Oddly enough, some folks like doing this because of the dopamine hit of really understanding how something works. FWIW I'm that kind of person :-). But I certainly understand it isn't for everyone.

The platform I've been using to explore stuff like this is an Ultra96 board with a LimeSDR as the receiver. The Ultra96 has the Zynq Ultrascale FPGA on it. Given that USB3 latency is < 10 uSec I am guessing (hoping?) I can implement it in the Ultrascale fabric which is fed IQ data from the USB 3.1 port.

For DSSS demod, my guess is a 20 tap match filter designed from a resampled RRC filtered barker sequence. Haven’t looked at the VHDL, but that’s how I’d do it.

> Why can’t they make visible light with an antenna?

You can. The antennae are the size of molecules. LEDs emit light, your retinas receive them. The retinal dye molecules are tuned for specific frequencies just like you'd tune a macroscopic antenna.

Richard Feynman has a short book on Quantum electrodynamics for dummys. Otherwise known as QED: The Strange Theory of Light and Matter. Read that as many times as you need to break you of any preconceived notions about how light works.

As for sin waves etc. It's better think of light oscillation between it's energy as an electric field and a magnetic field at right angles to each other. Although I think that's kinda gross and wrong.

As for an antenna producing visible light, in theory there isn't any reason. Because radio waves are light. The problem is practicalities.

> But I have a hard time visualizing RF light.

The best things I can recommend is to play with GnuRadio, IQ data, false color representation of a freq or set of frequencies bound to color/decibel strength, and ParaView and importing said IQ data for graphing.

Think of the frequency of light as its color, and dB strength of how bright it is. However at cm and m wavelengths, voids happen much more regularly. That light can 'bend' around, and go through objects.

> There aren’t any good videos out there of just a straight forward visualization of RF light.

There's quite a few false color images. If you look at hydrogen line radioastrometry, there's a ton of false color images. There's also this HackADay that mapped wifi for a wide area covered by their CNC gantry https://hackaday.com/2015/02/17/mapping-wifi-signals-in-3-di...

> And also, why can visible light pass through other visible light without causing interference?

EM primarily only directly affect things with an electric charge. EM radiation itself doesn't have an electric charge, therefore EM usually doesn't affect other EM. However, if we include gamma (+10^19 Hz), then if those collide, they can create an electron and a positron. But that's only theorized with energy vector diagrams and not actually seen.

> Why can’t they make visible light with an antenna?

You can... You just have to pump enough energy in it to make it glow! /hahaha

> Why are large arrays of RF receivers not more widely used for small-scale RF imaging?

That's primarily a cost issue. Go look how much a single RF frontend chip and an a/d chip costs. (Price gets to stupid levels at, say 24 bit A/D).

Now instead, lets look at human vision. Humans can see (eyes are receptors of radio from 380nM to 720nM). When converted to Hz, we're talking 372.55 THz wide spectrum vision.

My SDR on the table can see 112 MHz, or .0000112 THz

Now, in order to replicate what's going on in the eye, you'd need millions of antennas AND data acquisition (of some sort). And then, even with current SDRs, these generate 60GB/min - you need the disk, memory, and CPU to do stuff with that. It's NOT a trivial problem.

Now there are some RF arrays out there. KerberosSDR is one such array. However, its max bandwidth is 3 MHz @ 8 bit. And it can only do 4 inputs, which is enough to do geographical tracking of radio signals (within 24MHz to 1.7GHz). I know of one person who's trying to do some VR work with a KerberosSDR.

The other problem, once you have the millions of antennas and data acquisition, is a matter of synchronization. Timing is also another stupidly hard area, which increases geometrically with more sensors. And remember that 1nS = 11.8 inches deviation.. So whatever processing you're doing had better be time consistent and local to the device.

Eventually, we'll get to what you're proposing. A lot of us are wanting that. But we're decades away.

> Why can’t they make visible light with an antenna?

Why can't an oscillator and antenna emit matter? Is some matter transparent to other matter in the same way that some matter is transparent to light? This idea that matter and energy are equivalent and that the same wave physics underlies it all doesn't seem to be true in reality. Instead it seems that phenomena are divided into regimes and in each regime behavior is very different. Sure it's tough to build an oscillator at the frequency of matter, but is it really impossible to build a matter emitter? I only have a BA in physics but I remember while learning (and in later reviews of physics) that it seems like theory doesn't always scale and there are a lot of very weird assumptions and "explanations".

> And also, why can visible light pass through other visible light without causing interference?

Radio passing through radio doesn't interfere with thr signal. Its just more likely that the interfering signal will also be picked up by a receiver.

Think about a bright white lighbulb next to a small red LED; you won't be able to see the small LED.

The other part is that out eyes are essentially 2d grids that are fairly large in both directions with respect to thr wavelength of light. Radio receivers are essentially a single point.

We can also bend and focus light with lenses fairly easily with many different substances, due to the refraction index of the material.

