Over on his YouTube channel, Aaron, creator of DragonOS and WarDragon has uploaded a video showing how it is possible to decode Meshtastic with an RTL-SDR and GNU Radio project called Meshtastic_SDR.
If you weren't aware of it, Meshtastic is software that enables off-grid mesh network based communications and can run on cheap LoRa hardware. The mesh based nature of the system means that communications can be received over long distances, without any infrastructure, as long as there are sufficient Meshtastic nodes in an area that are able to route the message to the destination node. One example application of Meshtastic is to use it as a mesh-based text messaging system. This might be useful for teams of hikers, pilots, or skiiers who operate in remote areas without cell phone coverage.
In the video, Aaron shows how to install the Meshtastic GNU Radio software on DragonOS (Linux), and how to run the GNU Radio flowgraph and Python decoder script. Later in the video Aaron shows some test text messages being received by the software.
The Meshtastic_SDR project can also be used to transmit Meshtastic messages with an appropriate TX-capable SDR.
Welcome back to Sarah from the SignalsEverywhere YouTube channel who has recently returned to producing videos from a hiatus. In her latest video, Sarah shows off her new OP25 Mobile Control Head Android App which allows you to implement a full P25 digital radio scanner at a fraction of the cost of a commercial digital scanner. In the past, Sarah had released a similar application written for the Raspberry Pi but has decided to shift her focus to writing an equivalent Android app that is less clunky and can be deployed for a lower cost.
The app controls and displays information from the OP25 software that runs on a Raspberry Pi with RTL-SDR connected. It works by using a server application on the Raspberry Pi that manipulates the OP25 instance and its configuration files.
Sarah writes:
The application is a wrapper for OP25 that uses a raspberry pi and an android device to provide users with a mobile control head for their OP25 P25 scanner setup. Currently it's just a basic application but I'll be adding features like automatic site switching, etc.
OP25MCH: https://github.com/SarahRoseLives/OP25MCH
There is also a separate application I call the OP25Display which is just a display for a users existing OP25 instance.
OP25Display: https://github.com/SarahRoseLives/op25display
On this blog I have posted three previous entries related to the RX-888 (Mk2) which may be of interest to the reader:
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| Figure 1 - The (stock) RX-888 |
The "Thermal Dynamics" page referred to reliability issues experienced - possibly heat-related, but this page discusses vulnerabilities and repairs that may be needed if - when using an external clock - the device has stopped functioning.
Comment: There are many reasons why an RX-888 may not produce signals. One of the better, easier tools to diagnose/test an RX-888 is to use the SDDC "ExtIO" driver along with a problem like "HDSDR" on a Windows machine: A fairly fast computer (at least a quad-core Intel i7 at 3 GHz or better) is recommended and a USB3 port is required.
How did I know that the problem appeared to be due to no clocking of data from the A/D converter? On my Windows 10 machine I could see that the USB PHY enumerated properly, but the results of the waterfall/spectrum plot from HDSDR - and the fact that there was no difference in the (lack of) signal regardless of the frequency or sample rate - caused me to suspect such.
What finally clinched it was partially disassembling the '888 and probing it with an oscilloscope and finding the clocking to be absent from the A/D converter - and subsequent removal and probing under the shield covering the clock section.
Using the external clock:
Note: If your RX-888 failed after you have used an external clock, the damage described on this page may have happened to your device. If you have disabled the onboard 27 MHz clock (e.g. removed the jumper) you may wish to (temporarily) restore its operation for the purposes of diagnosing the problem and subsequent testing - and doing so is strongly recommended as it allow one to rule out other issues - particularly those that may be related to external clocking.
While the internal 27 MHz oscillator seems to be quite stable, there are instances where you might want to reference it to an external and more stable source such as that derived from a GPS or an atomic standard. Most commonly, this is done using one of the Leo Bodnar GPS-stabilized references allowing sub-milliHertz accuracy and stability across the HF spectrum.
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| Figure 2: Board |
Just above this is a jumper (green, in this case) which, when removed, disables the on-board clock so that the externally-applied oscillator does not conflict.
