LF (Low Frequency) - 2200m Amateur Band

Created: 29 July 2012

This band is a reasonably new allocation in Australia.
(image stolen from VK2XV)

Regulations limit the radiated power output to 1W EIPR, but due to antenna inefficiencies it takes 100s of Watts to get to this limit.


My transmitter is based on a design by DF3LP, documented here. My design does away with the exciter, and instead uses a DDS (AD9850, using a cheap PCB bought on eBay) and a micro (Atmel atMega8).

The PA was adapted from the DF3LP design:
  • The 3mH inductor was made using 37 turns on an FT140-77 toroid.
  • The 13.5uH inductor was made using 34 turns on a T200-2 toroid.
The  13.5uH inductor needed some adjustment. On examination, the MOSFET output (orange) appeared to be switching too late - it should switch when the voltage has reached zero. 
The fix was to add 5 turns to increase the inductor to 18uH. See the before and after traces:

Interestingly others have found the same need - such as VK1SV

My transmitter is built into an old transceiver case:

  • It has an optical serial connection (which is another project in itself!). This provides full RS-232 connectivity, including control lines, over TOSLINK optical cables. This permits electrical isolation from the controlling PC.
  • There is a 1PPS (pulse per second) input to allow connectivity of a GPS for frequency calibration (yet to be fully implemented).
  • It is powered from 13.8V. 
  • There is an antenna connection as well as a receiver connection - the transmitter includes a transmit/receive changeover relay.
At this stage the control firmware is minimal. It permits setting of the frequency and on/off control, but has yet to have any modulation modes (such as CW). Testing to date has used PC software that keyed the transmitter using the RS232 control lines.

Output is about 45W at 75% efficiency.

Loading Coil

LF antennas are very short. Mine is 40m long - only 1.8% of a wavelength! To correct for this a loading coil is required to add inductance to resonate the antenna.
My loading coil is made form twisted-pairs recovered from CAT-5 network cable. 60m of cable gave 240m of twisted pairs. This was basket wound onto a former made of wood and conduit. Regular taps are made of the coil, and connected to banana sockets.Inside the main coil is the variometer coil which allows fine adjustment of the inductance. Originally the variometer coil was wound in the reverse direction to the main coil. This needed to be fixed.


The final unit provides a variometer range of 9.35mH to 10.60mH. Overall inductance can be reduced by shorting out the taps.

More details to come:
  • Receiver (Softrock SDR based)
  • Scopematch device
  • RF Amp-meter
  • Matching Transformer
  • Antenna
  • Results