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Small Loop Antenna: Background and Construction

 

Figure 1: The upright, crosspiece, and notched end pieces to hold the 125 turns of magnet wire in place.

Figure 2: A close-up of one of the end pieces.

 

 

 

 

 

 

 

Figure 3: Assembling the loop.

Figure 4: The loop structure, after being screwed together.

 

 

 

 

 

 

 

Figure 5: Winding the loop with 24 awg enamel-coated magnet wire.

Figure 6: The small loop antenna, initial construction phase complete!

 

 

 

 

 

 

 

The electromagnetic spectrum is our window to the cosmos – our eyes and optical telescopes can detect visible light, but that is only a tiny fraction of the whole spectrum. Higher frequency than visible light is ultraviolet, X-rays, and gamma rays; these types of high-energy radiation certainly provide interesting insight into our universe, but to observe in this frequency range requires complicated equipment that most amateurs don’t have access to. However, if you look toward the lower-frequency end of the light spectrum, there is infrared, microwave, and radio radiation; of these types, radio is perhaps the easiest – and, I think, the most fun – part of the spectrum for amateur astronomers and technology enthusiasts to explore for themselves. The radio end of the spectrum is full of opportunities not just for telescope (antenna) design, but also for studying things about outer space which can’t be studied with other kinds of telescopes. Sudden Ionospheric Disturbances (SID’s), caused by solar activity, are just one of the many interesting phenomena you can detect from your backyard with simple, homemade receiving equipment, and this is thanks to a neat coincidence of nature and technology.

An interesting property of radio waves is that the longer the wavelength, the further the waves propagate, or travel, through the atmosphere. This is very useful for communication; a familiar application of this is AM and FM radio stations, which are broadcast to a wide area. A less commonplace use for this propagation property of radio waves is in submarine communications, which are conducted on Very Low Frequency (VLF) stations. These stations propagate much farther than the typical AM/FM stations do – in fact, they are known to travel from 5000 to 20000 km. Thus, even if we are far away from a VLF station, we can receive its signal and monitor its strength, which is a necessity for monitoring SID events. So, why would we want to listen to submarine communications if what we are really trying to do is monitor solar activity? Well, VLF waves produced on Earth are prevented from escaping the Earth’s atmosphere by reflecting off of the ionosphere, and this reflection is highly dependent on the level of ionization in the ionosphere. When X-ray or UV radiation from solar wind or a solar flare strikes the ionosphere, the disturbance increases ionization and thus enhances the propagation of the VLF station’s signal, which we detect as an increase in the strength of the signal. In other words, by monitoring changes in the strength of the signal from a submarine station over time, we can figure out when solar activity has occurred.

I decided to focus my attention on trying to monitor the signal from the 24 kHz communication station in Cutler, Maine, simply because that is the nearest station to me. To detect such a low frequency signal, you will need to construct a Small Loop Antenna.

A small loop antenna consists of several turns of wire wound around a small frame a few feet in diameter. When insulated wire is wound into a coil, it acts as an inductor. Magnetic fields caused by radio waves near the detector induce a current in the inductor, so by monitoring changes in the voltage detected by the antenna, you can observe changes in the radio waves you are receiving. In addition to the inductor – which is the antenna – there are two other main hardware components you will require for SID observing; an amplifier – to strengthen weak signals so that they are detectable – and a tuner, to make sure you are getting optimum performance at the frequency of the VLF station you choose to monitor. Once you have your electronics hardware setup in place, the final piece of the puzzle is the software and signal processing phase – this is how you convert your analog signal of wiggling electrons into meaningful information and graphs, and contribute the data you gather to research groups which will add your observations to their data collections so that they – and you – can learn more about the Sun and the ionosphere.

Now that you know what a loop antenna is and what it’s used for, we can move on to the actual construction and implementation process! In this blog post, I will walk you through the steps I took to construct the loop antenna itself. In later posts, I will add discussions of the more complicated electronics of tuners and amplifiers, and finally move on to software and analyzing the science results you get from your observations.

I constructed the loop antenna itself from wood scraps I rooted out of my dad’s supply of deckbuilding material (this was not exactly a high-budget operation). As you can see from Figure 1, I measured an upright post 1.5”x1.5”x23”, a horizontal post 1.5”x1.5”x21”, and 4 notched end-things to hold the wire in place 1.5”x1.5”x3”. “Notched end-things” is not an extremely     helpful descriptor, so you should look at Figure 2 to see what that means. Assembly was pretty straightforward overall; really, the pictures are much better help than any words I could say here.

After that part of the structure was completed, I started on what was probably the most tedious part of the initial construction process; I wound the loop with 125 turns of 24 awg magnet wire (available online; I found it difficult to locate the required length of wire from any local retailers). I found it more convenient to wind the loop *before* mounting it to the baseboard, because that allowed me to spin the loop around like a top so I could wind more quickly. (I wouldn’t advise doing that on a nice countertop!)

Finally, I (actually, my dad) prepared the baseboard for the antenna; we cut a piece of 1” thick board into a 21”x21” square, then lopped off the corners. The mitered corners are (obviously) not necessary to the function of anything; I just didn’t want to impale myself or anyone else with a sharp corner when carrying the antenna around. The antenna is attached to the baseboard with L-brackets sized appropriately to the width of the upright; I used four brackets – one for each side – but you could probably get along perfectly well with two brackets on opposing sides. Once you’ve done that, you’re pretty well finished with the initial construction process!

Check back soon … my next post will outline in detail the electronics portion of the project.