Reverse-engineering the clock circuit:
As
schematics for the RX-888 (Mk2) are not publicly available, exactly how
it worked was unknown and thus the type of external signal to be used
was unknown, found with trial and error. In the process of this repair I
had to figure out how the circuit worked, so here is a brief outline:
What seems to go wrong:
In the description you may note that the external clock input goes directly to the output of the crystal oscillator and also to the clock input of the Si5351 with no
blocking capacitor: There's the BAT99 dual diode that ostensibly
offers protection - but this is probably not the appropriate protection
device as we'll see: The BAT99 in conjunction with an appropriately-specified TVS diode (e.g. 4-5 volts) would have been better.
 |
| Figure 3: The clock section - under the shield. |
An RX-888 (Mk2) crossed my workbench that seemed "dead" - but critically, it would enumerate on the USB and would load the firmware, indicating that one of the apparent issues - that of the FX3 interface chip - appeared to be working OK. A quick check with the oscilloscope on the clock pins of the A/D converter showed that it was completely absent even with the internal clock enabled (jumper pins shorted). This indicated that the clock generator had failed in some way.
On
the RX-888 (Mk2) all of the clock generation circuitry - the 27 MHz
TCXO, the Si5351 synthesizer, the LVDS driver and a "local" 3.3 volt
regulator for the aforementioned devices - is located under the metal
shield. This was removed carefully using a hot-air rework tool and some large-ish tweezers to expose (and not disturb) the components underneath.
Figure 3 shows what's under the shield:
With the shield removed, I could see that the 27 MHz clock (which was enabled by bridging the jumper) was making it to the input pin of the Si5351 synthesizer, but nowhere else. I could also probe the data and clock lines used for programming the Si5351 and when the firmware was loaded, I could see a brief string of pulses on each line indicating that the FX3 was attempting to program it.
At the time I had some "wrong" Si5351s available: I'd previously ordered a pre-programmed version (fixed frequency, non reprogrammable) by accident so I dropped one of those on the board (hot-air rework soldering) and was greeted with output signals (at the wrong frequency) but I observed that they stopped at the LVDS driver chip indicating that it, too, was dead: A signal was on its input, but only one output had anything at all and its output was only a few 10s of millivolts - possibly due to leakage from the input rather than the device actually doing anything.
Placing an order with DigiKey, I soon had in hand some proper Si5351s and a handful of the SN65LVDS1DBVR driver chips and dropped them on the board as well, restoring operation of this RX-888 (Mk2).
A few notes on chip replacement:
A modest hot-air rework station was used in the repair of this '888.
For removal of the defective parts, the board was set on a heatproof, stable service: My station has a set of aluminum bars with ridges to allow a board to be secured and sit flat. Using a pair of curved, ceramic-tipped (which have lower heat conductivity than metal) tweezers, just enough heat was applied to remove the defective device(s) once they had been appropriated warmed by targeted air from rework station's hot-air wand.
The defective devices remove, a very thin layer of solder past was added to the pads after removing the defective chip(s) and the new device was placed in position, being sure that the pin orientation was correct. Applying heat - but not enough air flow to cause the part to be blown out of position - the device will center itself once the solder melts and surface tension takes over.
Closely examining the part for solder bridges (magnification is helpful for this) and if there are some, apply a small amount of liquid solder flux (a low-residue "flux pen" is good for this) apply some heat from a clean, tinned iron through a small piece of "solder wick" whetted with flux should remove them.
What likely happened:
The key to the mode of failure is noting what had failed and how the components were related. As mentioned earlier, there is a "protection" diode (BAT99) connected between ground and the local 3.3 volt supply - but while this will prevent negative-going excursions, it is less effective in positive-going swings that exceed 3.3 volts as it dumps that energy into the local 3.3 volt supply. As the clock, the Si5351 and the LVDS driver are all on that same supply, it appears that much more than 3.3 volts appeared there, blowing up the '5351 and LVDS driver - and it is only by serendipity that the 27 MHz clock survived - likely due to its ability to handle much higher voltages by design (e.g. it may be 5 volt tolerant, built using a much larger fabrication process, etc.)
Obviously, the local 3.3 volt regulator survived as well - but one should remember that it, too, can take rather higher voltages on its input. Also note that typical regulators like this will only source current - they have no circuitry within to sink or clamp higher-than-expected voltages on their output so when the "high" voltage was applied to the clock input the BAT99 diode - and the protection diodes on the oscillator and Si5351 - shunted it to the 3.3 volt supply which is how the LVDS driver - which has NO direct connection to the external clock input - got destroyed as well from high supply voltage.
