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New to this community and also a newish Ham. Hello everyone.

This question could easily have been posted under 'Astronomy', but I thought I would start here since Hams tend to be quite knowledgeable about many things that have to do with radio.

I am interested in building a backyard radio telescope. The radio window that allows in radio frequencies through the Earth's atmosphere have wavelengths between about 0.5 cm to about 30 m. If I got this right, a quarter wavelength receiver would need to be about 7.5 m in length.

I have seen the radio dishes that SETI uses and I can tell you right now that my HOA will not allow any sort of radio dish in the backyard, especially one that is 7.5 m in diameter. As part of my amateur license, I learned about radio antennas for transmitting and receiving.

Question 1: Is it possible to use a radio antenna like a Yagi to pick up interstellar emissions between 0.5 cm and 30 m with any kind of sensitivity?

Question 2: What about using an antenna like the one in the image? Would this enhance sensitivity? Would this antenna still function if it was laid flat on a field?

Radar Antenna

I can't suspend any antennas in the back yard, and I can't place any radio dishes. However, I can place quite a large antenna installation as long as it is at ground level and not readily visible from the road. This fixed installation would receive signals as the Earth rotated. I imagine that with the relatively clear view we have from the backyard, the right antenna installation could pick up emissions from a large part of the sky at the same time. I would be sacrificing the ability to pinpoint where the emission was coming from, but gaining the ability to monitor a large part of the sky for a relatively long period of time. Am I right with this train of thought?

My plan is to connect (several?) SDRs to the antenna and monitor frequencies between 10 MHz and 100 GHz. I would also have a dedicated SDR that would automatically tune to any areas that have a high power signal. So one group of SDRs would focus on monitoring the entire frequency range, while one SDR would serve as the fine tuned receiver.

More details to come (with probably a million questions). Thanks for reading this long post.

-- Edited on Feb 3 --

Thanks to some of the answers and comments below, I was inspired to look in a different direction. Ideally, I am looking for an antenna that offers a broad bandwidth so that I can pick up on as much of the cosmos as possible. In my reading, I came across biconical antennas.

Biconical antennas Biconical antennas are a modified type of dipole where the two elements form a roughly conical shape. This change allows them to have a wider bandwidth versus a regular dipole. The cones used on these are rarely solid and are often made of multiple elements, making them easier to fold or transport. Their broadband nature allows quick testing without having to adjust or change the antenna. They are linearly polarized and typically work in frequency from 20 MHz to 300 MHz, but when designed for it, they can work as high as 18 GHz.

The frequency response for biconical antennas is pretty close to the radio window (which is about 10 MHz to 60 GHz, but more practically 10 MHz to 6 GHz).

So the answer to question 1 is possibly yes using a biconical antenna. Am I understanding this correctly before I go too far down this rabbit hole?

-- Edit #2 on Feb 3 --

In a different thread I asked a question related to the feasibility of building a biconical antenna and putting it in my attic. Based off feedback from @Phil Frost - W8II, it looks like that won't work due to the amount of background noise it would pick up - especially from terrestrial sources.

I'm back to looking at directional antennas again. Ultimately, I'm looking for what is beginning to sound like a pipe dream - a wide bandwidth antenna that will pick up stellar emissions but filter out terrestrial noise.

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    $\begingroup$ Welcome to hamSE! Please tell us more about the antenna in the picture. This large array and dish antennas can be very directional. Is this what you want? I read that radio astronomy installations tend to focus on specific wavelengths, e.g. 21-cm for hydrogen, rather than "DC to daylight." What do you hope to observe? $\endgroup$
    – Brian K1LI
    Commented Feb 2, 2021 at 16:25
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    $\begingroup$ 0.005 meters to 30 meters is a very large span...a single antenna can't efficiently manage this span. You might consider concentrating on a smaller span for antenna, even though SDR can manage a larger frequency range. $\endgroup$
    – glen_geek
    Commented Feb 2, 2021 at 17:01
  • $\begingroup$ Hello! Thank you for your comments! My intention is not to have a directional antenna. Ideally, I want something lying flat on the ground and capable of receiving radio transmissions from any part of the sky... and for the huge frequency span I mentioned. I'm ignorant of the challenges involved and am very interested in learning more. @glen_geek - why can't a single antenna manage this span? $\endgroup$
    – FontFamily
    Commented Feb 2, 2021 at 17:06
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    $\begingroup$ You might look at Nasa's radio Jove project. They outline a backyard antenna that covers the low-frequency end of your range (20M) radiojove.gsfc.nasa.gov/telescope $\endgroup$
    – glen_geek
    Commented Feb 2, 2021 at 17:40
  • $\begingroup$ @glen_geek Thanks for the tip. NASA's Radio Jove project was a very interesting read. The 20M they choose appears to be along the "water hole" that picks up emissions from hydrogen. $\endgroup$
    – FontFamily
    Commented Feb 3, 2021 at 15:26

