One of the interesting things you can do with a wideband loop antenna is listen to 630 meters ham radio activity. So, what the heck is 630 meters?
In the origin days of ham radio, more than a century ago, all we had for frequencies were the long and medium waves, below 3 MHz. The 630 meters frequencies are from 472 to 479 kHz, just below the AM broadcast band. Starting in 1906, frequencies in this range were used for maritime distress communications. Unfortunately, lots of interference took place between amateur and commercial operations.
Starting in 1912, hams were banned to higher frequencies above 1.8 MHz. We all know this was great for discovering HF radio. But it was more than a century before amateur radio returned to medium waves. For the past six years, hams have been able to operate with low power in the 630 meters ham band again.
Few actually do, but activity is growing and quite interesting. Signal modes are mostly digital, including Morse Code, but mainly low power digital modes. Recently, we have been talking about weak signal communications software. That’s the place to start if you want to hear 630 meters ham radio using your magnetic loop antenna.
Begin by running WSJT-X software and tune to 474.2 kHz USB. Most days, in this passband, you will find FT8, JT9 and finally, WSPR digital signals. Very slow and very low level. You will see some traces in your waterfall display. And the software will decode them just fine.
Just like nearby non-directional beacons (NDB), most long distance results will be found in the hours around sunset and sunrise.
630 meters Ham Radio – Antenna Challenges
While magnetic loop antennas are great for receiving 630 meters ham signals, transmitting is another story. Keep in mind these are extremely long wavelengths. A half-wave dipole would be over 1,000 feet long and your quarter wavelength vertical would top 500 feet. Not practical.
Most hams get on this band with an inverted-L antenna strung around their property, fed against a lot of radials. You would be lucky to achieve 1% efficiency to reach your maximum radiated power of 1 to 5 watts. If you can make a really big tuning capacitor, a magnetic loop would work as well. Either way, watch out for high voltages.
You should do some listening before thinking about building a transmit antenna for this band, and a wideband magnetic loop is a great starter.