# Tag Info

21

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, ...

19

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. ...

18

You are going to have a very difficult time achieving the first 4 with any band on a low budget. But in general, I'll say a few words to get you started. You probably want to be able to use digital modes. Your best bet to get consistently across the country will be via digital modes, as they can add something like 20-30 dB effectively to your signal. Olivia ...

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 ...

12

If anything, living in an area with high metal deposits near to the ground will improve your conditions by improving the conductivity of your ground. That said, unless it's very near to the surface and high concentration, I doubt you'd notice any difference. Note that when we model the ground as an ideal ground plane, anything underground has no impact on ...

11

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 ...

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

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 ...

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 ...

9

This is called the "FM capture effect" and is mostly a characteristic of the demodulator design. Demodulators with a strong capture effect detect the zero crossings of the signal. The zero crossings are only slightly affected by mixing two signals until they are nearly equal amplitude. The steepness of the waveform at zero crossing means that the stronger ...

9

I understand why you think it should mix - that's what happens with SSB, for example. The difference with FM is that in FM, at any instant, each station's signal is only at one frequency. If you slowed the signals down and watched through a spectrum analyzer, you could watch the two signals, and you'd see two peaks at constant amplitude, moving independently....

9

In general, my experience is that band conditions are best for long-distance communication at sunrise and sunset. You generally want the sunset to be somewhere in between you and the person you are talking to, so probably something like 9 PM EST. Beyond that, things like solar weather can make a big difference. I tend to use VOAProp for predicting when and ...

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 ...

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

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

According to NASA TP 2000 209756, particularly section 2.5, it shouldn't be too different from similar propagation on Earth during daytime, except we'd need significantly lower frequencies than we are used to for similar results. According to that report, The Martian dayside ionosphere at solar maximum has a peak density similar to that of the Earth ...

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

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

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 ...

8

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 ...

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 ...

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

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 ...

7

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/...

7

We're about half way down in the waning years of Cycle 24. We're currently on a 9 day stretch with zero sunspots, the first such stretch since 2010. With the number of sunspots dwindling, we've seen little F layer propagation on frequencies above 25MHz in the last 2 months, and that downward trend is likely to continue. As we approach minimum in 2018(ish), ...

7

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 ...

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