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9

Ham operators often tell me that comparing gain to an isotropic radiator isn't much use because it's only a theoretical antenna, is this true? No, that is not true. I generally find that hams that denigrate or choose to ignore the isotropic antenna or references to it, simply don't understand its central place in antenna engineering. Certainly the isotropic ...


7

The antenna you describe is "omnidirectional" only in the xy-plane; it has zero radiation along the z-axis. Thus, your dipole is mounted vertically; i.e., x=0 and y=0 for all segments. According to Antennas by John Kraus, the far E-field for a center-fed $\lambda/2$ dipole in free space is: $$E = \frac{\cos\left({\pi\over 2}\cos\theta\right)}{\sin\theta}$$ ...


6

dBi is useful for comparing gains of real world antennas. For instance, an 80m mobile antenna may be -10 dbi, a 1/4WL vertical ground plane may be 0 dbi, a dipole may be 6 dbi, and a beam may be 10 dbi. So we can say that the beam has 20 dB gain over the 80m mobile antenna using dBi as the reference dimension.


6

In your particular case, I'd note 2 things: The aluminium is directly above the whip, where the antenna's fields are the weakest. Aluminium is a good conductor, so unless there is something about it that will increases losses, it won't decrease the radiated energy. It might change the feedpoint impedance, and it might change the direction in which energy is ...


5

In general for a given antenna evaluated on a given frequency, if whatever is changing the Q occurs uniformly or universally and does not result in a change to the effective aperture, phasing, or geometry of the antenna then the pattern will not be altered in a meaningful way. An example of a uniform change would be changing the wire gauge of a wire ...


5

On HF (1.5 to 30 MHz) amateur radio operators use antennas with the major directive lobe close to the horizon in order to maximize the distance the signal will travel. At these frequencies the signals bounce off of the ionosphere - so by minimizing the incident angle, the distance of communications tends to be maximized. Multipath signals are rarely a ...


5

If the container is small relative to wavelength, then it won't have much effect at all. If it's very large (say, infinite) then it will reduce ground losses by providing a more conductive ground plane. It doesn't matter if it's actually connected to the soil or not. This is a good thing. The container will be closer to the antenna, which will change the ...


4

First a bit of context regarding terminology. In antenna engineering, we use the term "loss" to refer to RF energy that is lost as heat. This is typically an undesirable loss that creates inefficiency in the antenna system. This is the loss that Phil addressed in his (now deleted) answer. When we speak about the directionality of an antenna, we use the ...


4

Those that complain that dBi isn't useful because an isotropic antenna is only theoretical often advocate an alternate unit such as dBd, or decibels relative to a dipole. There are a couple arguments that could be made from here. One argument is that a dBd is typically defined as 2.15 dBi, because that's the directive gain of a half-wavelength dipole in ...


4

Free space is (under normal conditions of frequency and power and atmosphere) a linear medium, so waves propagating in it follow the superposition principle, so they do not interact just because they occupy the same space. The situation in your diagram is not particularly significant. What you do need to consider is the rest of the radiation pattern of your ...


3

If the only thing the material did was "reflect", and it was like a flag on a vertical antenna, then I guess it wouldn't have any effect on the radiation pattern. The problem is that no such material exists. One problem is that for a material to "reflect", it has to be many (at least 10, say) wavelengths long. Any less than this, and effects like ...


3

No real world antenna exhibits "zero" power in any particular direction. Careful design can minimize the power in the unwanted direction but not eliminate it. In a yagi antenna, the front to back ratio is the difference of the decibel gain of the forward, major lobe less the decibel gain of the lobe 180 degrees from the forward lobe. Thus a yagi antenna ...


2

The NEC4.2 study/graphic first below shows how a structure comprised of conductors to (roughly) emulate the a-c power lines in a wood-frame house might affect the radiation patterns of a 40m-band vertical monopole located six feet away from one side of that house. The effects of this may be different than those expected. Below are the patterns for that ...


2

From the picture, it looks like you have a log-periodic antenna rather than a Yagi. In either case, optimizing some parameter like front-to-back ratio involves optimizing the spacing and length of the elements. In the case of the log-periodic antenna this is more difficult given the broad-band nature. Such optimization can be done by trial and error, ...


