Episode: 4526
Title: Baofeng and SDR++
Source: https://hub.hackerpublicradio.org/ccdn.php?filename=/eps/hpr4526/hpr4526.mp3
Transcribed: 2026-07-31 16:13:18 (official HPR transcript)

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This is Hacker Public Radio Episode 4526, for 2025-12-08
Today's show is entitled, "Baofeng and SDR++"
The host is Lee and the duration is 00:09:33
The flag is Clean, and the license is CC-BY-SA
The summary is "Lee tries transmitting and recording on VHF and UHF"
Hello, I'm Lee.
Sturmgren's talk about dipping my turn in the water of Imterradio.
I took the UK Imterradio Foundation license exam in March 2025, for a while I've been
experimenting with software to find Rageau and had been trying to pick up signals such
as commercial FM and DAB Rageau with my SDR dongle on my PC running event too.
But up until now I've not even tried transmitting.
Back in 2024, I'd obtained two handsets that look like walkie talkies.
After Mr. X showed me this type of unit, that old camp, the making model of the ones
I had, is a bow-fing UV-5R, six mobs after taking the exam.
I finally got around to finding out my cool sign, and downloading my license from off-com,
who are the regulators.
I record from my study for that exam, that according to the license terms, is mandatory
to announce your cool sign in the station, which is basically meaning where you're transmitting
from, when commencing a session that includes transmissions on amateur bands, also why
you can transmit live signals and conversing real time.
We've even then coded a bit not encrypted non-voice signals, actual broadcastings such as playing
pieces of music, but commercial radio station is not permitted.
Furthermore, the power of the transmissions should not exceed 10 watts, and a handset
like the ones I have, indeed, does full below this level, even at the highest transmit
power setting.
For this initial experiment transmitting, I wanted to speak a voice message from my handset,
and pick it up on my PC with the real-tech chipset SDR.com, and even record it.
To decide how to transmit and receive my signal, first I need to consider its parameters.
First the distance.
This would be very short, indeed, within another room in the same building, a power much
less than the allowed ten watts would be more than adequate, than the frequency range.
To transmit spoken voice signals clearly, frequencies up to 8 kilohertz need to be accurately
conveyed.
The Vows are low frequency, say, less than 1,000 hertz, consonants in general range up
to around 2,000 hertz, those specific consonants could civilen some more like 4,000 or
more.
So this set my approximate bandwidth I would need to be of the order of magnitude of
10 kilohertz.
Then I wanted to choose what channel and modulation scheme to use.
Frequency modulation seemed most apt, as it's a fairly simple mechanism that offers reasonable
protection against distortion.
Then the two meter wavelength band, with frequency 145.5Mhz, is channel V40, which can
be used for FM.
Alternatively, in the 70 cm band, with frequency 4, 3, 3.5Mhz, those channel U280, which
can also be used for FM.
Both these channels have made bands for UKM to radio, license holder, transmissions.
Channel V40 is apparently what used, so I decided to try U280 at first.
This my transmission was for test purposes, rather than deliberately contact other amateur
radio uses.
While using FM, there is narrow band or wide band FM, wide band as you would expect
uses more bandwidth, and this hence use commercially for music, which tends to need frequencies
up to 44 kilohertz.
For my purpose, it's narrow band was fine, and it's more conservative in its use of
the radio spectrum, hence impacting any other uses less.
What I was aiming for was to communicate in one direction from transmitted to receiver.
This mydi is nine as simplex, with two-way comms, nine as duplex.
I started my experiment setting up STR++, which is some software capable of reading data
from the USB-Dongol, and extracting specific radio signals, then plotting as both a real-time
horizontal spectrum and a vertically scrolling calicoid frequency trace.
It also demodulates the chosen signal, so the actual audio of it can be listened to or
recorded for further processing.
I plan to set the frequency modulation scheme and bandwidth for other receiver, the 433.5
narrow band FM, and a standard 12.5 kilohertz respectively, then on the handset set this
same frequency and try transmitting and see what happened.
