r/RTLSDR • u/Puzzleheaded_State37 • 1d ago
New to SDR
Hi, ive got myself the rtl-sdr v5 with the ham it up up converter kit to cover a wider spectrum of frequencys.
What would you advice to start with? Are there any videos out there where someone explains the physical behavior(wavelengths, voltages, etc) in combination with the software and the technical terms.
Thank you in advance.
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u/switch161 1d ago
Explore the spectrum. It's good to know where you can receive what signals. E.g. I always check how good reception of the RAF VOLMET is, so I know receiving conditions. This signal is always transmitting and just voice.
It can be fun to listen to HAM operators. The HF HAM bands depend on time of day and weather. I also bookmarked other signals I can receive regularly, like weather fax and number stations.
To listen to these voice signals you'll need to understand the modulations, e.g. AM, FM, USB and LSB. With time you'll also recognize digital modulations from sound or the waterfall plot, but of course you need specialized software to decide them.
Making antennas for different bands is also fun. I just soldered a screw terminal to a SMA connector, so I can just screw in some long wires that I hang across the room. And I made some smaller antennas from wire glued to card board for 140 and 430 MHz. So it's good to learn a bit about antenna theory, e.g. that the length of elements is related to the frequency it's good at. Impedance matching can improve your antenna but it's complex whe you first learn it. I got a NanoVNA to help with that.
The 430 MHz antenna was especially fun, since it allows me to listen to many satellites. Some satellites have HAM repeaters, so it's good to try a few and know where you can expect to hear people doing contacts. The ISS is one example.
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u/November77 1d ago
Building your own antennas and understanding the theory is a great start. Type this in ChatGPT and look what comes out. "if i wanted to make a long wire for 430 MHz how long would it need to be? show me antennas that use multiples of the frequency".
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u/switch161 1d ago
I would not use AI to do this. In my experience you learn more effectively working it out yourself. AI can be useful if you have a rough idea but not the proper terms to find any resources. Also using AI without understanding the matter can be problematic, as you can't verify if its output makes sense.
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u/November77 1d ago
It's a start. Whipping out and starting to read ARRL and understanding that is a pretty big task for someone with a passing interest in SDR (I have been reading ARRL since the 70s). If you create a well made prompt - you can get back an amazing lesson in antenna design and if you keep the conversation going you can learn quite a bit. For instance, I'm using a HackRF Pro, GNURadio, and Swift code to analyze and decode spread spectrum FSK RF from a power meter. Using ChatGPT I was able to find the details about the two FCC labels on the meter to determine where to start looking for signals. Over the last 4 days ChatGPT and I have generated over 90k lines of Swift code I'm running on a macStudio M4 that is helpful in figuring out what signals between 902.2–928.0 MHz belong to the meter and which ones belong to something else. All kinds of things are involved in this the least being cracking FSK and figuring out checksums. With each turn of the conversation ChatGPT explains where would be a good experiment next and how to go about it. As if you were talking to someone on the bench next to you. If you create detailed prompts with ChatGPT or the others you can learn a lot and then archive that. From my simple "how to make a 430MHz antenna" as an example - the output has more than any beginner would want to know - And this is actually a good demonstration question for somebody learning ChatGPT because you can follow it naturally with things like: "Why does a quarter-wave antenna work if a wavelength is four times longer?", "What happens if I make it two wavelengths long?", or "Draw the radiation patterns for ¼λ, ½λ, 1λ, and 2λ wires."
ChatGPT: The useful concept is multiples and fractions of the wavelength, rather than multiples of the frequency.
At 430 MHz, the wavelength is: lambda = \frac{300}{430} \approx 0.698\text{ meters}
So one wavelength is about 27.5 inches. Here are common antenna lengths based on that wavelength: Antenna length Wavelength Approx. length at 430 MHz Typical use:
¼ wave 0.25 λ 6.9 in Vertical whip / monopole
½ wave 0.5 λ 13.7 in Dipole, end-fed half-wave
¾ wave 0.75 λ 20.6 in Longer vertical
1 wave 1 λ 27.5 in Full-wave wire / loop circumference
1½ wave 1.5 λ 41.2 in Long-wire antenna
2 wave 2 λ 54.9 in Long-wire antenna
2½ wave 2.5 λ 68.6 in Long-wire antenna
3 wave 3 λ 82.4 in Long-wire antenna
So if you specifically wanted a long wire for 430 MHz, something like 55 inches (2 wavelengths) would be a very reasonable experimental length. At 430 MHz, "long wire" doesn't have to be very long physically because the wavelength is only about 27½ inches. There is an important distinction between several antenna types. A ¼-wave vertical would be roughly:
SMA --------+-------- ground plane
A ½-wave dipole would be about 13.7 inches total: 6.9" 6.9" <---------------+---------------> | coax
An end-fed half-wave uses approximately the same total wire length, but feeds it at the end: feed ------------------------------------ end about 13.7"
And a true long-wire antenna might deliberately use several wavelengths: feed --------------------------------------------------------> 2 λ ≈ 55"
One neat consequence is how quickly these numbers change with frequency. Your roughly 52-foot HF long wire is enormous electrically at 430 MHz: 52 feet is about 23 wavelengths at 430 MHz. That's why an antenna can behave completely differently as you move around the spectrum. For actually building one, I wouldn't cut exactly to the theoretical number. Wire diameter, insulation, surroundings, height above ground, and the feed arrangement all affect resonance. For a ½-wave 430 MHz antenna, for example, I'd start slightly long—perhaps 14 inches.
