# Can I sense a bright star pointing an eight foot antenna towards it?

If I connect an eight foot Yagi or other comparable sized antenna to my oscilloscope and point the antenna at a bright star will I see a voltage on my oscilloscope?

I am not interested in turning the voltage into an image just wondering if I would see a voltage increase when it is on a bright star. I’d like to know your thoughts before I take the time to build the antenna. I’m thinking about in the 25cm range. I’ve heard that’s an active area. My oscilloscope will read down to about 20 millivolts.

Stars are too dim for amateur radio equipment. There are two possible radio sources that you can detect: the sun and Jupiter.

Jupiter is particularly interesting as interactions between Io and its magnetic field produce beams of radio waves that sweep past earth every 10 hours. These are detectable in the amateur range, at about 20 MHz.

Nasa make a kit for detecting these radio signals, or it is possible to use a ham antenna, but of course it must be cut for the frequency of operation. The Nasa kit uses a phased dipole antenna which must be set up in a field or similar as the antenna is about 7m long.

Stars are not very good radio sources. Supernovae remnants such as Cassiopeia A or the Crab nebula are much brighter at radio wavelengths. Most supernovae are too distant to be powerful radio sources; radio supernovae are rare. A local supernova would be a radio source but we haven't observed a supernova in the milky way for several hundred years.

• Thank you. Would and amateur setup get a signal from supernova? – Lambda Oct 12 '18 at 3:48
• @Lambda that's an interesting question, so it's better to ask as a new question so that someone will have room to post a new answer. – uhoh Oct 12 '18 at 4:40
• @Lambda: When its near enough, we all get a signal from a supernova... – PlasmaHH Oct 12 '18 at 14:58
• @PlasmaHH a very scary thought indeed. – Carl Witthoft Oct 12 '18 at 15:51
• Regarding Jupiter: Jupiter changes it's distance from Earth by 2 AU (about 300 million kilometers) and back over the course of about 13 months. This represents a difference of over 15 light-minutes. If you map the phase of the Io's radio transmissions over this time, you will see it advance 15 minutes ahead or behind "expected arrival time" over these 13 months. Thus proving that radio waves travel at the speed of light, or alternatively, that electromagnetic radiation has a finite speed. – dotancohen Oct 13 '18 at 1:35

As others have noted, you will not be able to detect a star using an oscilloscope and an antenna. The received signal level is too low, and the oscilloscope not nearly sensitive enough.

A radio telescope consists of an antenna, an amplifier, and a receiver (that incorporates other amplifiers and other stuff besides - like filters and mixers to select the desired frequency range.)

An antenna by itself wouldn't pick up enough signal to be directly useful.

The oscilloscope lacks amplification and filtering needed to make the antenna signal useful.

As others have said, you can use commercial antennas and receivers to pick up the signals. There are kits you can buy with everything you need, or you can get the components piece at a time from various sources.

As an alternative, you might consider building a small radio telescope using standard satellite TV components.

I have one, and besides the sun and the TV satellites, it can detect the moon. I haven't gotten around to trying to detect smaller or less intense things. I do have it mounted on servos, though, and have made pictures of ambient RF signals. Houses and trees are surprisingly "bright" sources of 13GHz RF.

The folks here have instructions for building one, as well as examples of what you can do with it.

Here is another example of making such a small radio telescope.

I think both projects link back to the same original source.

You can usually get all of the needed parts at any store that sells satellite TV receivers. I bought my stuff on Amazon, but most of the hardware stores here stock those things as well.

All you need is a dish, an LNB, (both can be bought in a set) and one of the little gadgets that helps you aim the dish properly. And a few feet of cable and connectors, of course.

The dish has high gain.

The LNB contains amplifiers and filters to make the signal strong enough to be useful.

The alignment device is the final bit. It has yet more amplification, and converts the received radio signal to a (somewhat noisy) voltage that represents the strength of the received signal.

The signal strength indication is shown on a small meter. You can also open the box, and add a couple of wires - you can then connect that to your oscilloscope and see how strong the signal is that you are picking up from the sun or whatever. The two wires driving the meter are the correct place to connect to.

My profile picture is an image I made in my garage using my servo aimed satellite dish. Not terribly impressive, but that was made without any kind of additional "lighting." All just ambient RF.

If you have a fluorescent light, you can pick up 60Hz modulated RF by pointing just the LNB at the light. Fluorescent lights cause broadband RF interference, and the LNB can pick it up at 13GHz. The signal strength meter demodulates it, and you can see a nice 60Hz signal if you connect an oscilloscope to the meter.

My detector is a little more advanced than just the little meter. I built a controller out of an Arduino.

It uses a MAX2015 as a signal strength detector, and has a 24 bit analog to digital converter. It also has a chip to generate control signals for the LNB.

The LNBs can actually receive two bands, and can use horizontal or vertical polarisation. My controller lets me switch among the various combinations.

The Arduino operates the hardware (it also drives the servos,) makes measurements, and delivers results to my PC over the serial port. It also takes commands as to what to do. The smarts are all in the PC - an Arduino just hasn't got what it takes to build an image out of a bunch of measurements.

• Very good information. I think that is the route I will take. The stars are out of my reach, but what you have described sounds like a good doable project. Thank you. I’ll check out the links. – Lambda Oct 12 '18 at 16:54

Connecting an antenna directly to oscilloscope will not give reception, even with a strong radio source.

First problem is the power level. Typical received power from antenna would be around -100 dBm, i.e. $$10^{-10}\,\textrm{mW}$$. A typical oscilloscope has an input impedance of 1 Mohm, which means if all the received power went there, it would give a voltage of $$\sqrt{10^{-10}\,\textrm{mW}\cdot1\,\textrm{Mohm}} \approx 0.3\,\textrm{mV}$$. With the minimum scale of 20 mV, you wouldn't see much anything.

The second problem is mismatch loss. Most antennas are matched to 50 ohm impedance instead of the 1 Mohm. The mismatch means that only about 0.01% of the power would actually go into the oscilloscope, the rest would reflect back.

• But the objective isn't to transfer power to the oscilloscope, but voltage. en.wikipedia.org/wiki/Impedance_bridging – Phil Frost Oct 12 '18 at 12:51
• @PhilFrost Depends completely how you think about it, result is the same. For impedance bridging, you could calculate the voltage over the 50 ohm impedance of the antenna and arrive at same tiny result on oscilloscope. But usually in radio receivers, the goal is to use all the received power - and oscilloscope is not a radio receiver :) – jpa Oct 12 '18 at 13:30
• My point exactly: an oscilloscope is not a radio receiver. So why bring it up? The question asks specifically about using an oscilloscope, and measuring a voltage. The higher the scope impedance, the better. You say it's a problem but it's not. – Phil Frost Oct 12 '18 at 13:37
• @PhilFrost Maybe I understood the question wrong then. To me "If I connect .. antenna to my oscilloscope" sounds like connecting it directly, without any amplifier in between. Then it would be a case of trying to use an oscilloscope as a radio receiver, and my answer explains why it will not work. An oscilloscope with 50 ohm input impedance (they do exist) would work much better as a radio receiver. – jpa Oct 12 '18 at 13:46
• The typical minimal scale of modern scopes is around 1mV/div so even with an 8bit one you would see .3mV and with 10 or 12 bit ones definetly. The problem here is noise, it will be in the same order of magnitude or worse. – PlasmaHH Oct 12 '18 at 15:01