If you move the red LED away, you will be able to see it more easily. This is because we can focus the light onto more than just a point receiver so that the the bright light doesn't entirely overwhelm all the sensors. If you do photography, you know that overexposure is problematic and can cause areas around bright areas to be overexposed as well.

While we cant use glass to bend radio waves, they can be be bent in the same way as visible light. Ham radio operators do this all the time by using the ionosphere to bounce radio off of to make long distance contacts that are beyond normal line-of-sight.

Radio and visible waves are also subject to gravitational lensing.

They are also both subject to diffraction, where the light will bend slightly around the endge of an object. Ham radio operators also make use of this to make contacts near the edge of obstructions, like mountains.

Visible light can be used in fiber-optic cables, and waveguides are a similar device used for microwave-frequency light.

> Why can’t they make visible light with an antenna?

In terms of normal radio antenna designs, you could, I think, but antennas need to be sized proportional to the wave length of the light.

However, since visible light is higher energy than radio light, and near many of the transition energy found in atoms, we make extensive use of that additional energy when detecting (e.g. photoreceptors in our eyes and photodiodes), emitting (e.g. LEDs and incadescence), and using (e.g. photo etching and film photography) visible light.

> Why are large arrays of RF receivers not more widely used for small-scale RF imaging?

You can only image something down to a resolution of the wavelength used.

You do see some applications of this, viz millimeter wave imaging at airports.

I can't find the link, but a week or some ago, there was an article on hackernews about radio imaging of cities from satellites? If I find it I'll edit it in. Resolution on the order of meters is adequate for that purpose. (For reference: WiFi is roughly 12½cm. Fm radio stations are roughly 3m. Am radio is roughly 300m.)

Also, this is a limitation of visible light microscopes as well. We can't imagine, say, molecules, because theyre smaller than the wavelengths of visible light.

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One of the biggest difference between visible light and radio light when it comes to thinking about propogation is what absorbs it. Your WiFi router can be on the otherside of a wooden wall because wood doesn't absorb or reflect substantial amounts of rf energy; a metal wall does. 5ghz doesn't propogate as well as 2.4 because it's absorbed by water in the air and things more than 2.4ghz is. Visible light is absorbed by most things some amount.

So, the big differences are how the physical world affects the light, but in broad strokes it behaves the same way to the same situations, its just that those situations (e.g. what can reflect, refract, diffract, and absorb it) are frequency/wavelength dependent.

If you like direwolf you might want to take a peek at the M17 project[1]. They're looking to do the same thing with what's happened with proprietary radio protocols and looks really promising:

> M17 is a new digital radio protocol in development as an alternative to those currently available, with freedom in mind. Freedom in the code, protocol, voice codecs, and hardware. The goal is to provide a better option for digital radios in the future.

[1] https://m17project.org/

I feel like it's easier to toy around on Windows, but if you want to do anything (and I mean anything) serious, you'll need Linux. That's been my experience anyway.

This is a guess, but it is a good one: no. It may not even be possible to get it certified (wrong interpretation of the law to me, but the government has more lawyers than anybody else). Many SDR boards don't have the power required to do actual transmission as it is hard to design a front end that pleases everyone, but equally as important, any real transmit power could mean the device is construed as non test equipment.

Why do you care? Just buy it and flash the bitstream to it, no one will ever find out.

But are you willing to part with $720 to play with it?

It is somewhat natural to just refer to 802.X standards as 802.X when implementing them as that is the correct name of the standard and you need to read most of it in order to implement it. If you want to talk about distinct added features it makes sense to use the name of TG (ie. 802.[number][letter]) that drafted the specification of said feature, but it is mostly only an handy shorthand.

https://www.nuand.com/product/bladerf-xa4/ $480 for the smallest.

And while too rich for most is one way to look at it, another is that $1K would not even buy you a proper monitor all that long ago, and that this is a very low volume production. I think the price point is actually quite impressive, I expected it to cost substantially more, especially given the BOM, there are some pretty impressive bits on there, and those connectors and PCB are also not exactly free.

My very limited knowledge of SDR’s from when I was researching a project is that for a lot of use cases, the cheap ones just don’t have the bandwidth or frequency range. I ended up getting a limesdr which was enough to serve my needs, but it was still around $400 for the usb version.

Looks like modelsim

https://github.com/Nuand/bladeRF-wiphy/blob/main/fpga/models...

https://www.mentor.com/products/fv/modelsim/

While receiving analog radio is trivial for the processing required, it is fairly low frequency transmission which requires antenna sizes which are not entirely practical for phone-like device. I would even assume that many phones contain the required hardware (which usually involve using headset cable as antenna) but don't have any kind of software support for it because of combination of minimal demand for such a feature and the fact that the entire experience is not exactly user friendly. Notably, Raspberry Pi uses wifi/BT radio which apparently either includes FM radio or used to in previous revisions (and apparently the radio chip is capable of receiving FM and directly sending the audio to BT headset, which is somewhat interesting feature given the usual requirement of wired headset as antenna).