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Small Loop Antenna: Electronics

Figure 1: the “Ham it Up” upconverter, connected to the NooElec dongle and the coax from the antenna.

Figure 2: the tuning/amplifying circuit, attached to the antenna and a coax which connects to the upconverter.

 

 

 

 

 

 

 

 

Figure 3: A schematic for the tuning/amplifying circuit, drawn by Steve White

 

Figure 4: a screenshot from SDR#; the lines in the waterfall plot are VLF stations.

 

 

 

 

 

 

 

 

With the woodwork and physical labor part finished, the next step of the antenna-building process was to work out how to make sure that it was capable of receiving the faint, low-frequency submarine communication signals and transferring those signals to the radio software on my laptop. Setting up this connection was interesting, but to me quite confusing due to my very limited electronics know-how, so I will try to retrace all my steps here for those of you who might find it helpful when constructing your own loop antenna.

Before we get started, a quick note on software: for testing purposes, I found SDR# a very simple and easy-to-manipulate program; although for later data processing GNURadio is more useful, I would recommend you install SDR# just to determine that your antenna is working properly. Here is the SDR# install page: http://airspy.com/download/

And here is a page describing how to run SDR# with the equipment we will be using (scroll down to the heading “SDR# (SDRSharp) Set Up Guide”): http://www.rtl-sdr.com/rtl-sdr-quick-start-guide/

The first thing I bought – even before I started this project – was an RTL-SDR TV tuner (“dongle”) from NooElec, which is a simple USB plug-in software defined radio (see Wikipedia entry here). There is a female MCX socket on the dongle which allows you to connect an input lead from an antenna, so you would think that it was simple matter – just plug the antenna into the dongle, and you’re good to go! But, as it turns out, the dongle has a frequency range from 25 MHz to 1750 MHz, well above our desired frequency of 24 kHz for the Cutler, Maine VLF station. So, I had a few options – get an entirely new SDR with the appropriate frequency range, or figure out how to shift the signal I was receiving from my antenna to a frequency range which I could detect with my original dongle. After investigating the options available, I decided to go with the second route; I purchased a NooElec Ham It Up Upconverter (meant for Ham radio users, apparently), which was advertised to work for signals down to 200 kHz, but which I found works perfectly well at very low frequencies. (Note: if you decide to use this tool, be sure to order a male SMA to male MCX cable to connect the upconverter to the dongle, as well as a USB to USB-B cable to connect the upconverter to your laptop’s power supply).

It is a good idea to test that the upconverter is working before moving forward, because it is a crucial part of the setup; I did this by simply plugging it into the dongle and power supply (my laptop), running SDR#, and flipping the switch on the upconverter from “Passthrough” to “Upconvert”; a green light should turn on when you do this, and you should see a large spike appear on the SDR# plot at about 125 MHz; that is the Local Oscillator (LO) frequency. All of the VLF stations will show up a few tens of kHz above 125.1 MHz; and to deduce their actual frequency, simply subtract the frequency you observe from the frequency of the LO. To understand how this works, check out this article on local oscillators.

With the upconverter working, your dongle is now capable of receiving signals at a frequency low enough for our purposes. The next thing you must address is the fact that the signals we want to pick up are very weak – too weak to pick up by simply connecting the antenna to the upconverter and moving along. Therefore, it is important to optimize the signal your antenna is picking up by building a circuit to tune and amplify your radio signal. Being entirely new to hardware electronics, I was unfamiliar with how to build such a circuit, but fortunately Steve White – engineer at the Green Bank Observatory – was able to build one for me, and was kind enough to explain it to me and provide some schematic diagrams so that I – or any future builders – could try to construct one on their own later. Here is a link to a webpage with a schematic on it which provided some inspiration: http://www.vlf.it/easyloop/_easyloop.htm

Figure 3 shows a scanned copy of a drawing of the circuit Steve built. The signal is amplified by using operational amplifiers, a type of integrated circuit component well-described in the article here.

The tuning aspect of the circuit is slightly more complicated, but pretty interesting; so I don’t bore you with a long description here, I’ll give you a link to another helpful article. In short, tuned circuits optimize a system’s performance at a particular frequency, and are often called “LC” circuits – this is because inductors (L) and capacitors (C) work together to tune the signal. Since the antenna itself is a coil of insulated wire, it acts as an inductor, so all the circuit needed to tune the antenna were some carefully chosen capacitors. Steve gave a pretty good demonstration of how this actually works by running a function generator and plugging the antenna into a spectrum analyzer. When he changed the frequency of the test tone, we could see that as the frequency approached ~24 kHz, the peak of the signal was higher, and as the signal’s frequency got further away from the tuned frequency, the peak became weaker. If you build an LC circuit for your antenna, you should give this test a try; it is really neat to see for yourself!

Once this circuit was completed and tested, it was time to put together the full set-up – in the Jansky Lab at the Green Bank Observatory, we attached the ends of the antenna’s wires to the circuit, ran a coax cable from the circuit board to the upconverter, plugged the upconverter into its power supply and dongle and switched it on, then started up SDR#. Sure enough, we saw some peaks at 45 kHz and a faint line near 20 kHz – definitely something! When you get to this point, it is important to know how to make sure the signal is coming from outside your system and ensure that it is either: a) a radio station, or b) external interference. You want to rule out any effects from the circuit board, your computer, or harmonics from the Local Oscillator in your upconverter. It’s pretty simple to rule these effects out – just disconnect your antenna cable from your upconverter, and the signals you see (except for the Local Oscillator at 125.1 MHz) should disappear. If they don’t disappear, then they were probably harmonics from the LO, or some other artificial effect and are definitely not radio stations. Radio frequency interference (RFI) is a non-systematic effect which is a bit harder to rule out; it could be caused by something in your house or near your observing spot. The best way to identify RFI is to simply move your setup to a different location to see if the signal still appears; if it vanishes, it could have been caused by RFI.

When I took my antenna home to experiment with, I noticed that the signals I was picking up were pretty weak, which I didn’t understand because Steve’s circuit was certainly capable of amplifying the signals enough for them to be quite visible. I asked him about this, and he suggested it could be due to interfering effects from my laptop. I moved the antenna about 10 feet from my laptop (that is the width of my room), and the signals strengthened quite a bit. That was a relief! So, the moral of the story is, if you have your antenna close to your computer and are not picking anything up, try moving your antenna away from your PC.

Another strange thing that happened in the course of testing my antenna was the discovery that if I touched the wires of my antenna, all of the weaker signals I was picking up strengthened and jumped into view. Being a child of the 21st century, this completely astonished me – but my mom was not surprised, and said when she was a kid people held on to their TV antenna or wrapped foil around it to try to get better reception of the 6 o’clock news or whatever. I checked with Steve, and sure enough this is due to some kind of shielding effect. I wrapped my antenna with foil, which seemed to improve the signal (somewhat), and made the antenna look like a bit of an eyesore.

Anyway, the point of telling you about my experiences with troubleshooting and improving my antenna’s reception is not to bore you to death – I’m hoping that from these stories you’ll see that sometimes what seems like a major setback is actually just a bump in the road, and with a little persistence and creativity (and maybe some help from someone with more experience), it is almost always possible to isolate the problem and figure out a solution. That’s what engineering is all about!

Ok, I’ll leave you with that, and post again soon about the software and digital signal processing (DSP) part of our antenna pipeline. Stay tuned…