Or the quality of cable. RG 213 is really the minimum spec for VHF/UHF for a stretch of more than about 5m. If you know of a mobile mast decomissioning somewhere, you may even be able to get hold of some heliax for cheap :-)
dublinpilot wrote:
I had to get about 1km away from the airport before the ATIS stopped spilling over onto every frequency.Perhaps with more knowledge of the setting, I’d be able to tidy that up somewhat.
Basically the RF front end of your SDR is getting overwhelmed. You might be able to improve things somewhat at the DSP end by using decimation (which to put it simply, is trading bandwidth for dynamic range). See if you have decimation settings and try to increase them.
If you have a problem band, e.g. if you’re in some location and finding that an entire band is a problem (e.g. broadcast FM) you can make a notch filter for that band very easily by putting a T-piece in your feedline and cutting a piece of coax to 1/4 the wavelength of the problem frequency, and connecting this to the bottom of the T. (Remember to use 0.66 times the speed of light to work out the right length of coax – the signal travels slower in coax than the speed of light). This is called a stub filter. The filter will be quite wide so you won’t really be able to do this if the problem frequency is close to the one you want to listen to, but it’s an easy way to get around 40+dB of attenuation on a problem band.
alioth wrote:
This is the key to a good VHF installation, really. An antenna designed for the frequency in use, placed somewhere high up and free from obstructions. This will solve 90% of airband reception problems right off the bat.
And another 9% will likely be solved by shortening the length of the RF cable, ie. placing the RPi in the attic, close to the antenna…
varj1 wrote:
Please update us on your progress!
Sorry for taking so long to update this thread.
I followed Ultranomad’s advice and bought a different antenna.
Actually not knowing much about them, I bought two different ones on AliExpress.
This one was described as having a Frequency range: 118-136 (MHz). Cost €4.19 delivered.
This one was described as having a Frequency Range: 25MHz – 1200MHz; Cost €4.51 delivered.
Then I also bought a connector to allow these to be connected to the SDR. Cost €1.64 delivered.
The outcome was dramatically different from using the original antenna! Now both performed very well and I had no difficulty now picking up the local traffic from my home. I’m about 2km direct distance from two local airports. Using both of these antenna’s I could hear the local traffic without difficulty. I could also hear ATC from the airport that I have a better line of sight from, with both antennas, sitting on my desk on the first floor of my home.
The first one had marginally better performance. Interestingly I could also pick up (just about) the ATIS from the airport with a worse line of sight using this antenna, but not the second one.
The clarity was not near as good as with my airband transceiver, but it still was plenty good enough. I’m sure it could be improved a bit with a better knowledge of the different settings in Sharp SDR software. I was just trying things without really knowing too much about what I was setting. But it was perfectly understandable.
Where things got really interesting was when I took these to one of my local airfields. Now the antenna were pickup up too much! It didn’t matter what frequency I was tuned to! I could hear the ATIS on EVERY frequency! And the tower too! So it radio clearly lacks some of the signal cleaning the comes in the dedicated transceiver.
I had to get about 1km away from the airport before the ATIS stopped spilling over onto every frequency.
Perhaps with more knowledge of the setting, I’d be able to tidy that up somewhat.
The first antenna is a bit more awkward to use as it’s not free standing.
It’s length is 26cm retracted and 123cm when extended. (All test were done extended).
The second antenna is 30cm from base to tip. Obviously the two curly bits makes the reception length longer.
So if you could make a airband receiver for very little money (SDR, Antenna, Connector) would be about €12-15. It would also be set up as an online server if you wanted to put it online. But the quality probably won’t be as good as a dedicated airband receiver unless you really know what you are doing or buy more expensive SDR equipment. But still pretty useful.
Oh, working on my phone or tablet battery, it uses up a battery pretty quickly!
Thanks for all the advice, and hopefully the report is useful for someone else.
Mateusz wrote:
We bought a proper air band antenna and mounted in on the club building roof
This is the key to a good VHF installation, really. An antenna designed for the frequency in use, placed somewhere high up and free from obstructions. This will solve 90% of airband reception problems right off the bat.
One of the radio amateurs in the Isle of Man has an installation receiving AIS and feeding into aprs.fi – his house is 500’ above sea level, and he has a good antenna installation. From the Isle of Man he occasionally receives AIS packets from the English Channel (although these undoubtedly happen when there’s the right propagation conditions). He regularly receives ships tracks down to the toe of South Wales. This is with an inexpensive SDR setup.
At EPKM we use such a USB “TV” dongle and a Raspberry Pi to provide a live MP3 feed available to the Aeroklub members. It uses this software: https://github.com/szpajder/RTLSDR-Airband – which is surprisingly reliable. The operating system on the RPi is set up read-only, making it more resistant to power failures. We bought a proper air band antenna and mounted in on the club building roof. Some generic use adapters were also needed to connect the antenna to the receiver’s input. No issues with the reception were reported to me over the course of last three years.
dublinpilot wrote:
Thanks guys. I’ll try a better antenna and the software tweaks mentioned. Much appreciated!
Please update us on your progress!
Thanks guys. I’ll try a better antenna and the software tweaks mentioned. Much appreciated!
Peter wrote:
What would be the benefit of say 12 bits? Presumably, recovering weaker signals?
Dynamic range (the difference between the “quietest” and the “loudest” signal the ADC can deal with). Each bit you add increases the dynamic range of the ADC by 6dB (so going from 8 bits to 12 bits increases the ADC’s dynamic range from 48dB to 72dB, which is quite a lot (and if the 0dB point of both is the same, this translates to better sensitivity for the 12 bit ADC).
You can increase dynamic range by oversampling and decimation too, but you’re trading off bandwidth to do that.
They are 8 bits
What would be the benefit of say 12 bits? Presumably, recovering weaker signals?
As a software developer I’m extremely surprised that software gets an easy ride in certification.
Yes; that’s totally contrary to everything I have come across before. But there are different levels of software QA depending on what it is. None of them prevent bugs, of course.
