How To Install Parabolic Grid Antenna

We've recently been testing methods to help budding amateur radio astronomers become into the hobby cheaply and easily. We take plant that a depression price ii.4 GHz 100 cm x sixty cm parabolic WiFi filigree antenna, combined with an RTL-SDR and LNA is sufficient to detect the hydrogen line peak and doppler shifts of the galactic plane. This means that y'all can create backyard hydrogen line radio telescope for less than The states$200, with no complicated construction required.

If you don't know what the hydrogen line is, we'll explicate it hither. Hydrogen atoms randomly emit photons at a wavelength of 21cm (1420.4058 MHz). Usually a unmarried hydrogen atom will only very rarely emit a photon, but the milky way and even empty space is filled with many hydrogen atoms, so the average effect is an appreciable RF power fasten at ~1420.4058 MHz. By pointing a radio telescope at the night sky and averaging the RF power over fourth dimension, a power spike indicating the hydrogen line can be observed in a frequency spectrum plot. This can exist used for some interesting experiments, for example y'all could measure the size and shape of our milky way. Thicker areas of the galaxy will have more hydrogen and thus a larger fasten, whereas the spike volition be significantly smaller when pointing at empty space. You tin likewise measure the rotational speed of our galaxy by noting the frequency doppler shift.

The 2.four GHz parabolic WiFi filigree dishes tin can be plant for a cheap at US$49.99 on eBay and for effectually US$75 on Amazon. Outside of the United states they are typically carried by local wireless communications stores or the local eBay/Amazon equivalent. If you're buying one, be sure to get the 2.iv GHz version and NOT the 5 GHz version. If you can detect ane.nine GHz parabolic grid dish, then this is also a good pick. Although we haven't tested it, this larger 2.iv GHz filigree dish would probably also piece of work and give slightly better results. WiFi grid antennas have been unremarkably used for GOES and GK-2A geosynchronous atmospheric condition satellite reception at two.4 GHz with RTL-SDRs as well and we take a tutorial on that available on our previous mail service.

These dishes are linearly polarized only that is okay as hydrogen line emissions are randomly polarized. Ideally we would have a dual polarization (Not circular polarized) feed, but linear appears to be enough and is much simpler. In addition, the ii.4 GHz feed is obviously not designed for 1420 MHz, but but like with GOES at 1.7 GHz the SWR is low enough that it still works.

The Gyfcat animation below shows a hydrogen line "migrate" scan performed with the 2.iv GHz WiFi dish, an RTL-SDR Blog V3 and a NooElec SAWBird H1 LNA. The scan is performed over one twenty-four hours, and we simply permit the rotation of the earth allow the Milky Way to drift over the antenna. The Stellarium software on the left shows the move of the Galaxy/galactic plane over the course of a 24-hour interval for our location. The dish antenna points straight up into the sky, and we accept ready Stellarium to look straight up too, then Stellarium sees exactly what our dish antenna is seeing.

via Gfycat

You can clearly meet that there is a lump in the radio spectrum at around 1420.40 MHz that grows when parts of the Milky Way pass over the antenna. This lump is the hydrogen line being detected. As our Galaxy galaxy is filled with significantly more hydrogen than empty infinite, nosotros run across a larger lump when the antenna points at the Milky Manner, and simply a very modest lump when it points away.

It'southward important to ignore the very narrowband spikes in the spectrum. These narrowband spikes are merely radio interference from electronics from neighbors - probably TVs or monitors equally we notation that nearly of the interference occurs during the day. In that location is also a large constant spike which appears to be an artifact of the LNA. The LNA we used has a 1420 MHz filter built in, merely LCD TVs and other electronics in today's suburban environment spew noise all across the spectrum, even at 1420 MHz.

You can too note that the hydrogen line peak is moving around in frequency as different parts of the galaxy pass overhead. This indicates the doppler shift of the part of the galaxy being observed. Because the arms of the galaxy and the hydrogen in it is rotating at pregnant speeds, the frequency is doppler shifted relative to us.

Using the power and doppler shift data of the hydrogen line is how astronomers first adamant the properties of our galaxy like shape, size and rotational speed. If we continued to scan the sky over a few months, we could somewhen build up a full map of our galaxy, like what CCERA have done as explained in this previous post.

