I'm starting to study RF propagation at college, and I'm thinking about what antenna I should choose for this scenario:

enter image description here

The distance between the transmitter and receiver is just 700 meters, and the link is not line-of-sight because there are 2 buildings in the way.

The frequency is 408.00625 Mhz and the bandwidth will be 12.5 Khz FM.

Do you think a 1/2 wavelength dipole (omnidirectional) will work for this? Or would a Yagi (directional) be better? I think that a Yagi would oversize the link[?].

Which one of these antennas would be the best choice?

  • 1
    I took the liberty to edit your question to fit the guidelines of this site. A dipole is not omnidirectional; rather, it has maximum gain off the sides (broadside) and minimum gain off the ends. And what did you mean by "a Yagi would oversize the link"? – Mike Waters Nov 13 '17 at 2:06
  • Well a Yagi antenna is mostly used in TV and systems with more bandwidth, also more distance between the end-points. Viewing the 3D radiation pattern of the dipole it seems omniddirectional like a toroid. – EduardoG Nov 13 '17 at 2:10
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    In general, a yagi has much less bandwidth than a dipole. But in your case this doesn't matter, your frequency is known and fixed. – tomnexus Nov 13 '17 at 4:35
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    It would be nice if some more information could be added about the assumptions in the problem. Do we have only the shown buildings and nothing else? This is important, because we need to know if signals can reflect from something else. Is a specific power level required at the receiver? Is there a power level which will saturate the receiver? Do we have transmit power and feedline losses? – AndrejaKo Nov 13 '17 at 6:22
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    The problem with questions such as this is that they rely on a whole bunch of assumptions which the professor might or might not have explained to students. Often, in real world, different assumptions might be more justified, leading to different answer. – AndrejaKo Nov 13 '17 at 6:24
up vote 3 down vote accepted

A complete analysis of this situation first must consider the propagation path performance, and then the types(s) of antennas to use w.r.t. the losses on that path. This short propagation path has nearly a fixed propagation loss regardless of the antennas/patterns in use.

Antenna characteristics depend on path losses, the Z-matched power at the tx antenna input, tx antenna gain toward the rx antenna, rx antenna gain toward the tx antenna, the r-f signal needed at the receiver input connector for acceptable performance of the overall link system (including fade margin), and signal reflections from other structures and surfaces in the propagation environment.

Below is a graphic showing general considerations for point-point link paths.

enter image description here

  • Yeah Sir you are right I've been reading about the Fresnel Zones and they will be my propagation model. After losses calculation I will choose the antenna. My question was just to know the most used anntenas for some ranges. – EduardoG Nov 13 '17 at 18:52

What you need is a link budget. A higher gain antenna (such as a Yagi) adds gain to your budget and makes the link more likely to work, or allows for less transmit power.

I'm not sure what you mean by "oversizing" a link. Using an antenna with more gain than necessary has no particular disadvantage other than the antenna might be larger or more expensive than necessary.

With buildings blocking line of sight like that, you might be better off looking for a repeater or passive reflector.

Taking this as a theoretical question, and not knowing anything else such as how the buildings are oriented, what they are made of, or how much power you are using, I will step out and say that in general you will get maximum signal from A to B by using a pair of Yagi-Uda antennas.

The trick, due to scattering, or knife-edge effects, plus the fact that building faces tend to change the polarity of reflections, will be the orientation of the respective antennas.

That answers your question as asked.

To get a more precise answer would require more information.

  • Well in my country the frecuency band I selected can be applied to point-to-point consulting the Frecuncy Allocation Table So it can be licensed.Trust me I m sure it can be licensed. – EduardoG Nov 13 '17 at 2:18
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    Note to SDsolar: The FCC is a US body, which oversees communications in. the USA and some other North American countries (and US overseas possessions) – Scott Earle Nov 13 '17 at 2:27
  • Good point. The answer is still valid. You'd want Yagis for directionality and gain, and the buildings can create strange reflections that impact both direction and polarization. Using dipoles could work but would not be optimum. – SDsolar Nov 13 '17 at 6:20

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