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Ground-mounted horn antennas seem a popular choice for amateur radio astronomy.

enter image description here

"Track the Movement of the Milky Way With This DIY Radio Telescope" on the IEEE Spectrum website and the DSPIRA Guided Tour on the Digital Signal Processing in Radio Astronomy (DSPIRA) portal describe how to build and use ground-mounted horn antennae for frequencies around 1420-MHz.

enter image description here

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  • $\begingroup$ This is a really interesting concept and I saw this in some of my early research. I elected not to go with this approach since these antennas appear to be highly directional and also responsive along a narrow frequency range. I'm curious about whether this can somehow be adapted to fit my needs. $\endgroup$
    – FontFamily
    Commented Feb 3, 2021 at 15:27
  • $\begingroup$ I edited my original question to include more details. What do you think about biconical antennas? $\endgroup$
    – FontFamily
    Commented Feb 3, 2021 at 15:57
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I imagine that with the relatively clear view we have from the backyard, the right antenna installation could pick up emissions from a large part of the sky at the same time. I would be sacrificing the ability to pinpoint where the emission was coming from, but gaining the ability to monitor a large part of the sky for a relatively long period of time. Am I right with this train of thought?

Not really. The sky is full of noise sources, and the Earth is full of even more. The point of a directional antenna isn't just knowing where a signal is coming from, it's excluding everything else, to lower the noise level to the point where you can actually make out a particular signal. Otherwise, it's a lot like standing down the hall from a crowded party — there are lots of voices, but you can't understand any of them, you just hear a roar.

You can get some directionality even with antennas that are relatively small (and in any case probably most of what you're interested in will be wavelengths of 2 meters or less, which makes the problem much easier). I'll let other answers deal with that.

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  • $\begingroup$ This is a really good point and one that I considered in thinking about the design of this radio telescope. My goal is to record as much of the sky around me 24/7/365. I am sacrificing localizability in order to hear just that roar. The idea is to then use signal processing and machine learning to dampen the background noise. Interesting signals would be flagged. Theoretically, if more than one SDR was used and there was more than one array, I could narrow the source down somewhat (?). $\endgroup$
    – FontFamily
    Commented Feb 3, 2021 at 15:31
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    $\begingroup$ Theoretically, that's right. You're referring to a technique known as (very) long baseline interferometry. I don't know what software is available for amateur radio astronomers. $\endgroup$
    – Brian K1LI
    Commented Feb 3, 2021 at 16:50
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Crossed-dipole style antennas, sometimes used by hams for satellite reception, might fit your requirements. Their low directivity obviates the need for azimuth and/or elevation rotators to keep them pointed at their targets.

Examples are described in, "Double Cross — A NOAA Satellite Downlink Antenna" by Gerald Martes, KD6JDJ,

enter image description here

and "The Lindenblad: The Ultimate Satellite Omni Antenna," by Howard Sodja, W6SHP.

enter image description here

While the published articles on the Lindenblad antenna orient the folded-dipole elements 30-degrees from the horizon, I found through NEC-2 modeling that increasing the angle to 70-degrees dramatically reduced the overhead null, which might make the antenna more useful in your application.

Analysis with NEC-2 modeling software indicates that either antenna could be useable over at least a 2:1 range of frequencies. These antennas are not trivial for a beginner to construct, but you might garner assistance from members of your local ham radio club, which might also be fertile ground for recruiting budding radio astronomers!

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  • $\begingroup$ WOW. This antenna design is fascinating - not just for my radio telescope application but also for a home antenna for the Ham. This solution might just work. I can't have any obvious antenna installations on the property, but I read in the second article that there was an installation done in the attic. Very interesting. $\endgroup$
    – FontFamily
    Commented Feb 3, 2021 at 15:39
  • $\begingroup$ Bummer - I was reading about the frequency range and it is based off the dipole length. It's a really neat design but it appears that this antenna type operates only within a narrow frequency range. $\endgroup$
    – FontFamily
    Commented Feb 3, 2021 at 16:13
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    $\begingroup$ The frequency range has two aspects: SWR and pattern. Yes, the SWR range may not be very great, but you won't be transmitting, so that doesn't matter very much. I wrote in my answer that the antenna should be useful over about a 2:1 frequency range, by which I was referring to the pattern. The pattern's not constant over the range, but useable. $\endgroup$
    – Brian K1LI
    Commented Feb 3, 2021 at 16:48

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