630 Meter Project

I have been interested in getting on the 630 Meter (472-479 KHz) band ever since the FCC released it to us in 2017.  It is a challenging band to get on.  A lot of things you take for granted on HF are wrong on this band. It has been a long time since hams had access to these frequencies. We were running spark gap transmitters then. Technology has improved a little bit since then.

This is my first step in getting on the new band. It is a home brew WSPR beacon transmitter. It puts out 12 Watts.

The core is an Arduino microcontroller board driving an Adafruit Si5351 frequency synthesizer module. The Arduino actually controls the Si5351 frequency output to generate the WSPR signal.  The output of the Si5351 goes to a FET driver that drives an IRF510 general purpose FET running class D.  Most of the similar designs on the Internet drove the FET with CMOS logic chips. The key to efficiency and reliability with switched FETs at high frequency is a solid gate drive. I felt the gate drive circuitry on most of the published designs were a bit soft. I bought a tube of the IRF510s figuring I would blow up a lot of them while getting the circuit going. I’m still using the first one! Not bad for a $1 output final transistor!

The remaining inductors and capacitors transform the FET output to 50 ohms and filter out the harmonics to meet the FCC -43dBc regulations.

The Arduino software was found on Github and written by NT7S. I made some changes. The original software only covered the HF bands so I had to make changes so the Si5351 operates at 475 KHz. The original version used a GPS module to control transmit timing. I changed it to accept an external signal from a PC running WSJT. The PC USB-serial port RTS line is normally used to drive the transmitter PTT. This signal is used by the Arduino to start a new transmission sequence.

Beacon information like call, grid square, and power output are hard coded in the program.

The current antenna is an inverted L. The vertical part is about 40’ high and the horizontal part goes about 45’. A full sized inverted L for this band would be about 510’ long. It obviously needs a lot of loading. The ground system is lacking as well and the antenna is very inefficient. I estimate I have an ERP of about 100 mW. 

Variometer. This is a variable inductor.  The internal coil rotates and its field either adds or subtracts from the inductance from the outer windings. The main form is a plastic five gallon bucket and the internal assembly is made from PVC tubing. The variometer provides the extra inductance required to resonate the short wire.

The variometer is a variable inductor that was very popular in the early days of radio. It has found new life on the MF and LF bands.  This one was built around a 5 gallon plastic bucket.  It has a range from about 960 to 1240 uH.  I built it with lots of extra inductance along with taps.  Some of the inductance was removed during tuning by shorting turns out. 

I used a Rig Expert AA-30-Zero VNA to tune the antenna. It is a very good deal (~$85) for a VNA that covers 100 KHz to 30 MHz.  Most antenna analyzers do not go low enough for 630 meters, and the null is way too sharp to tune with an SWR bridge.  My antenna has a lower Q than I would like and barely covers the 7 KHz bandwidth with an SWR less than 2:1.

The system was given its first extended work out the night of December 22-23, 2018. The beacon was picked up by 54 different stations in 21 states plus VE3, VE6, and ZF1.  The most distant station was in California at a distance of about 1700 miles.

Tuning the 630M inverted L antenna. The VNA and variometer made tuning to resonance easy.