RTL-SDR potential?

[Samibotss]

Hi all,

I'm looking to get into Radio Astronomy to fill the time when the weather is . I've heard of the RTL-SDR and was wondering what king of things can be achieved with it in terms of RA. I'm looking to get readings on comets primarly. Any advice on where & how to get started?

P.S : was wondering wether or not a satellite dish could help boost my reception

[7170]

On 06/11/2017 at 17:30, Samibotss said:

Hi all,

I'm looking to get into Radio Astronomy to fill the time when the weather is . I've heard of the RTL-SDR and was wondering what king of things can be achieved with it in terms of RA. I'm looking to get readings on comets primarly. Any advice on where & how to get started?

P.S : was wondering wether or not a satellite dish could help boost my reception

Hello, (in my mind at least) a good starting point is to try and categorize the different types of radio astronomy that an amatour is lickly to have access to and then briefly discuss the equipment needed.

1) VLF SID detection

2) Meteor detection

3) Detection objects in our solar system (the sun, moon, jupiter's storms)

4) Detection of stronger objects outside our solar system (Cygnus-A and Cassiopeia-A for example)

5) Neutral Hydrogen emission line (1420.4MHz) detection

6) Polsar detection

When I refer to RTL-SDR below I refer to standard dongles as well as better modified versions such as RTL-SDR V3 which have better components etc, obviously there are some out there which have extended frequency range down to lower HF but they are more expensive. Also I am assuming you are not using an up-converter.

For (1) you are looking at frequencies under 50khz. An RTL-SDR dongle doesn't go down that low so is no use here. The usual approach is a large loop antenna connected to your PC soundcard - just be careful about lightning!

For (2) you are tuning to a distant VHF radio station, beacon, radar or carrier wave which is ordinary out of range due to the curvature of the earth. The idea is that when a meteor hits and goes through the atmosphere it leaves an ionised trail of particles which the normally out of range radio waves bounce off for a few seconds and your receiver picks up these. If you are tuned to a radio station you can actually hear the station properly for a very short period of time and can be quite a bizarre experience. This has been a popular pursuit since the early 80's (and maybe before), as you can calculate a lot of information, e.g. frequency, meteor speed, direction etc. The move to digital has reduced the options for distant stations as you want one which is high powered and consistent (e.g. an analogue tv carrier wave) . If you are in the UK the most popular option today is the GRAVES radar sight in France at 143.050 MHz, for which you can use a 2m 3 element yagi antenna and an RTL-SDR (though depending on location you may need a preamp). In other words an SDR dongle is perfect.

For (3) you are likely to be looking at different frequencies. Detecting storms on Jupiter is on 20.1Mhz so is out of range of RTL-SDR. You can monitor the sun on all sorts of frequencies , many of which are within the RTL-SDR's range, e.g. around 70mhz, 613mhz etc. For the moon you can use C-Band or K-Band satellite equipment so is out of range of an RTL-SDR unless you use a downconverter.

For (4) you setup a stationary antenna or dish pointed in the direction of where your object will be in say an hour's time and let it sweep across while you take readings. 613Mhz works and in the past it was popular to modify RF tuners to turn off AGC and add an AM Detector and record the change in sound level on a chartplotter or PC. An RTL-SDR dongle should in theory work and this is in fact on my list of things to try again next year. All sky surveys at various frequencies within the range of an RTL-SDR will also work with varying levels of success.

For (5) you really need to use a dish greater than 2m in diameter and a setup with a low noise figure combined with good filtering. RTL-SDRs have a fairly poor noise at 1420MHz but there are articles out there showing how you can still use them with with two LNAs near the antenna. So this is a possible one but with lots of work, and it helps if you are passionate about radio!

(6) is the holy grail of amatour radio astronomers in my opinion and there is not that long a list of people who publicly claim to have managed to detect them from their back yard. The British Astronomical Association had an article where Vela was detected using an RTL-SDR using a 30m dish, so it is technically possible. (6) is an exercise of engineering, mathematics and computer programming more than anything.

Hope this helps. RTL-SDRs are great but you must use filtering, e.g. FM Broadcast band stop as they suffer from overloading very easily. I'm always amused when people complain about a repurposed <£20 tv dongle which can do all this, forgetting how much you would have paid for the equivalent radio say a decade ago.