Receiving DVB-S Signals – Hardware
The most ideal way to receive signals from this payload is with a dedicated DVB-S receiver like the Minitiouner Express (~AUD$150 shipped – www.datv-express.com/ ), however it’s also very possible to receive it with Software Defined Radio receiver setups, such as the ubiquitous RTLSDR devices. You will also need a 70cm antenna with some gain, and a low-noise preamplifier.
While some DVB-S set-top boxes may tune down to 445 MHz, the very low bitrates that we will be transmitting from the payload will likely be incompatible with them.
To get an idea of what kind of equipment is required to receive signals from the payload, we’re going to run through a brief link budget for the entire system. We’ll make the following assumptions:
- Transmitter Power: 500mW (+27 dBm)
- Transmitter Frequency: 445 MHz
- TX-RX Range: 100km
- Transmit Antenna Gain: 0 dBi
- Symbol Rate: 1 Msps (QPSK)
- Effective Bitrate: 2Mbit/s
- Occupied Bandwidth: ~1.4 MHz (measured)
So, onto our link budget:
- Transmit Power: +27 dBm
- TX Antenna Gain: +0 dBi
- TX EIRP: +27 dBm
- Path Loss: 125.4 dB
- RX EIRP: -98.4 dBm
Let’s factor in a nominal 70cm receiver setup: a 7-element yagi (~9dBi), and a MiniKits PGA103 preamp (2 dB NF):
- RX Antenna Gain: +9 dBi
- Power into RX: -89.4 dBm
- RX System NF: 2dB
- RX Noise: -174 + 10log(1.4 MHz) + 2 = -110.5 dBm
- C/N: 21.1 dB
- Eb/N0: 21.1 – 10log(1e6 / 1.4e6) = 22.6 dB
- Required Eb/N0: 4.5
- Link Margin: ~18 dB
So, from this, we can see that even a fairly modest receive setup may be able to receive video from this payload at a distance of 100km! It’s worth noting that these calculations assume no impairments from local noise, and no implementation loss (which is unlikely to be the case). Fading caused by the payload swinging will also eat into the link margin.
It’s important to note that this payload is very much experimental – it may not perform as well as expected, or it may not work at all!
Software-Defined Radio Receivers
There’s a huge range of SDRs available which are fit-for-purpose in this application, provided you put a low-noise-figure preamp in front of them – ideally one with band-pass filtering. Examples include:
- RTLSDR v3 – www.rtl-sdr.com/buy-rtl-sdr-dvb-t-dongles/ – ~AUD$40 shipped.
- Airspy Mini – www.itead.cc/airspy-mini.html – ~AUD$150 shipped
Note that I’ve only tested DVB-S reception under Linux using RTLSDRs. The Windows software (SDRAngel) has support for a wider range of SDRs, though I haven’t specifically tried reception using anything other than a RTLSDR.
Dedicated DVB-S Receivers
The ‘Minitiouner Express’ ( www.datv-express.com/ , ~AUD$150 shipped) is a dedicated DVB-S receiver. It has pretty good receive performance out-of-the-box compared to the RTLSDR (~3dB better) and is more targeted at those users interested in experimenting with DVB-S/S2.
It only works with the ‘Minitiouner’ software ( wiki.batc.org.uk/MiniTioune_software ), which is Windows only.
I won’t be covering how to use this bit of hardware in this guide. If you decide to purchase one of these and need assistance setting it up, please contact me.
Antennas & Preamps
Regardless of what receiver option you go with, you will have the best chance of receiving signals from the payload if you have a yagi antenna with reasonable gain, and a preamplifier.
I highly recommend the preamplifiers from MiniKits. A few suitable preamps include:
To use these you may also need a 12v ‘Bias-Tee’ unit: www.minikits.com.au/EME195-Bias-Tee
A yagi antenna is highly recommended for best results. The transmission will be vertically polarised for the majority of the flight (some tumbling after burst).
There are a range of homebrew 70cm yagi designs out there, including:
- Designs based on VK5DJ’s yagi calculator: www.vk5dj.com/yagi.html
There are also comercial yagi options available:
- Cushcraft 11-element – https://www.strictlyham.com.au/browse-by-type/antennas/beam-antennas/cushcraft-a43011s
- Cushcraft 6-element https://www.strictlyham.com.au/browse-by-type/antennas/beam-antennas/cushcraft-a4496s#.X0DKI5MzZ24
- 7-element yagi from element14 – https://au.element14.com/lprs/yagi-434a/antenna-yagi-7-element-434mhz/dp/2096215
If you have other suggestions to add to this list, please let me know!