So as soon as I told software to start listening, I got a load of white noise coming in.
So options to adjust the gain as well as scale and offset white plot, so the background noise
would be near the bottom, and then he signal near the top of the fluctuating graph.
It seemed to be very little else I was seeing using this part of the spectrum.
As soon as I pressed the push to talk button, my handset, I saw the noise disappear, and
the signal centered on the carrier frequency of 433.5 mHz.
Actually, after zooming in later, I realized it was slightly off-center, but the software
allows the centering to be slightly adjusted for a closer match.
The spike I saw was clearly the carrier wave.
I knew the signal itself would manifest the symmetrical bumps to the left and right of the spike.
What surprised me though, was there were slightly diminishing amplitude, go signals appearing
at regular frequency intervals, falling off an either side of my actual signal.
After a little research I summarized this was to do with eye-strike U imbalance, an artifact
of the signal demodulation process.
Those are setting in the software that's meant to counter this.
For the gain, which is how much the signal will be amplified and diminished to fit the
desired output range, there were options to handle this automatically via the real tech
chip set of either software, or alternatively it could be set manually, so from zero
decibels, meaning a 1 to 1 input to output correspondants up to 48 decibels, that corresponds
to the output being about 150 times the input.
The result of my experiment was I got a very quiet reception of why I spoke into the transmitter.
To the extent I had not noticed it was there until I happened to share the signal with
a naïe, tell me troubleshoot, or surprised when it was able to tell me exactly what I'd
said in the transmission.
After this I realized I could use the compression filter, or amplification filter and
audacity, and did actually see the spoken message was there.
The troubleshooting I went through almost every relevant setting on the transmitter, as well
as receiving software.
I actually even switched band down from UHF to VHF, and was using the 145.5MHz channel called V40,
thinking this would make a material difference.
Also I tried to adjust in the bandwidth for the receiving end, thinking the signal may
be being restricted by sampling to a narrow band.
Offerating the handset menu, meant pressing menu, then up or down or key numerically,
the menu option desired, then pressing menu to edit, up or down to choose the setting
value, then menu again to save, and exit to go back to the frequency display.
There was also an orange key to the top left, this switch between choosing a frequency
numerically, or letting the up and down keys page free set standard channels, the punctuate
the whole spectrum.
One of the things I tried was checking on my handset, if the squelch was set up, which
silence is noise when nothing is being said, or whether there was a transmit tone out
of audible range being said, called CTCS or DTS.
These features are used to signal to a receiver, something is on air, even if that's
something is silence.
This allows automatic activation of squelch, or can help a repeat and note the signal
is there, and the SFTD is another setting that determines the offset frequency to allow
it to be relayed slightly further up or down on the spectrum, while repeat achieving
greater range.
But these settings were all turned off, there was a setting for busy channel lock-out
to stop accidentally transmitting over the top of someone else.
But this was off too, there were a load of other settings in the menu, the related
tenset operation rather than characteristics of the radio signal send.
But I eventually figured out, it's my problem was this, before transmitting there was
a lot of loud white noise, just random radio noise in the air, for picking up my voice
on the recording, I had the recording sensitivity set low, otherwise this white noise
was incredibly loud, but as soon as I transmitted the white noise was gone, replaced
by my signal, however, because the recording level was very low, my voice came through
very, very quietly, while I had to do was counting intuitively, so the recording level
up to their maximum, even though this meant very loud white noise prior to an after transmission,
only then when I did actually transmit was my voice order one recording, I had to cut
out the very loud noisy bit before an after transmission in audacity, before then amplifying
the recording to something I could hear easily, anyway, here's the recording I made of my signal.
So that was my first experience transmitting on nanometer bands, and now now a lot better
how to operate the handset, as well as making the fairly theoretical stuff I learned for the
exam more tangible to me. Anyway, with that I'll sign off from hack public radio, and thank you
for listening, over and out.
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