What probably happened to damage the '888 likely occurred when the external clock was being connected/disconnected. Typically, an SMA connector is used - mounted on one of the end panels - to feed the external clock into the unit but a problem with this type of connector (and others like the type "F" and "UHF") can make a connection with the center pin BEFORE the ground/shield is firmly connected.
What this means is that if there is a "ground" differential between pieces of equipment of several volts, this voltage can be dumped into the high-impedance and poorly protected input of the RX-888 (Mk2) as the connector is mated and tightened.
This voltage differential between pieces of equipment is actually quite common. Let us consider a possible scenario in which we have the following:
In the above we have four different "grounds" connected between the pieces of equipment:
The problem here is that what is called a "ground" colloquially does not mean that they are at exactly at the same potential: It is very common for a "ground" on an RF coaxial cable grounded some distance away nearer the antenna has a slightly different voltage on it than the wiring "ground" in a building: Ground has finite resistance and currents are always flowing around through the earth - and this is especially true during lightning storms where two "grounds" could be hundreds of volts apart for a brief instant if there is a nearby lightning strike.
The
other problem is that many computers may not be "grounded" in the way
that you think - particularly laptops small desktops powered by a remote
supply (e.g. a "wall wart"). Sometimes, these power supplies do
not have a DC connection between the "ground" pin of the mains supply
and the DC output meaning that they are "floating": Often - usually due
to EMI filtering of the switch-mode supply - this causes the DC output
to float at some (usually AC) voltage that may be many tens of volts away from the ground - a phenomenon usually caused by the (needed!) capacitors in the filter circuit. As these capacitors are often coupled in some way to the mains, they will conduct a small
amount of current - but if it's shorted to ground at the instant that
the mains voltage waveform is at a peak, the energy of the capacitor may
instantaneously be dump through that connection resulting in a very
brief - but surprisingly high - current spike, even if the capacitance
is quite low.
While the amount of current of the "floating" supply between its output and the "ground" (third prong on the outlet) is likely to be quite small, it can easily be enough to induce small currents through interconnecting cable. What's worse is that if you have two pieces of equipment - one being firmly grounded through its antenna such as the RX-888 and the source of the external clock which may be powered from a source that is "floating" as well - that when the connection is made between the output of the external clock and the signal source is made that there will be an elevated voltage: As it's common for the center pin of the cable to make contact first, this voltage - and the capacitors in whatever EMI filtering may be present on the "ground" of the device powering/connected to the external clock - will dump into the clock input of the RX-888 - and from there, into the other circuitry of the '888's clock circuit.
How to drive the '888 to prevent this from happening again?
As it happens, I have already produced a blog entry on this very subject, so I'll leave it to the reader to peruse that article, found here:
Thank you to RTL-SDR.COM reader Lee. who found a recently released program called "gypsum" which enables an RTL-SDR or HackRF to be used as a GPS Receiver when combined with a GPS antenna. Phillip Tennen, the author of Gypsum notes that Gypsum can obtain a fix within 60 seconds from a cold start and that it has no dependencies apart from numpy. We want to note that it appears that Gpysum has no live decoding ability yet, as it works from pre-recorded GNU Radio IQ files.
In the past, we've shown in a tutorial how GPS can be received and decoded with GNSS-SDRLIB and RTKLIB on Windows. The new Gypsum software should work on Linux and MacOS too.
What's more, Phillip has written an incredible 4-part writeup on how Gypsum was implemented from scratch. In the write-up, Phillip introduces GPS and explains how it can even work with such weak signals that appear below the thermal noise floor. He then goes on to explain how the detected signal is decoded and turned into positional information, and how challenging it was to propagate the accurate timing information that calculating a solution requires. The write-up is presented with clear visualizations to help readers intuitively gain an understanding of the advanced concepts involved.
Thank you to Adrian Musceac (author of QRadioLink) for submitting his article about how he implemented an amateur radio DMR Tier III Trunked Radio Base Station with a LimeNet-Micro software-defined radio. DMR Tier III is a digital voice trunked radio system that employs Time Division Multiple Access (TDMA) technology. Tier III is largely based on Tier II, but adds trunking abilities which enable efficient channel access and resource allocation.
The LimeNET Micro is a software defined radio based on the LimeSDR, but it has some upgraded specifications such as an embedded Raspberry Pi Compute Module 3+ that make it easier to deploy as a base station.
Adrian writes:
The Tier III extension (trunked radio) to the DMR standard is defined and specified by the European Telecommunications Standards Insititute (ETSI) in the TS 102 361-4 document.
