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Software Defined Radio

Has anyone managed to use a SDR dongle to listen to airband transmissions?

I mean one of these type of DVB-T dongles.

I have a spare one and trying to see if I could use it to hear items on the airband. But am really struggling.

I brought it to the local airport today and could just barely make out the ATIS….but only barely. I’ve no idea if the one I have is just crap or (probably more likely) my settings are wrong. I really don’t know how to adjust the settings for airband.

Just wondering if this is a fools errant or if others have managed it ok. If they have managed it, any clue to the setting to use would be greatly appreciated!

I should say that FM radio is no problem…..easy to receive. But everything else, I’m struggling to hear clearly.

EIKH Kilrush

I used it Linux to receive 1090ES – digital broadcasts by the transponders, but was unable to receive any audio on the right frequencies.

EGTR

Firstly, make sure your antenna is resonant on air band, or it won’t be efficient.

Secondly you might need to filter out strong commercial (FM radio) broadcasts. The front ends on these things are not that great, and they can easily be desensitised (or flat out overwhelmed) by powerful commercial FM transmitters nearby. This will need an external high pass filter, which should be something you can buy off the shelf (I think they are pretty often required for inexpensive SDR dongles).

Thirdly they don’t have a lot of dynamic range, if it’s 8 bit that’s only 48 dB of raw dynamic range (a typical “traditional radio” will have around 90dB dynamic range by comparison, which is hugely more – dynamic range is basically the difference between the weakest signal and strongest signal the radio can work with. In fact you can kind of see that from where your noise floor is). The upshot of poor dynamic range is that it’ll be pretty “deaf” especially if there are strong nearby signals like that FM radio broadcast, or other airband transmissions, near the frequency of interest. You can improve the dynamic range somewhat by increasing the decimation settings and narrowing the bandwidth (although these will all be software – hardware decimation tends to come with the SDRs that cost a few hundred euros or more).

So I suggest first try increasing the level of decimation (at the expense of CPU) and decreasing the bandwidth; you should probably not expect to be able to receive the whole air band at once with any kind of sensitivity, making sure your antenna is resonant in the airband too. If that doesn’t work, try adding a filter to reduce possible desenitisation from broadcast FM (which is not far away from the air band).

Andreas IOM

This antenna is OK to pick up ADS-B transmissions, but for the VHF air band it is direly inadequate. If I remember correctly, antenna rod is screwed into the foot – for a very basic fix, make a 10-12 times longer rod out of any wire. Or find a MCX to BNC cable and connect a “rubber ducky” antenna from a handheld VHF radio.

Last Edited by Ultranomad at 04 Apr 20:07
LKBU (near Prague), Czech Republic

Also to add to the antenna, if you’re indoors, it won’t be all that great (and ATIS transmissions aren’t all that high powered). Even with a proper airband transciever, I can barely get Ronaldsway ATIS (only ~6km or so away) if I’m indoors, but with an external antenna the signal fully deflects my signal strength meter and is loud and crystal clear. The coax with the antenna pictured is also pretty awful (high loss).

Last Edited by alioth at 04 Apr 21:35
Andreas IOM

What is the A-D converter resolution in these?

Administrator
Shoreham EGKA, United Kingdom

Peter wrote:

What is the A-D converter resolution in these?

They are 8 bits (hence 48dB of dynamic range – each bit is good for 6dB). You can improve things somewhat using decimation (basically oversampling, and increasing dynamic range at the cost of bandwidth (and CPU usage)). You get a 3dB improvement with each decimation step (which goes up in powers of 2, e.g. 2,4,8,16).

Andreas IOM

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.

Administrator
Shoreham EGKA, United Kingdom

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.

Andreas IOM

Avionics certification moved here

Administrator
Shoreham EGKA, United Kingdom
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