This is a really good thing, but for non-radio people it can be confusing.
In the 80's, the FCC put limits on digital signals below 30MHz based on "baud rate". A baud is a raw bit in a digital data stream, it can either be data or part of the channel protocol. For example, a typical serial port like you have on a PC or an Arduino or something might operate at 9600 baud, each "chunk" consisting of a start bit, 8 data bits, and a stop bit. That is a total of 10 bauds, two of them, the start bit and the stop bit, are part of telling the circuit where the data starts and stops. So 9600 baud sends 960, 8 bit bytes per second over the line or only 7,680 bits per second. With me so far?
Okay, so the reason baud rates were used is because digital modes were modulated using a technique calls "frequency shift keying" or FSK. Frequency shift keying would send one tone for a zero bit, and one tone for a one bit. Those tones were detected with a circuit called a tone detector circuit and typically they needed a few tens of cycles of the tone to reliably detect the tone. A higher frequency tone meant you could detect it sooner (shorter time for the detector to latch on to the frequency) and that would give you a higher baud rate. But if you're modulating a higher frequency tone on to an RF carrier, it creates a wider impact on the spectrum and everything else was predicated on 2.5kHz max width voice channels. So allowing a faster baudrate, using FSK modulation, would result in digital modes taking up way more spectrum and thus limit the number of users.
But between then and now, there has been a freakin' Cambrian explosion of modulation techniques because digital signal processing is just math. We have a whole stable of techniques in the barn because of this, And as a result, you can put a lot more bits on a channel without pushing the spectrum bandwidth out.
A lot of people have pointed out to the FCC that making the limit baud rate based was silly if they really wanted it to be a spectrum bandwidth limit. Just make it that, and the experimental folks will compete to see how many bauds they can fit into that space.
I will admit I am biased, I'm one of those folks who got back into Amateur Radio because I was playing around with SDRs and wanted to start trying new modulation techniques. I am not motivated by "QSOs in every state" or every country, I'm motivated by "I just pulled an image off a weather balloon over the Atlantic ocean on 20 meters!" and "I can see my beacon 500 miles away on the KiwiSDR network!" things like that. So this change is really going to open up a lot of space for experimentation for me and I can't wait.
> A baud is a raw bit in a digital data stream, it can either be data or part of the channel protocol.
If we wanna be pedantic, a baud is not a bit, it's a symbol. It may be equivalent in some modulation schemes but in things like quadrature amplitude modulation, 1 baud > 1 bit. It goes up to 32768-QAM at 15 bits per symbol.
You are absolutely correct. My experience, which is by no means exhaustive, is that "symbol rate" vs "baud rate" vs "bit rate" and "protocol encoding" vs "data bits" are topics that go deep but can quickly overwhelm someone thinking about this stuff for the first time (or at least nearly so). So to reassure you I was trying to be more "accessible" than "precise" in my writing here. And yes, I often miss the mark and go too far one way or the other.
That said building large constellation QAM modulator/demodulators is a lot of fun I've discovered, but building real world modems that can deal with fading, reflections, and multipath takes away the fun pretty quickly :-).
Yes. Suppose you're communicating with signal flags. You have a black flag and a white flag, and assign them to represent 0 and 1 respectively. If you can change flags once per second, you have a bit rate of 1 bit per second, and the baud rate is also 1 (1 flag change per second).
Now suppose you have a red flag and a green flag as well, and assign black = 00, white = 01, red = 10, and green = 11.
If it still takes you one second to change flags, the baud rate is still one. However, each symbol change now encodes two bits, so the bit rate has doubled, to two bits per second.
There are lots of ways, one possible way being an indication in the beacon package via APRS or WSPR that a new image is available. "SSTV" is a protocol for sending "Slow Scan TeleVision" images over the air on 10 meters which is 28 MHz which has decent propagation. Generally such transmissions are energy intensive and might blip on then blip off given a solar charged power source. Most Amateur balloons just tell you where they happen to be at the moment via GPS co-ordinates.
> But certainly HF stuff, especially below 30MHz, is like any old wire will do
I've got a couple mostly horizontal wires running along the length of my living room--the speaker wires from my A/V receiver to my rear surround speakers.
I've wondered if that could be used as an HF antenna while simultaneously still being used for the speakers?
The audio signals for the speakers are all below 20 kHz. The radio signals I'd want to receive (and maybe transmit) are all about 1 MHz.
The idea then would be to put low pass filters on the audio connections and high pass filters on the radio connections. The cut off for the filters could be somewhere in the middle between 20 kHz and 1 MHz, so that both the audio signals and radio signals are far enough way from the cut off that the filter is flat.