The project uses LimeNet-Micro, LimeSDR-mini or Ettus USRP hardware to set up such a base station for experimental and amateur radio digital voice communications purposes. The core components of this project are MMDVM, MMDVMHost (both under the form of forks supporting communication via ZeroMQ and pseudo-TTY), GNU Radio, DMRGateway, QRadioLink and the DMR trunked radio controller GUI.
Since DMR trunked radio is not very well known and used in the amateur radio world, I hope this will bring some new information to amateurs interested in these digital voice communication technologies. All code used is available as free and open source software (FOSS). A demo of the project used with real world amateur radio communications can be found on the page.
After initially finding that I couldn’t tune the 868Mhz ground plane antenna with the radials bent down at 45 degrees I decided to experiment to find out why.
Initially I had the radials connected to the 4 corners of the base of the chassis mount N Type socket. This works great if you have the radials completely horizontal and gives an SWR of 1.1:1 but, with the radials bent down at 45 degrees the best SWR is around 2:1.
Removing the radials from the base of the N Type chassis socket and soldering them to the outer of the N Type plug at the same level as the feed point for the radiating element I found that an almost perfect SWR can be achieved very easily.
It seemed weird to me that such a small change could have such a big effect on the obtainable SWR for the antenna but, as can be seen in the image below with the radials soldered to the N Type plug and bent downwards I immediately got an SWR of 1.07:1 and a much wider SWR curve.
By making my own antennas I’m learning a lot about antenna design for the 800-900Mhz frequency range. Minor changes seem to have a much bigger impact than they do at much lower frequencies.
More soon …
In my quest to improve my Meshtastic signal range using home-brew antennas I’ve finally put together a neat little ground plane vertical antenna for the 868Mhz ISM band.
The design follows the normal ground plane simplicity using 4 radials and a vertical radiating element albeit on a tiny scale. The radiating element is 82mm long and the radials are each 92mm long.
Initially I modelled the antenna at a height of 3m above the ground with the radials tilted downwards at 45 degrees. I took this approach as this is how I have built ground plane verticals for the 70cm band in the past and so I thought I’d try the same approach on the 868Mhz ISM band. (I later found this to be detrimental to tuning!)
The 3D far field plot for the antenna shows it has a very nice, relatively high gain lobe at just 2 degrees elevation with a number of lower gain lobes higher up.
Looking at the 2D far field plot you can get a better understanding of the radiation pattern and gain figures at various angles. At 2 degrees there is 6.7dBi gain with the next major lobe being at 8 degrees with 4.36dBi gain, far more than I imagined I’d see for such a simple antenna.
Putting the antenna together was easy enough with particular attention being paid to the measurements of both the radials and radiating element. I soldered some lugs to the ends of the 2.5mm diameter solid core wire radials to enable easy attachment to the N Type chassis socket that I decided to use as the base for the antenna. This worked out well and provided a good solid mechanical and electrical connection for the 4 radials.
For the radiating element I used an N Type plug with the vertical 2.5mm solid core wire element soldered to the inner centre pin of the male connector. I also slid a small piece of insulation down the wire to stop it from shorting against the metal outer of the plug and then pushed in a tight rubber plug to stop water ingress.
Connecting my VNA I found the antenna was mostly resonant at 790Mhz with an SWR of 2.5:1. I knew this would be the case and that the wires would need a little trimming.
Trimming the wires a couple of times in 1mm nibbles I got the point of resonance up to 868Mhz but, the antenna was still exhibiting a lot of reactance that was keeping the SWR above 2:1. Trimming the radials reduced this slightly but, I could not get an SWR much lower than 1.95:1.
Scratching my head I decided to try moving the radials back up so that they were horizontal rather than at 45 degrees downwards, this had the immediate effect of the SWR dropping to 1.1:1.
The SWR stays below 1.2:1 from 868Mhz to 871Mhz which is plenty wide enough for the Meshtastic devices. Why there is so much reactance when the radials are bent down at 45 degrees I am not sure, but it was easy enough to resolve.
The finished antenna is tiny but, seems to work well. Signals from my other nodes are up by 6-9dB according to the SNR reports in the Meshtastic app. I now need to make a couple more of these for my other nodes and then hope to hear some other nodes locally once they appear on air.
Remodelling the antenna in EzNEC with the radials as shown above the gain at 2 degrees is now 5.5dBi, down 1.2dBi but, the overall radiation pattern is identical to the original.