Hardware Required

A 2.iv GHz WiFi parabolic grid dish. (~$50)
A low dissonance amplifier (LNA). This is required to become the noise figure of the receiving system depression enough, and the gain loftier enough.
1. We recommend using a hydrogen line specific LNA. Good models include the NooElec SAWBird+ H1 ($44.95), or the GPIO labs Hydrogen Line pre-filtered LNA ($49.95).
  Using a specially made hydrogen line LNA with filtering built in will go yous meliorate results compared to a general purpose wideband LNA. It may as well exist mandatory to use one of these LNAs for those living in areas with stiff interfering signals from things similar cellular and broadcast FM/TV etc.
2. If yous're on a budget, and don't have many strong interfering signals around you, then you get away with using an unfiltered full general purpose wideband LNA like an LNA4ALL or our $19 RTL-SDR Web log wideband LNA.
  Nosotros can generally get away with an unfiltered LNA if we point the antenna straight upwardly towards the sky, or at a high tiptop. This avoids most terrestrial sources of racket from leaking into the antenna. However, the H-Line specific LNAs are usually very loftier gain, and very low noise figure, so tin can work improve for this type of experiment.
An RTL-SDR Blog V3, or whatsoever other RTL-SDR with a congenital in bias tee (~$21.95). An Airspy is as well a practiced choice with proficient supporting software, just costs a lot more.
A Type North Male to SMA Male adapter (~$7 on Amazon, cheaper elsewhere). Most WiFi filigree antennas have an Due north-female person connector so we need to convert to SMA to connect to the RTL-SDR.
A high quality USB extension cable (~$10), just long enough to get to your PC/laptop. We recommend a high quality USB3.0 spec cable, as these have much lower voltage loss over longer runs. If you lot're using an active cable, make sure it can handle the voltage drop.
Some sort of tripod ($39.99) to mountain your dish, or another way to mount information technology. You lot could probably even merely lay it on the ground.
A l Ohm terminator ($5.50) (optional but recommended)
A Windows PC or Laptop (for this tutorial). A Raspberry Pi could also piece of work with other software or as a TCP server.

Full price (not including the PC): Us$179.40, and probably less if you already have some parts or find similar items priced cheaper elsewhere.

Hardware Setup

The recommended setup is unproblematic. Antenna pointed direct upwards -> LNA -> RTL-SDR -> USB Cable -> PC.

Cheap and Easy Radio Astronomy Setup with an RTL-SDR and 2.4G WiFi Grid Dish — Cheap and Like shooting fish in a barrel Radio Astronomy Setup with an RTL-SDR and 2.4 GHz WiFi Grid Dish

Detailed instructions below:

Construct the WiFi dish. This is merely a matter of putting in a few screws to join the 2 panels and feed. Make certain the feed is mounted with the long axis matched with the grid direction. Besides ensure the reflector is installed.
Mountain the dish exterior pointing straight upwardly into the heaven. One time you are a little more than avant-garde, you could endeavour other elevations or even motorize information technology, just start with directly upward for now. The rotation of the dish does not really matter as hydrogen line emissions are randomly polarized.
Connect the RF side of the LNA to the antenna cable via the Due north-SMA adapter.
Connect the RTL-SDR to the RF+DC side of the LNA.
Connect a high quality USB cable from the RTL-SDR to your PC. We don't recommend using anything more than a few meters of coax between the LNA and RTL-SDR in guild to optimize the indicate levels.
1. Do not employ coax betwixt the antenna and LNA. The LNA should exist straight continued to the antenna output.

Information technology may also be wise to waterproof your LNA and RTL-SDR if kept outdoors. This can be as unproblematic as putting it in a plastic bag, or former coke canteen sealed with some putty.

Software Setup

In order to discover the hydrogen line we need to use software capable of integrating/averaging many FFT samples over time. Averaging the samples reduces the SDRs quantization noise, allowing the weak hydrogen line peak to exist seen. Because the milky way is moving adequately slowly in the sky, we tin safely average for five-ten minutes at a time.

For Linux, at that place are diverse programs that tin exist used. PICTOR, and rtl-obs are some good choices, only are a trivial more complicated to ready. But they have some skilful features like the ability to properly calibrate the results, and some interesting algorithms that could increase the SNR of the hydrogen line detection.

For this tutorial we will keep it as simple as possible, and we will use Windows, with SDR# and a SDR# plugin called "IF Boilerplate". We will also utilise a free astronomy plan called Stellarium for tracking the Milky Way's galactic plane across the sky.

Stellarium Setup

Download Stellarium from https://stellarium.org, and download the Windows version using the push upwards the top.
If you opened Stellarium during the day y'all won't come across whatsoever stars due to atmosphere simulation. Hit the 'a' key on the keyboard to disable atmosphere.
Hitting the F4 fundamental to get into the options menu. Here we recommend increasing the brightness of the Milky Way to 6.0, to brand information technology really obvious.
Nosotros also suggest going to the markings tab, and turning ON the Azimuthal grid, which will provide a mark to Zenith (direct up in the heaven).
Cheque the location shown in the bottom left. If information technology'due south non right for you, press F6 to set the right location.
Employ the mouse bicycle or pinch controls to zoom out so that the entire sky is visible. Elevate the mouse then that the camera is looking at Zenith (directly up into the heaven).
Equally Stellarium will take opened by default in full screen fashion, press F11 to go to Windowed mode.

By clicking on an object within the Milky Mode or behind information technology, you can detect out the Galactic coordinates of where in the Milky way you are pointing. This could be useful for comparing with already known results like those shown here. Right click to remove the info text about that object.