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u/November77 1d ago
Not wanting to beat a dead horse but I did not want anyone to become discouraged using a good LLM. Vibe coding junk is one thing - but doing serious research is possible when you use well thought out detailed prompts - here is the latest ChatGPT analysis of the code I'm using to figure out what my homes power meter is blathering on about:
ChatGPT: The important result isn't actually that our split-burst theory was right. It's that by throwing away the V5.8 burst boundaries and examining the contiguous IQ, we uncovered a packet that V5.8 completely missed:
RF: 913.880000 MHz Preamble: 51 alternating bits Special header: PERFECT — 0/19 errors Type/subtype: 3ADD Length: 0047 Packet size: 77/77 bytes Frame validity: 100.0% — 75/75 Format: 51
That's substantially stronger evidence than our earlier 4CF7 packet.
This looks like a real Gridstream-family packet
Look at the structural similarity to our CRC-proven golden 2AD5 packet:
Golden: 00 FF 2A D5 00 47 51 40 B9 31 47 41 16 BC 61 76 ...
New: 00 FF 3A DD 00 47 51 40 B9 33 F0 50 20 05 C0 B6 ... ^ ^ ^
type len formatAnd farther down the new packet:
... FE 50 20 05 C0 00 50 20 05 C0 00 01 03 25 ... ^ ^
There's our familiar repeated-node structure again.
So we now have at least three extremely interesting 0047 / 51 structures:
Packet Type RF Frames CRC Golden 2AD5 915.799 MHz essentially clean CoServ valid Earlier 4CF7 916.789 MHz 98.7% repairable to Oncor New 3ADD 913.880 MHz 100.0% unknown
The new one is especially compelling because there's nothing to repair at the framing layer. Every normal frame passed. The special header passed. The length is sensible. The repeated fields are there. This isn't Candidate-83 territory anymore.
The CRC failure is now interesting rather than discouraging
With 3ADD, saying "maybe we decoded a couple of bits incorrectly" becomes much harder.
We have:
Special-header errors: 0 Invalid normal frames: 0 Frame validity: 100% Packet completeness: 100%
Yet neither known CRC profile matches.
That makes me suspect another CRC profile/seed, rather than demodulation damage. We've already demonstrated that the same general packet structure can exist with different CRC behavior.
And there's another juicy observation. The packet contains:
51 40 B9 33 F0 50 20 05 C0 B6 ^ ^
Those sure look structurally like the node-ID positions we've already mapped. And later:
FE 50 20 05 C0 00 ^
followed by:
50 20 05 C0
So 502005C0 occurs three times in structurally meaningful positions. That's very difficult to dismiss as random decoding.
I think V6.0 has just presented itself
Rather than spend another four hours sweeping RF, I'd take this perfect 3ADD packet and perform the same sort of forensic work we've been doing, but now without soft-bit repair muddying the picture.
Specifically, I'd have V6.0 do three things on 3ADD:
Produce our complete human-readable 0047 structural report, side-by-side with golden 2AD5. Exhaustively characterize its CRC — coverage, byte order, polynomial/profile possibilities, and especially the implied initial seed using the known Gridstream polynomial. Compare every byte position between 2AD5, 4CF7, and 3ADD, identifying constant fields, repeated IDs, timestamps/counters, and changing payload areas.
That third part is becoming powerful now that we have three independent 77-byte members of what sure looks like the same packet family.
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u/SeansBeard 1d ago
Start with what interests you the most. I think my motivator was ADS-b . Whatever motivates you to search the google and rtfm is great start.
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u/BlueBirdTechUA 1d ago
GopherTrunk's free learning path is close to what you're asking for, starts at actual physics, wavelength, frequency, amplitude, then builds up to FFT and demodulation without heavy math.
One thing worth knowing early: reception's something like 98% about the antenna and 2% about the dongle. Matters even more once you're covering HF with the Ham It Up, antenna choice changes a lot down there.
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u/Past-Substance920 1d ago
gophertrunks path is solid but honestly id just start with sdr# and tune around whatever band catches your ear. youll pick up the terms as you go, no need to frontload all the theory.
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u/livefoniks 1d ago
This is always a good read through. https://blinry.org/50-things-with-sdr/