Total cost of the build is about £1 and an hour of my time tinkering with it, bargain!
More soon …
Following on from my last article on improving the Heltec ESP32 v3 antennas I found during the installation of the 90 degree SMA connector that the device was very sensitive to stray capacitance from things around it. After reconnecting my VNA I found the SWR curve would change substantially depending on what the device was near and so I set about rectifying this.
I decided to remove all the insulation from the single radial inside the unit and then added two more radials to increase the ground for the antenna to tune against. I then removed the N type plug with the antenna connected to it and made a new antenna from a piece of 1.5mm solid core insulated mains wire connected directly to the N type socket, without using an N type plug. Tuning to resonance was much easier than before and I soon had the SWR down to 1.2:1. Moving the device around and placing near to other objects the SWR curve was now much more stable than before with only very slight changes in curve shape.
Making this change to the 868Mhz antenna has shown an improvement in signal strength from my node-1 device of almost +0.5dB, every dB counts when you only have 100mW to play with!
The Bluetooth antenna update has made a massive improvement to the usability of the device via the iOS Meshtastic app. Being able to have a reliable, solid connection from anywhere in the house is great and I no longer lose messages because I’ve strayed outside the range of the Bluetooth connection.
I now have 2 new Heltec ESP32 v3 devices on the way to me and will be getting those configured and operational outside with external antennas in the hope of hearing some nodes locally to me.
More soon …
The Heltec ESP32 v3 LORA devices have a coil type Bluetooth/Wifi antenna on the PCB from the factory. This antenna doesn’t work particularly well and has very limited range so, I decided to do something about it.
Getting out the calculator a quarter wave at 2400Mhz is 29.7mm. Looking at the coil antenna on the PCB I decided the best way to connect the new antenna would be to solder it to the coil of the existing antenna. This would short out the coil completely whilst creating a solid mount point for the new antenna.
After a little measuring I decided to use a 31mm long piece of 1.5mm hard core mains cable for the new antenna. I stripped back the insulation from one end of the wire so that the exposed copper wire was exactly the length to short across all the windings of the coil antenna on the PCB.
Attaching the the wire to the coil was easy enough to do but, it’s worth pointing out that you need to be quick so that the heat doesn’t transfer down onto the PCB desoldering the coil antenna from the device.
Whilst tinkering with the Bluetooth antenna I decided I would also make a neat little quarter wave 868Mhz vertical antenna for this device whilst I had it all apart. This is my Meshtastic node-2 and it’s sole purpose is to allow me to use my iPad to send/receive messages via bluetooth which are then forwarded on to my base node-1 in the house. Node-1 is connected to the house wifi and the Meshtastic MQTT server. This combination allows me to message people on the mesh even though there are no local nodes within RF range.
Running the numbers for the 868Mhz antenna the vertical will need to be around 82.1mm long with a radial of similar length. I had to hand a very nice SMA to N Type chassis mount socket that would be ideal to mount the antenna to the case. I drilled out the holes in the case, measured out the wires and attached it all to the case. Connecting the antenna to the N Type socket I connected my VNA and set about tuning the antenna to resonance.
Squeezing the radial and SMA connector into the case I realised I really could do with a 90 degree SMA connector so, I quickly ordered one from Amazon which will be delivered tomorrow. Connecting up my VNA, I had to trim the antenna down to get it to resonance. The SWR ended up at 1.2:1 which is ideal. I ended up cutting off more wire than I thought I would to get the antenna to resonance but, this is due to the extra capacitance caused by the insulation on the wire. If I had used bare copper wire then I wouldn’t of had to cut so much off. I eventually ended up with around 72.9mm of wire for both the antenna and radial.
Putting the device back into the case and connecting the USB battery the device fired up and immediately connected to my node in the house. Checking the signal strength of node-1 in the house I could see a 7dB increase in signal strength compared to the little wire antenna that comes with the device. This is a significant improvement for such a simple antenna and well worth the effort.
Next I had to drill a hole in the front of the Heltec case so that the Bluetooth antenna could poke out the front and be bent up vertically. This worked out really well and improved the Bluetooth range massively.
Putting the node back in the house and taking my iPad down to the end of the garden some 30m away I could instantly connect to the device via Bluetooth from my iPad, something I’d not been able to do prior to adding the new antennas. I can now use the Heltec device via Bluetooth from anywhere in the house or garden making it much more accessible.
