Recent Radio and Radio Astronomy Projects
by Dave Benham
Tom Hagen and I have been working with LF, VLF and ELF-ULF for the past year. Imagine our surprise when we learned that some of this activity qualifies as radio astronomy! Here’s what we’ve been doing:
To begin with, what are LF, VLF AND ELF-ULF? Quite simply, they are descriptors for ranges of radio frequencies. See the table below.
|ELF (extremely low freq)||3Hz to 30Hz||100,000km to 10,000 km|
|SLF (superlow frequency)||30Hz to 300Hz||10,000km to 1,000km|
|ULF (ultralow frequency)||300Hz to 3000Hz||1,000km to 100km|
|VLF (very low frequency)||3kHz to 30kHz||100km to 10km|
|LF (low frequency)||30kHz to 300kHz||10km to 1km|
|MF (medium frequency)||300kHz to 3000kHz||1km to 100m|
|HF (high frequency)||3MHz to 30MHz||100m to 10m|
|VHF (very high frequency)||30MHz to 300MHz||10m to 1m|
|UHF (ultrahigh frequency)||300MHz to 3000MHz||1m to 10cm|
|SHF (superhigh frequency)||3GHz to 30GHz||10cm to 1cm|
|EHF (extremely high freq)||30GHz to 300GHz||1cm to 1mm|
As a frame of reference, all the activities discussed here occur below the frequency of the AM broadcast band.
On LF, we were using a Rycom 6040 “selective level voltmeter” (i.e., receiver) to receive NDBs, non-directional beacons used for navigation at airports around North America in the general frequency range of 190 KHz to 530 KHz. We built various tunable loops for antennas. They are built to a certain number of turns, with equal wire spacing, and tuned with variable capacitors. In the end, the best overall antenna was a commercially built Palomar wirewound ferrite rod loop. It’s tunable, directional and mounted on a gimbal mount which can help in eliminating ambient electrically generated noise. Surprisingly, we heard stations with as little as 25 watts output from British Columbia to Central America, to above the Arctic Circle (see Fig. 1 for a map of stations heard). Although this activity is not directly related to astronomy, it served to give us experience with our loop antenna builds and receiving equipment.
Figure 1: LF Non-Directional Beacons
On VLF, we have been using two receivers that we built to monitor US Navy submarine communications stations in the 20 KHz to 40 KHz frequency range. Surprisingly, this qualifies as radio astronomy. Again, we made our own wirewound loops for these receivers, some as small as 16” square to as large as a 5’ span hex shape. The receivers are called the Gyrator II (Fig. 2) and the Gyrator III (Fig. 3) which are relatively simple to build devices. We are also experimenting with the Rycom 6040 (Fig. 4) and another Wandel and Goltermann selective level voltmeter and a McKay-Dymek DR55. We have received transmissions from Hawaii, Washington State, North Dakota and Cutler, Maine.
In the 1950s, scientists at McMath Hulbert used a SEA receiver to monitor the noise level at 27 KHz (or thereabouts). Often, when a solar flare occurred the atmospheric noise at this frequency became enhanced, hence the moniker, “SEA”, or Sudden Enhancements of Atmospherics, and this noise level was plotted. See Fig. 5 for an example of charting done in the 1950s; see Fig. 6 and 7 for the SEA receiver.
Today, VLF monitoring of these naval stations qualifies as radio astronomy because during the daytime, the ionosphere can become disturbed by solar activity. These disturbances are called SIDs or Sudden Ionospheric Disturbances. The ionosphere is comprised of, roughly, 3 layers – the F (highest), E (middle) and D the lowest. Normally during a quiet solar day, the VLF signals go through the D layer to the higher mid-level E layer, then lose energy as they pass back through the lower D layer on their way back to earth. SIDs are caused by solar activity ionizing (energizing) all layers of the ionosphere. The normally less active, and lower, D layer of the ionosphere is energized to the point that it becomes the daytime bounce point for VLF signals rather than the E layer. Signals bounce cleanly off the D layer during the SID, increasing the received signal strength on the monitor. When we monitor, the signal strength is plotted through a DATAQ analog-to-digital converter to a computer. The resultant plots can be analyzed for spikes during the daytime for this solar activity (Fig. 8 is an example of a daily plot) and compared to the GOES satellite data.
Our monitoring provides an indirect method of quantifying SIDs, similar to what was done in the 1950s when looking for “SEA”. See Fig. 9 for a photo of one of the loops we use for this monitoring. Fig. 10 shows the Standard Signal Generator 1001-A which we used for tuning loop antennas and building/aligning the Gyrator receivers. In the 1950s, this generator was used in the McMath-Hulbert electronics lab. Amazingly, this device is still spot on with its alignment after all these years.
Figure 2: Gyrator II receiver
Figure 3: Gyrator III receiver
Figure 4: Rycom 6040 used for LF and VLF monitoring
Figure 5: Paper chart recording done in the 1950s. The upper plot is of
18 MHz and the lower plot is of 27 KHz (marked “27 KC level”).
Figure 6: SEA receiver used in the 1950s
Figure 7: Top view of SEA receiver
Figure 8: Daily recording of NAA on 24 KHz
Figure 9: Loop antenna used for VLF monitoring
Figure 10: Standard Signal Generator 1001-A used for loop tuning and receiver builds
We have recently become approved to be a SuperSID monitoring station in the SuperSID Project which is run by Stanford University and SARA (Society of Amateur Radio Astronomers). We have received our SuperSID radio, 96 KHz sound card, software and materials to make a loop with 400’ of wire. Ken Redcap procured a computer for us, which will be dedicated to this project. This project will serve to supply Stanford with daily data as well as educate schoolchildren. Fig. 11 shows our first full-day plots of four US Naval radio stations.
Figure 11: First full-day plot of stations NAA in Maine, NLK in Washington, NPM in Hawaii and NML in North Dakota
On ELF through ULF, Tom Hagen built what is called a “natural” radio. This is a simple, easy-to-build receiver that receives electromagnetic energy (radio waves) in a frequency range that is typically attributed to audio frequency. This radio will receive atmospheric phenomena (“sferics”) with odd names like “whistler”, “tweak” and “click” which are generated by lightning strikes around the planet and propogated via a natural waveguide between the ionosphere and the Earth. Clicks are quite common. Tweaks are the next most common and whistlers are quite rare. The captured sound files are converted to sonograms by computer software for analysis (see Fig. 12 and 13).
Our radio is mounted away from the MHO buildings (see Fig. 14) in order to escape the electrically generated noise to which these radios are quite sensitive. This radio is powered by a gel cell battery which is continually recharged by a solar cell. We intend to run the audio output of this radio from our remote location back to the office area of Tower 2 where we can save it to a computer for playback and analysis. Our eventual goal is to hear the Schumann Resonance, which is an approximate 8 Hz signal that represents the resonant frequency of the Earth.
Figure 12: Sonogram of a tweak
Figure 13: Sonogram of a whistler
Figure 14: Tom Hagen holding audio output speaker of the "natural" radio
Possible future radio astronomy projects are:
- Using a small satellite TV dish to receive SID signals directly from the Sun
- Monitoring 18 MHz – 20 MHz for “space noise” from Jupiter, duplicating a project done in the 1950s here at McMath Hulbert
- Rejuvenating the SEA receiver and using it to collect data
- Becoming an AWESOME receiver site for Stanford