the near field. > > 4. Is the "ground loss" frequently referred to related or confined to > the Brewster angle consideration? No... The pseudo-brewster angle is a far field pheonomenon. > > 5. Again, what is actually going on by ionosphere and ground > reflection--is it similar to metal interception radio waves and becoming > a generator? Ionospheric propagation is more of a refraction as the ray passes through an area with changing index of refraction, so it bends (just like light in a lens). The index is different for different angles of incidence and polarization, and, is dispersive (different with small changes in frequency) and nonuniform to boot. The ionosphere isn't a very good conductor, but there's a lot of it. More than reflecting from metal, it's more like reflecting from water, or, even better, like a mirage, where you see an image of the sky on the road in front of you that looks like water. However, from a "large scale" ray tracing standpoint, you can consider it as a reflection from a plane at the "virtual height" of the ionosphere. Ground reflection is similar.. it's the propagation of an EM wave across an interface with different indices of refraction, which makes for a big reflection coefficient. If you like, you can think of the path being like a transmission line with a really bad VSWR at the air/ground interface. > > thanks > k7puc > _______________________________________________ See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA. _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk ```
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