It’s amazing the difference an hour and two little pieces of wire can make to these devices and is well worth the effort.
More soon …
Since setting up the new HAM station here in the UK the one band I’ve not yet got back onto is 160m, one of my most favourite bands in the HF spectrum and one that I was addicted to when I live in France (F5VKM).
Having such a small garden here in the UK there is no way I can get any type of guyed vertical for 160m erected and so I needed to come up with some sort of compromise antenna for the band.
Only being interested in the FT4/8 and CW sections of the 160m band I calculated that I could get an inverted-L antenna up that would be reasonably close to resonant. It would require some additional inductance to get the electrical length required and some impedance matching to provide a 50 Ohm impedance to the transceiver.
Measuring the garden I found I could get a 28m horizontal section in place and a 10m vertical section using one of my 10m spiderpoles. This would give me a total of 38m of wire that would get me fairly close to the quarter wave length.
For impedance matching I decided to make a Pi-Network ATU. I’ve made these in the past and found them to be excellent at matching a very wide range of impedances to 50 Ohm.
Since I still had the components of the Pi-Network ATU that I built when I lived in France I decided to reuse them as it saved a lot of work. The inductor was made from some copper tubing I had left over after doing all the plumbing in the house in France and so it got repurposed and formed into a very large inductor. The 2 x capacitors I also built many years ago and fortunately I’d kept locked away as they are very expensive to purchase today and a lot of work to make.
Getting the Inverted-L antenna up was easy enough and I soon had it connected to the Pi-Network ATU. I ran a few radials out around the garden to give it something to tune against and wound a 1:1 choke balun at the end of the coax run to stop any common mode currents that may have appeared on the coax braid.
Connecting my JNCRadio VNA I found that the Inverted-L was naturally resonant at 2.53Mhz, not too far off the 1.84Mhz that I needed. Adding a little extra inductance and capacitance via the ATU I soon had the antenna resonant where I wanted it at the bottom of the 160m band.
With the SWR being <1.5:1 across the CW and FT8 section of the band I was ready to get on 160m for the first time in a long.
Since it’s still summer in the UK I wasn’t expecting to find the band in very good shape but, was pleasantly surprised. Switching the radio on before full sunset I was hearing stations all around Europe with ease. In no time at all I was working stations and getting good reports using just 22w of FT8. FT8 is such a good mode for testing new antennas.
As the sky got darker the distance achieved got greater and over time I was able to work into Russia with the longest distance recorded being 2445 Miles, R9LE in Tyumen Asiatic Russia.
In no time at all I’d worked 32 stations taking my total 160m QSOs from 16 to 48. I can’t wait for the long, dark winter nights to see how well this antenna really performs.
The map above shows the locations of the stations worked on the first evening using the 160m Inverted-L antenna. As the year moves on and we slowly progress into winter it will be fun to start chasing the DX again on the 160m band..
UPDATE 6th October 2023.
Been using the antenna for some time now with over 100 contacts on 160m. Best 160m DX so far is RV0AR in Sosnovoborsk Asiatic Russia, 3453 Miles using just 22w. Pretty impressive for such a low antenna on Top Band.
More soon …
I’ve been waiting for over a week so far for a male to male SMA connector to arrive from Amazon so that I can connect the 2.4Ghz up-converter to the 2.4Ghz amplifier. Since it still hasn’t arrived I decided to connect the up-converter directly to the IceCone Helix antenna to see if I could get a signal into the QO-100 satellite.
To my surprise I could easily hear my CW signal on QO-100 even though the total output from the up-converter is only 200mW.
I didn’t expect to be able to hear my signal since it’s a tiny amount of power that has to travel some 22500 miles to the satellite but, I could hear it and was amazed that it was peaking S8 on my SDR receiver.
Being excited I put out a CQ call that was soon answered by OH5LK, Jussi in Finland. Jussi gave me a 579 report which I was extremely pleased with. He was of course much stronger at a 599+ at my end. We had a quick QSO and exchanged details without any problems at all. Its really nice to get a QRPp contact without any QSB or QRM.
Neil, G7UFO who I chat with regularly in the Matrix Amateur Radio Satellites room has posted a connector out to me so I’m hoping it will arrive on Monday and then I’ll be able to connect the amplifier and hopefully get a few SSB contacts.
