Comments on Solar Radio Astronomy

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p_zetner
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Comments on Solar Radio Astronomy

Post by p_zetner »

Greetings All.

There's been some interest expressed in solar radio astronomy so I thought I'd throw in my two cents' worth. Many of you probably realize that there is an entire chapter (Chapter 13) dedicated to solar radio astronomy in Christian's book "Solar Astronomy: Observing, imaging and studying the Sun". I take responsibility for this chapter since what started out as an exercise in French to English translation ended up in a complete revision and rewrite of the chapter! There were quite a number of errors in the original French publication and the entire analysis of the Lucie radio telescope measurement was incorrect. I went through the original data and re-analyzed it correctly so the newly written English chapter is (hopefully!) pretty free from error. The equipment part of the chapter is almost solely devoted to the Lucie type of radio telescope, a repurposed satellite dish operating in the microwave portion of the spectrum. There are, however, a number of other radio observing techniques in use by amateurs.

My originally submitted version of Chapter 13 was quite a bit more extensive since I am interested in observing the Sun at much lower radio frequencies (tens of MHz) than the Lucie experiment (~ 11 GHz), largely because I expect there to be more radio action there. However, publication limitations meant that this extra discussion of lower frequency, longer wavelength techniques had to be cut. I thought I would upload the unpublished part of the chapter here. It might make a useful starting point for someone interested in looking at the Sun's radio emissions and it shows the direction I'm taking in my own radio studies. I've tried to cover the generalities as well as some of the specific technical details which can help an amateur to set up a radio frequency observatory. There are a number of references included, most of them still available online. If someone needs a pdf document not currently available online, please let me know and I can send it to you or upload it here as well.

The unpublished document is a bit "rough around the edges" but is pretty self contained with some references to the other sections of the "Solar Astronomy" book. There is a lot of disparate information on the web so I thought this might point the way to some fruitful project ideas. Someday, I may work it up into a better manuscript but have no immediate plans.

Cheers.
Peter


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Re: Comments on Solar Radio Astronomy

Post by Montana »

What a gem Peter :movie

I'll put a copy in the library too :)

Alexandra


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Re: Comments on Solar Radio Astronomy

Post by rsfoto »

Hi Peter,

Very interesting paper and will need to read it a few times.

As you possibly have read in another post I have a lightning station running in the Observatory and here I used Loop antennas.

Here https://frankfortweather.us/blitzblitz/ ... Recvd.html you can see in real time the signals my Lightning systems is capturing. The Big Magenta Wave is from time to time the impulse of my electric fence.

On this link you can see the real time and the FFT analysis. https://frankfortweather.us/blitzblitz/ ... vdFFT.html

As you can see the whole system detects at VLF range between 0 and 240kHz. Having 2 loop antennas as well as an E-Field probe.

Any information somewhere about how to calculate a Loop antenna for a specific frequency ?

Why are loop antennas not common in this cases. They are small and so require just a tiny space.

regards Rainer


regards Rainer

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Re: Comments on Solar Radio Astronomy

Post by marktownley »

Very good! Thanks all!


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Re: Comments on Solar Radio Astronomy

Post by p_zetner »

Hi Rainer.

Thanks for your comments.
"Any information somewhere about how to calculate a Loop antenna for a specific frequency?"
"Why are loop antennas not common in this cases. They are small and so require just a tiny space."
Loop antennas are specified by the number of wavelengths that fit into the circumference of the loop. For lower frequencies, tens of MHz and lower, the wavelength (lambda) is fairly large, requiring a large loop to satisfy a resonance condition like 1*lambda. Also, such large loops are often used with complicated "feed" arrangements to give a condition of constant current flow over the loop. Furthermore, the antenna pattern for a large loop can become quite complicated, especially when the presence of the ground is taken into consideration. That's true for long wire dipoles too, though, that are high off the ground.

Smaller loops (< lamdba / 10) are useful because they aren't resonant and they are easy to construct and mount. They have simple antenna patterns. Their efficiency (more specifically, their effective aperture) is small, meaning the ratio of incident electromagnetic power to electrical signal at the feedpoints is small...not ideal for radio astronomy. They can be made resonant by putting a capacitor in parallel, forming a resonant LC circuit, thereby improving the efficiency at the resonant wavelength. I've found a couple of papers which use this idea in conjunction with the Radio Jove receiver to observe Jupiter emissions around 20 MHz:

1. L. Laniak et.al. (2008) "The Observations of the Radio Storms on Jupiter with the Use of the SISI Radio-Telescope". They give a formula there to calculate the inductance, L, of the loop antenna so that, with the right value of C, resonance can be arranged to take place around 20MHz.

https://www.researchgate.net/profile/Ra ... escope.pdf

2. Hamilton David Galvis Rodríguez et.al. (2016) "Development of a Magnetic Loop Antenna for the Detection of Jovian Radiowaves at 20.1 MHz". They use an interesting double loop antenna, also with a capacitor to tune to resonance at 20 MHz.

https://www.researchgate.net/profile/Ed ... ion_detail

Both of these papers show some results attributed to Jovian noise storms.

An advantage of loops is that they respond primarily to the magnetic field, giving high rejection of nearby electric fields. This might be important near interfering sources (TVs, fluorescent lamps, mains wiring etc.) which radiate noise primarily in the electric field.

The poor efficiency of the small loop is sometimes improved using an active circuit (balun+LNA) to boost signal near the antenna feedpoints. A commercially available antenna is based on this type of active amplification for broadband operation between 20 kHz and 30 MHz:

https://www.wellbrook.uk.com/loopantennas/ALA1530AL

I ultimately chose the inverted V dipole of the Ellingson type because it's quite simple to construct and its operation is well characterized. It's also designed for the 29 - 47 MHz band which seems to be a little less polluted by radio interferences. Whether this is actually true at my site remains to be seen!

Cheers.
Peter


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Re: Comments on Solar Radio Astronomy

Post by rsfoto »

Hi Peter,

Thank you. I was thinking in small loop antennas. Look for example at my two antennas with a diameter of 38cm and wound with 20 turns of AWG 18 wire ~ 0.7mm Ø for VLF frequencies at around 30kHz


Image


How is the calculation done here ?

With a 38cm diameter the circunference is about ~ 1.194 m and 20 turns gives me a length of 23.88m.

The wavelength of 30 Khz is 10.000 meters and dividing that by ~ 24 gives me 1/418 of the wavelength ...

Need to continue to investigate :?


regards Rainer

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Re: Comments on Solar Radio Astronomy

Post by p_zetner »

Hi Rainer.

Your loop is ~ lambda / 8.38. It’s only the circumference of a single loop that matters. This would be considered a “small loop” with a null in the antenna pattern along the coil axis and maximum in the antenna pattern in the plane of the loop.
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Re: Comments on Solar Radio Astronomy

Post by Erwinux »

Hi all,
Thanks, Peter, for the chapter.

Rainer,
You can study your loop antenna characteristics using a nanoVNA. You will find the impedance and the resonant frequency. I'm not a specialist but I know this is basically an air core coil, so the number of loops will increase the impedance and lower the resonant frequency. Therefore, one can use basic math for coils calculations.


panta rhei
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