Stellarium looking straight up into the sky at the Milky Way — Stellarium looking straight up into the sky at the Galaxy

SDRSharp with IF Average Plugin Setup

Install SDRSharp, Blog V3 drivers, and the IF Boilerplate Plugin

Download the latest version of SDR# from www.airspy.com. Set up SDR# and the RTL-SDR every bit described in the Quickstart Guide at www.rtl-sdr.com/QSG.
For easily activating the bias tee on the RTL-SDR Weblog V3, download the special Blog V3 Windows drivers from https://github.com/rtlsdrblog/rtl-sdr-blog/releases.
Rename the original rtlsdr.dll file in the SDR# folder to rtlsdr_old.dll.
Copy over all the .dll files in the Release.zilch file.
Rename librtlsdr.dll to rtlsdr.dll.
Download the IF average plugin. Unfortunately the author of the plugin has not maintained his website, and the page is now offline. Merely the plugin is still available on his Dropbox. Go to Download->Direct Download to download it to your PC. We have also decided to mirror the plugin here on the blog server just in example the Dropbox file goes offline.
Extract the plugin files from the nix file into the SDR# folder.
Open the "Magic sentance.txt" file and copy the <add fundamental...> line.
Open up plugins.xml with notepad, and copy and paste in the <add fundamental...> line.

Receiving and Averaging the Hydrogen Line FFT

Open SDR#, select the RTL-SDR, press the outset push button.
Adjust the RF Gain slider to the maximum, and check the "Offset Tuning" checkbox to enable the bias tee via the V3 driver hack.
Melody to 1420 MHz and employ the heart tuning button to eye the frequency (the button next to the frequency input in SDR#).
Now on the left scroll downwards until you find the IF Average plugin that you installed earlier. (NOTE: On newer versions of SDR# the plugin is hidden by default! To unhide information technology, click on the top left 'hamburger' icon (the three horizontal lines to the left of the play/finish button). Then click on the 'IF Boilerplate' entry to enable information technology)
We used the following settings which results in a 6-7 minute averaging time (but shorter averaging times would probably also work - try reducing the dynamic averaging a petty):
1. FFT resolution: 1024
2. Intermediate Average: thousand
3. Gain: ~335
4. Level: grand
5. Dynamic Averaging: 902000
Calibration: Connect your LNA to the l Ohm terminator for initial scale. If you don't have a 50 Ohm terminator, only go out the antenna asunder.
Bank check the "Window" checkbox, and immediately printing the "Groundwork" button in order to generate a reference background browse. This scan will be subtracted from subsequent scans thus removing the unwanted curved shape of the RTL-SDR and LNA filters. The first browse will have 6-7 minutes.
Once the background scan is completed, you'll see the words "Corrected background!" in xanthous in the top left of the FFT boilerplate window.
You can at present reconnect the antenna.
Tip: If the FFT Average Window keeps disappearing behind the main SDR# window, push the main SDR# window to the right and bring the IF Average Window to the left so that it does not sit on summit of SDR#.
You may demand to adjust the Proceeds and Level sliders a trivial chip in gild to go the FFT graph on the screen. Attempt to continue the Proceeds large, every bit this increases the FFT proceeds allowing you to run into minor peaks more clearly.

IF Average SDR# Plugin — IF Boilerplate SDR# Plugin

At this stage you lot now just demand to await for the Milky Fashion to enter your antennas beamwidth, and watch for the H-line summit. The software will continually average the spectrum.

If you lot want to create a timelapse like the gif shown at the top of the mail nosotros tin recommend a program chosen "Chronolapse", which takes a screenshot every Ten minutes. Y'all can and then catechumen those images into a pic or gif. The IF average plugin can also output data files which could exist used for further analysis.

If you do not exercise the calibration at all, your spectrum will appear quite wavy. Exist sure to not confuse those waves with the hydrogen line peak.

If you neglect background calibration, the spectrum will be wavy and not flat. — If y'all fail background calibration, the spectrum volition be wavy and not flat.

Example Results

Hydrogen Line Peak (Ignore the Narrowband Peak) — Hydrogen Line Meridian (Ignore the Narrowband Pinnacle) Pointing at (-110°, 0°) Galactic Coordinates

Hydrogen Line Peak (Ignore the Narrowband Peak) Pointing at (-0°, 0°) Galactic Coordinates

Using a general purpose wideband LNA still works, but results in lower SNR peaks — Using a general purpose wideband LNA still works, but results in a much lower SNR superlative.

Other Notes

It is possible to get slightly higher SNR by covering the grid dish with foil, or a metal mesh. Nevertheless, the improvement appears to be very small, almost negligible since the WiFi feed is but linearly polarized.
Longer integration/average times volition spread the top out more. Smaller integration times may result in less SNR.
Yous may wish to experiment with an top that maximizes the time spent pointing at the Galaxy for your location. Use Stellarium and the time shift feature (F5, or CTRL+ClickDrag) to find the optimal acme. But lower elevations are more susceptible to man made interference.
A motorized antenna mount would permit you lot to scan more of the Galaxy in one day. An example build from this previous post here.