Could that actually work or is it likely to fry the A/V receiver and/or the speakers and/or the radio?
Heh, the answer is sort of. Your receiver likely already has a filter network to "ground out" any coupled electro magnetic interference (EMI) coming from those wires. All conductors "receive" to a greater or lesser degree and so people building things that connect to wires have to account for that and plan for it. So using your speaker wires directly would probably not work very well because of filter networks in the receiver.
For folks just playing around receiving HF I find the YouLoop[1] is a low cost and very effective magnetic loop antenna that you can set up on a wall in a dorm room (or living room :-) or pretty much anywhere. The Airspy HF+ discovery[2] is probably the best valued HF SDR you could use with that, but it works well with RTL-SDR V4 dongles[3] as well and the RT832 ones from NoElec[4] although NoElec and the V3 or earlier dongles don't go below about 55 MHz and need an upconverter like the HamItUp! [5] to move the bottom part of the HF spectrum up into a region the RTL832T receiver can tune them. (the V4 RTL-SDR dongle has an internal up converter for this purpose).
You still have to point it at things. I had the worst possible plot for my HF antenna which meant the lobes were in the wrong place for about 5 years :(
Be careful not to confuse antenna design with "needing to point". :-) There are antennas with pretty circular gain in azimuth but asymmetrical gain in altitude. I have a 08-ANT-0861 from MP Antenna that pretty much looks every which way. And I can recommend it for people who need "one" antenna for a bunch of different experiments.
Quick look of the datasheet suggests that's specified for three bands around UHF, VHF base stations. I mean I'm sure it'll sort of work outside those bands on receive to some varying degrees of success but on transmit, it's going to be a terrible terrible mismatch.
Would love to see an S11 sweep on a proper VNA for that antenna. I bet it looks like a polynomial with a googol order. On some bands the feed might be a better antenna.
My antenna comment was more about the launch angle. It was in inverted V which was steep aka a "cloud burner".
Fair enough, I've got an Agilent 8753A (3GHz version) that I can do an S11 sweep on. It that counts as a "proper" VNA :-) Drop me an email (contact is in my profile) if you would like me to send you a copy of it. (note it won't be right away as the bench is covered some different equipment at the moment)
That is about as proper as VNAs get :). Would be interested in that but will not share my email here as this is my trash account. If you can attach the sweep on imgur I can look at it there.
In the 80's, the FCC put limits on digital signals below 30MHz based on "baud rate". A baud is a raw bit in a digital data stream, it can either be data or part of the channel protocol. For example, a typical serial port like you have on a PC or an Arduino or something might operate at 9600 baud, each "chunk" consisting of a start bit, 8 data bits, and a stop bit. That is a total of 10 bauds, two of them, the start bit and the stop bit, are part of telling the circuit where the data starts and stops. So 9600 baud sends 960, 8 bit bytes per second over the line or only 7,680 bits per second. With me so far?
Okay, so the reason baud rates were used is because digital modes were modulated using a technique calls "frequency shift keying" or FSK. Frequency shift keying would send one tone for a zero bit, and one tone for a one bit. Those tones were detected with a circuit called a tone detector circuit and typically they needed a few tens of cycles of the tone to reliably detect the tone. A higher frequency tone meant you could detect it sooner (shorter time for the detector to latch on to the frequency) and that would give you a higher baud rate. But if you're modulating a higher frequency tone on to an RF carrier, it creates a wider impact on the spectrum and everything else was predicated on 2.5kHz max width voice channels. So allowing a faster baudrate, using FSK modulation, would result in digital modes taking up way more spectrum and thus limit the number of users.
But between then and now, there has been a freakin' Cambrian explosion of modulation techniques because digital signal processing is just math. We have a whole stable of techniques in the barn because of this, And as a result, you can put a lot more bits on a channel without pushing the spectrum bandwidth out.
A lot of people have pointed out to the FCC that making the limit baud rate based was silly if they really wanted it to be a spectrum bandwidth limit. Just make it that, and the experimental folks will compete to see how many bauds they can fit into that space.
I will admit I am biased, I'm one of those folks who got back into Amateur Radio because I was playing around with SDRs and wanted to start trying new modulation techniques. I am not motivated by "QSOs in every state" or every country, I'm motivated by "I just pulled an image off a weather balloon over the Atlantic ocean on 20 meters!" and "I can see my beacon 500 miles away on the KiwiSDR network!" things like that. So this change is really going to open up a lot of space for experimentation for me and I can't wait.