27

HF propagation over long distances is by skywave propagation, the reflection and refraction of radio waves between Earth's surface and the ionosphere. The ionosphere is a consequence of radiation from the sun ionizing Earth's atmosphere, so it changes significantly with time of day and sunspot activity. Although time of day is just one of many variables, ...


21

Mathematically yes, the value of that equation increases with frequency. However, that's not to say there's some physical mechanism for frequency-dependent attenuation in free space. Rather, the frequency term is in the equation due to the assumption of unity gain antennas at each end. A larger antenna is required to get the same gain at a lower frequency. ...


15

In general, the shorter wavelength HF bands are better during the day, and the longer wavelength ones at night. Although that depends a bit on what you want to do, and like all things propagation, it's subject to change. Let me try and give a rough mode of operations. Also take a look at the chart from eham. 6m- Randomly opened, in random directions. I ...


14

Radio waves don't stop at a distance, they just get weaker; you've read this correctly. The reason that communications stop working at some distance is that the signals are too weak to be understood. Besides distance (and being absorbed or reflected by objects in the path) causing the signal to be weak in an absolute sense (how much power there is), there ...


13

IMO, your question is too general to give a good, specific answer. Also, it would help if you elucidated the motivation for the question. I am a semi-retired electrical engineer, not a security or RF expert but I'll give it a go anyway. The first assumption is that the transmitting equipment is hidden from plain sight and other non radio-related detection. ...


12

Although much of the main page is not written in English, after you login most of the site is and the RMO Repeater coverage map maker is a wonderful and highly customizable tool even for multi-site repeaters. VA3XPR.Net: Three easy steps to creating RF coverage maps like a pro: Radio Mobile Online is a radio wave propagation prediction tool created ...


11

Another possibility to look into is meteor burst communication. This is a well-established technology. You beam a VHF signal up into the sky, it bounces off a meteor trail, and your recipient picks up the reflected signal. Since transmission is upwards, picking up the transmitter's location would require being above the transmitter at the time (making ...


10

This is a strange sort of question, as in reality the signal will go infinitely far (effectively) however you are really asking at what distance might a receiver be able to pick up the signal. In testing a 2.4GHz signal with a 100mW omni antenna, the furthest distance I could receive a signal with less than 5% retries (802.11b kit) was 2 miles with a 100mW ...


10

Satellites can and do use HF for communications. The first example would of course be Sputnik, which transmitted at 20 and 40 MHz. Amateurs use HF to communicate with satellites. According to Amsat, mode A: This mode requires a 2 meter SSB/CW transmitter and a 10 meter SSB/CW receiver... Mode K; This mode requires a 15 meter SSB/CW transmitter and a 10 ...


10

To send a message of around three bytes over such a long distance, would this require an antenna that uses alot of power? No. There's a huge variety of tradeoffs here, but as a quick calibration, WSPR sends 50 bit messages (so, a bit more than twice what you're asking) all around the world using power levels usually ranging from several milliwatts to ten ...


9

That's right, the 5 MHz band has approximately a 60 m wavelength. Back in the early days of radio people often measured where they were on the airwaves by wavelength rather than frequency. Nowadays the radio display shows the frequency, but amateurs still refer to bands by their approximate wavelength. Wavelength x frequency = speed of light, 2.99792e8 m/...


9

Other Effects are there any other geological properties that can affect radio waves? Yes, there are other effects. Basically, every material affects the propagation of radio waves. Absorption That one's kind of obvious: radio wave hits medium (e.g. soil), gets absorbed. That's the reason you can't look through a piece of coal – light is just a wave at a ...


9

This is an ill-posed question, since "best" could mean so many things. If VHF is so plainly best, why does anyone bother with UHF at all? If you mean what option has a lower path loss, then sure VHF is the better option. But then HF would be even better. And why not go with ELF? As the frequency decreases, so do free space path losses. Well, an ELF antenna ...


9

There is a special adaptation of the Friis equation, called the Radar Equation, that describes the range of a radar system. Here is a basic version that calculates the maximum range of a radar system: $$R_{max}=\sqrt[4] {\frac {P_tG^2\lambda^2\sigma}{P{_{r(min)}}*(4\pi)^3*L}} \tag 1$$ where $R_{max}$ is the maximum range in meters, $P_t$ is the transmit ...


