27 votes
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Which HF bands are best during the day and which are better at night?

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 ...
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22 votes
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Is free space path loss dependent on frequency?

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 ...
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15 votes

Which HF bands are best during the day and which are better at night?

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'...
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14 votes

How far do radio waves travel?

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. ...
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13 votes
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Non-traceable radio emitters

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 ...
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12 votes
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How do I make a RF propagation map for a repeater?

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 ...
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  • 1,246
11 votes

Non-traceable radio emitters

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 ...
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10 votes
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Can there be (long range) propagation of HF from satellites?

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, ...
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  • 8,171
10 votes
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Using shortwave communication, how difficult is it to transmit messages from the UK to the US?

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, ...
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  • 9,041
9 votes
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What is "Meter" referencing in "60-Meter Band"?

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 ...
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  • 8,922
9 votes

Does geology affect radio signals?

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 ...
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9 votes

Why is VHF better than UHF in this situation?

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 ...
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9 votes
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How can Ground Penetrating Radar work?

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: ...
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9 votes

Communications : Ground wave propagation

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 ...
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9 votes
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Do short waves pass through or reflect off concrete buildings?

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 ...
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8 votes

How do I make a RF propagation map for a repeater?

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 ...
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8 votes

Visible radio wave propagation

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'.
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  • 4,329
8 votes

Why can't VHF / UHF be used with ionosphere reflection?

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 ...
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8 votes

What's the difference between "maximum usable" and "critical" frequencies?

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: …...
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8 votes

Better explanation for beyond-line-of-sight VHF signal reach than "lower curvature to radio waves"

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 ...
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7 votes

How do I find out what bands are open?

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 ...
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7 votes
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What is propagation like when there are no sunspots?

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. ...
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7 votes

Better explanation for beyond-line-of-sight VHF signal reach than "lower curvature to radio waves"

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 ...
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7 votes
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What is this burst transmission sequence across the entire band?

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 ...
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7 votes
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Looking for a book that contained a circuit for receiving (literally) underground amateur radio transmissions

Yes, this most certainly was a thing, and people actually did it. This type of communication has been done at ELF frequencies in the 3-30 kHz range. Their linear amplifiers were ordinary Hi-Fi audio ...
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  • 7,515
6 votes

Is free space path loss dependent on frequency?

I'd like to offer a parallel and simplified explanation to W8II's correct answer above, for the mathematically-challenged VHF+ enthusiasts among us. :-) As was mentioned in recent threads here, ...
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  • 7,515
6 votes

What does multi-path sound like on FM?

Multipath distortion can be problematic on fast digital modulations since the receiver sees multiple copies of the transmitted signal, each delayed by some appreciable fraction of a symbol. For ...
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6 votes
Accepted

Where year 2016 falls in the sun spot cycle for HF propagation?

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 ...
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6 votes

How is it possible for an HF signal to go around the world when part of it must be in sunlight?

The D region does not affect all HF (3 MHz to 30 MHz) in the same way. For low frequencies, 2 to 4 MHz, the D region absorbs most signals during the sunlight times of the day. However, somewhere ...
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  • 2,868
6 votes

Can I talk 2 Meter VHF across the United States

There is a list of distance records on the ARRL website. Looking at these, I would say that it might be technically possible to hold the absolute minimum of what could be called a ‘contact’ (e.g. ...
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