UPDATE: I’ve since had 2 SSB contacts via QO-100 using just the 200mW O/P from the up-converter. Both times I got a 3/3 report not brilliant but, perfectly acceptable for the amount of power I’m putting out.
More soon …
For some time now I’ve been using my Funcube Dongle Pro+ (FCD) as my QO-100 downlink receiver. It’s worked fairly well and has given me the ability to listen to stations on the satellite over the last few months.
During this time I have noticed a couple of things about the FCD that has lead me to the final decision to change to a new SDR device.
The first of these ‘things’ is the fact that the FCD gets seriously overloaded when there are multiple large SSB signals within the receive pass band. The only way to manage this is to constantly keep changing the software based AGC, mix and LNA settings to reduce the levels of the incoming signals so that the overloading stops. This is great except when you tune to a quiet part of the satellite transponder you have to turn all the settings back up again to be able to hear the weaker signals. After a while this becomes tiresome.
The fact that there isn’t a hardware AGC in the FCD is a major drawback when being used for satellite reception especially when it’s on the end of a very high gain LNB and dish antenna.
The second of these ‘things’ is the fact that I can’t see the whole transponder bandwidth at one time with the FCD as it has a very small receive bandwidth capability. This means that I am constantly tuning up and down the transponder to see if there are any stations further up or down in frequency.
Talking to more experienced satellite operators in the Matrix Amateur Radio Satellites room they recommended replacing the FCD with a NooElec NESDR SMArt v5 that has hardware AGC and is capable of receiving and displaying a much wider bandwidth.
Looking on Amazon the NooElec NESDR SMArt v5 is only £33 so I decided to place an order for one and give it try.
In typical Amazon style the SDR receiver arrived the next day and I wasted no time getting it plugged in and connected to the QO-100 ground station.
The NESDR SMArt v5 is based on the well known RTL-SDR that came onto the market some time back but, has a number of improvements in it that take it to the next level.
The first thing that I was happy with was the fact that the GQRX SDR software I use recognised it immediately on startup, no configuration or drivers were required it just worked, straight out of the box. Since I use Kubuntu Linux on my radio room PC I did wonder if I would need to get into installing extra libraries etc but, thankfully none of that was required.
Looking at the signals from the QO-100 satellite initially they appeared to be nowhere near as strong as they were on with the FCD. Looking at the settings in GQRX I noticed that the hardware AGC was off and the LNA setting was back to it’s default very low level.
I switched on the AGC and then increased the LNA setting to 38.4dB and found that the signals were now plenty strong enough on the display but, not overloading the receiver.
I then went on to adjust the display so that I could see the whole satellite transponder bandwidth on the screen. This is great as it enables me to see the low, middle and high beacons that mark out the narrow band section of the transponder and at a glance see all the stations using the satellite. This was a massive improvement in itself and one that I am very pleased with.
Using the NooElec NESDR SMArt v5 SDR it very soon became clear that it copes with multiple large signals in the pass band so much better than the FCD did. There’s no more overloading of the receiver, no more ghost signals appearing on the waterfall due to the front end not being able to cope and no more having to constantly keep playing with the settings to get things under control. The hardware AGC built into the SDR device does a great job at keeping it all under control whilst receiving a much wider bandwidth than the FCD ever could.
The satellite beacons are now received at S9+15dB without the receiver being overloaded, the first time I have seen this since starting out on my QO-100 venture.
The other thing that became obvious very quickly is that frequency stability is much better than it was with the FCD, it doesn’t drift up and down the transponder now and stays tuned exactly where I put it. It’s also on frequency whereas, the FCD was always 1.7Khz off frequency.
The NooElec NESDR SMArt v5 is very well put together, it has an aluminium case that acts as a heatsink (it does get warm!) and overall the build quality is much better than the plastic cased FCD. When I think that I paid close to £100 for the FCD and the NooElec NESDR SMArt v5 only cost £33, I am amazed at the build quality.
Overall I’m extremely pleased with the purchase of the new SDR, it slotted in perfectly as a replacement for the FCD, works great with GQRX, my QO-100 Node Red Dashboard and performs considerably better than the FCD. Overall money well spent!
You can find the NooElec NESDR SMArt v5 spec sheet here.
More soon …
Hey there, fellow electronics enthusiasts! If you’ve ever tinkered with the signal generation, you’ve likely come across the AD9833 waveform generator chip. But did you know that this little gem can do so much...
The post Easy to use AD9833 C Library appeared first on Nuclearrambo.
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