9

So, I'll take this very smoothly. The idea is to take you from ray optics gradually to an understanding of radiomagnetic waves as actual waves, and show that it all makes sense. 1st idea: Refraction on a material interface (high->low density) We remember what happens when we have ray of light, leaving a optically denser medium (e.g., water) into a less ...


8

For the most part, propagation for VHF/UHF (1280 MHz is UHF) is line-of-sight because it doesn't tend to get the benefit of ionospheric reflection like HF does. This will also apply to your mentioned 433 MHz transmission, which is in the UHF range. While there are occasionally atmospheric events that result in indirect UHF/VHF propagation (e.g. tropospheric ...


8

It depends on what you mean by propagation. If you mean, does the modulation scheme affect the physical means by which EM energy gets from point A to point B?, then the answer is no. Mostly, EM propagation is linear, so the differences in modulation have little effect on how the wave propagate. However, if you expand propagation to include the ...


8

The short answer is no, there are no cases where radio waves would not have orthogonal magnetic and electric fields. In physics, a radio wave, indeed all EM radiation is called a transverse wave, meaning, by definition, that the oscillations of the waves are perpendicular to the direction of energy transfer and travel. The electric and magnetic parts of ...


8

SPLAT! is an RF Signal Propagation, Loss, And Terrain analysis tool for the electromagnetic spectrum between 20 MHz and 20 GHz. It is free and open source, and there are builds for Windows and Mac if you don't want to compile from source on linux yourself. I haven't used it, so I can't speak to ease of use, but it should generate propagation maps based on ...


8

There are visible radio waves. We call them 'light'. We can see a whole range of frequencies of these waves, translated by our brains into something we call 'colours'.


8

The ionosphere typically neither reflects nor absorbs waves with VHF or higher frequencies, but passes them through to space. There are no reflections back to the ground, so there is no useful propagation between stations on the ground. Tropospheric ducting is a different propagation mode which does carry VHF signals well, and is actually sought and used ...


8

The maps are related, but as this excellent posting describes: The 'critical frequency' is the highest frequency that gets reflected when it is aimed straight up at the ionosphere. However: …as the angle decreases from vertical the reflected frequency increases. And so therefore: MUF or MOF is a path dependent value. It depends not only on the ...


8

The concrete is relatively transparent to radio waves of such large wavelengths (it attenuates, it doesn't reflect). However, steel bars within concrete typically convert that concrete to a solid reflector from the perspective of a wave with such a large wavelength. Basically, that effect scales: Just as your microwave oven's front door has a metal plate ...


8

Please accept Phil's answer. It's the sanest one. Now, however to answer: what would be a better answer to this question? Why do VHF/UHF signals usually travel somewhat farther than visual line-of-sight distance between two stations? Because they are not the same as light; first of all, they aren't blocked by things like a thin cotton sheet....


7

There are a few radios that support diversity reception, but only to minimize effects of propagation rotation and multipath, not to increase data rates. This isn't true MIMO. It's more like SISO where the input is whichever one of two antennas has the strongest signal. Which brings up another point - data. There really isn't that much data being used on ...


7

I don't think anyone would be transmitting AM for weak signal work unless they were just fooling around. You'd get the same effect by cutting your power by 3/4s. In AM the carrier consumes 1/2 the power with the rest split between the side bands. So, SSB is not only more efficient use of spectrum, it's more effective use of your power.


7

If you are stuck on an island and have only enough battery for one minute of transmitting, the important thing isn't that propagation is good, but that someone hears you. Use your 30 minutes of receive time to find an area of some band that is very busy, with very many very strong signals. By reciprocity, the better you can hear them, the better they can ...


7

We went through quite an extended sunspot minimum before the current sunspot cycle started, and I well remember what it was like. 10m was open only when there was e-skip, which wasn't very often. 15m had openings from time to time, but when it was open typically you'd only hear stations in one direction, like from South America for instance, and they'd be ...


7

The full question, and possible answers: T3 C11 Why do VHF and UHF radio signals usually travel somewhat farther than the visual line of sight distance between two stations? A. Radio signals move somewhat faster than the speed of light B. Radio waves are not blocked by dust particles C. The Earth seems less curved to radio waves than to light ...


7

It's probably an ionosonde — sending out HF radio signals not to communicate but to measure the reflections and thereby determine the current characteristics of the ionosphere. The signal is probably not actually a series of brief transmissions, but a continuous chirp transmission with continuously increasing frequency. Two reasons: As far as I know, this ...


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