2

If the transmitted signal induces any currents in the "sheet", the sheet will affect the radiation pattern. One simple argument: if you run a wire parallel to the "flag pole", there will be induced currents. They may be small (if the wire is not resonant) but there will be some current. (Think of how a yagi antenna works with its "parasitic" elements.) ...


2

There is modeling software you can use for this. EZNEC and 4nec2 are popular among hams, but they only do wires. You could try modeling the aluminium as a bunch of wires - spacing them, say, 0.05 wl apart. If nothing else, the model might tell you when the effect becomes negligible. Software that handles conducting planes also exists, but is AFAIK fairly ...


2

Non-adaptive antennas in mobile systems are usually omnidirectional, meaning they radiate equally in all directions. They can also have directional antennas, if there is some means for getting them to point in the right direction. Usually that means the vehicle on which the antenna is mounted has to stop. Adaptive arrays are directional antennas that ...


2

The hex grids can fit more transmitter/receivers into the same space, resulting in greater power output per square meter. But more importantly, the one you pictured also has independent transceivers so they are individually field-replaceable. The computer that runs it all does diagnostics on startup and can flag the bad ones so they can be fixed right ...


2

The numbers arranged vertically are decibels. They tell you the relative radiant intensity in each direction. For example if in one particular direction you read 0 on that scale, and in another direction you read -3, then the power in the latter direction is half what it is in the first direction. Translating this into a range in meters depends on the link ...


2

The Longley-Rice model is a fairly common technique. However it's not a simple formula, and so requires some kind of software to implement. Unfortunately my favorite such tool, CRC CovWeb, has gone defunct. For a more simplistic prediction, you can try making a simple link budget based on the Friis transmission equation, or if you want to get a little more ...


2

There is a web application already: https://www.ve2dbe.com/rmonline_s.asp 73, Brian, ZL1IE


2

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 ...


2

The NEC4.2 analysis below compares the intrinsic fields/gains of the same monopole for the definitions and conditions shown in that graphic. Note that the monopole having a 10 Ω path connecting it to the ground plane does not display zero gain in the horizontal plane, then gradually rising to more positive values for greater elevation angles above the ...


1

In your example, the outer ring in the polar plot would be 12 dBi. So you are correct in thinking that any point -5 dB from the outer ring would represent a gain of 7 dBi. It works the same for referencing dB, dBi (isotropic), and dBd (dipole).


1

Define the Antenna structure into a free available NEC Software like EZnec, MMANA-GAL or others and you are able to calculate everything of the radiation patterns, gain, losses, currents, voltages, resistance, the complex and real resistance of the entire antenna Building and the rf-field (E/H) shape. Also, you can see shape of the E and H field and the ...


1

Yes it will probably have a profound effect on your radio signals. I was once involved in an experimental evaluation of some reflective window coating, to see if it would attenuate cell phone radio waves. This is a partially transparent shiny adhesive coating which can be applied to existing windows to make them more like mirror glass. The results were ...


1

Is it possible for a receiving station's antenna to be mounted at a certain height such that the radiation of the transmitting station misses the receiving antenna completely? In theory yes. But in practice not entirely. Real dipoles are not mounted over infinitely large, perfectly flat, perfectly conductive planes, so the nulls will not be so deep as ...


1

Much depends on the relative sizes. A metal flag on an HF vertical antenna is not likely to change the pattern in any way you'd notice. A gum wrapper near a 2.4GHz antenna definitely will change the pattern. I once did a design for a 2.4Ghz consumer product and ended up sculpting the antenna pattern with a block of nylon adjacent to the antenna, acting ...


1

Some information, since you asked for comments: 1) your rhombic isn't working properly. It shouldn't have a double beam like that, it should have a single forward beam. Something about the proportions is wrong. You can and should design a rhombic from formulas, and only simulate at the end. It's a well understood antenna, there is good design data for it. ...


1

antenna-theory.com provides some nice images of the ideal case. A half-wave dipole in free space along the vertical has a radiation pattern like this: A 1/2 wave antenna on 2m is a 3/2 wave antenna on 70cm. That radiation pattern looks like this: Of course you are not using a dipole, but a vertical. A vertical on an ideal ground plane has an identical ...


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