COMMERCIAL RIGS WITH LF/MF
While there aren’t any rigs available specificaly for LF/MF operations yet, there are some radios that can receive and/or transmit on LF/MF directly or with some modifications. The member testing each radio will be noted, and a set of more thorough LF/MF tests will be conducted.
- The Elecraft K3 and new K3S can receive on both LF and MF with decent sensitivity as tested by N4TLF. The K3 does need to have a newer synthesiser and a small filter modification, while the K3S can operate on LF and MF without changes. For LF/MF operation, the receive antenna needs to be connected to the Rx antenna port on the back of the radio, and that port needs to be selected with the “Rx Ant” option on the front of the rig. For transmitting, the xvtr output needs to be used, along with external amplifiers, as the xvtr output is only a low-level signal (0dBm?). Reception on a couple of K3 rigs has been used by the AMRAD/LCCC group. Some additional low-pass filtering to eliminate nearby AM broadcast signals would be an improvement.
- Ten-Tec Rx-320. Frank Gentges, K0BRA, came out with a modification to the Ten-Tec RX-320 (out of production) several years ago. It was a simple change of a front-end transformer, that allowed the RX-320 to work on LF/MF frequencies. This modification is documented here: rx320lfmod. N4TLF has verified that the RX-320 sensitivity seems good.
- The SoftRock series of SDR rigs do have an option for MF reception. The SoftRock Rx Ensemble II Receiver Kit ($67) has an option to receive from 180kHz to 3.0 MHz. Due to the limitations of using an Si570 as the local oscillator, it cannot tune below 180kHz without an external modification. This device has not been tested on LF/MF by an AMRAD member at this time.
- A Sony ICF-2002/7600D (out of production) was recently repaired and tested on LF/MF by N4TLF. It only tunes down to 150kHz, but its sensitivity in the LF and MF frequencies is rather good. It uses an internal ferrite rod antenna only, the external antenna connection will not work for LF & MF.
- Realistic DX-390 (out of production). coming…
- CountyComm GP5-SSB (currently available)($75). N4TLF has one of these on order. It covers LF and MF down to 150kHz. more info coming…
- SoftRock Ensemble II Receiver for LF. This an LF/MF variant of the standard Ensemble II receiver from Tony Parks at Five Dash. As designed, its lower-frequency limit is 180 kHz, and therefore cannot receive the 2200 meter band. However, the addition of one more dive-by-two flip flop would solve that issue. This receiver is reported to have good sensitivity by John Langridge, who uses it for his “MF Grabber” project. The Ensemble II for LF sells for $66 in kit form, or $97 assembled. AMRAD has not tested this unit at this time.
Many direct sampling SDRs can receive on LF or MF. There may be some overloading of the SDR ADC, however, due to nearby AM broadcast stations, requiring external low pass filtering.
- The Flex Radio 6000 Signature Series rigs have specifications that include LF and MF frequencies. Terry, N4TLF, has a Flex 6500, its receive LF/MF sensivity is good.
- The RF Space SDR-IQ (out of production) has been tested by Terry, N4TLF, and it directly receives LF an MF frequencies without any modifications. It uses USB of host connectivity. Several other RF Space models also cover LF and MF, down to 10 kHz reception!
- Our own AMRAD Charleston Rx1 SDR also works fine at LF and MF, as tested by N4TLF. It uses a Digilent Nexys2 FPGA board, with a small Charleston Rx1 RF front end. While the Charleston Rx1 originally included GNU Radio support, subsequent changes to GNU Radio have rendered that support nonfunctional. The Charleston Rx1 SDR currently runs with Quisk under Linux, and with some minor magic it also works with Quisk on Windows. Making the Charleston SDR work with other SR programs, such as SDR#, HDSDR, and Winrad is in the works, as is fixing the GNU Radio support. It uses USB for connectivity to teh host computer. Please see the Charleston SDR pages on the AMRAD website for more information on the Charleston SDR.
- A recent entry to the SDR hardware scene is the Red Pitaya board. This board is billed as a small, lab intrument, which sells for about $250 from Elektor (and others). It includes a Xilinx Zynq SoC, dual 14-bit, 125Ms/s ADCs, dual 14-bit DACs, and some otehr nice stuff. Host connectivity is via ethernet (wired or Wifi). The Red Pitaya itself has software/firmware to make it a dual-channel oscilloscope and/or dual-channel spectrum display, among other things. Pavel Demin has written software/firmware to change the Red Pitay into an SDR receiver or transceiver. His latest project actually takes advantage of the dual-channel hardware capabilities. A software DLL is available for HDSDR/SDR# as a receiver, or GNU Radio as a transceiver. While the Red Pitaya functions pretty good on the MF band (472 kHz), it generates a lot of internal noise at LF (137kHz), according to testing by N4TLF.
- Another recent LF SDR was created by Alberto di Bene, I2PHD, (author of Winrad and other useful LF/MF and SDR tools). It is called the Arm Radio (click on that name on the upper-left)which is implemented on an STM32F429 Discovery board. These boards sell for about $25 from a variety of electronics houses. Alberto’s design requires a minimum of additional hardware: an input low pass filter, and an audio output low-pass filter and amplifier. The Arm Radio could use an LF preamp in front of it. According to tests by N4TLF, the sensitivity is marginal for weak signal work at 136 kHz, and rather poor at 300kHz and up. Still, with a simple preamp, such as a J310, it should be more sensitive.
The openHPSDR devices should work fine on the LF and MF bands. AMRAD will be testing a Mercury/Penelope/Atlas system, along with a Hermes board in the near future. The upcoming Hermes-Lite should also operate on LF/MF without an issue.
There is a lot of SDR hardware and software coming out. Much of it should function fine on the LF and MF bands. Stay tuned for more information as it develops.