LF/MF UPCONVERTERS & TRANSVERTERS
New receivers or transmitters are not always necessary for operation on LF or MF. An existing radio can be used, along with an upconveter for receiving or a downconverter for transmitting. These devices “translate” the LF/MF frequencies of interest to a band that the exisiting radio is capable of operating on. There are commercially available units, kits, and designs that you must gather the parts for and build yourself.
Existing equipment could be a ham rig or general coverage radio with 80 Meters (3.5-4.0 MHz plus maybe 4.0-4.5Mhz), 30 Meters (or WWV) at 10MHz, or even 10 Meters (28.0-29.7 MHz). Both receive-only upconverters and transmit/receive transverters are available.
Another, newer option for receiving are the ubiquitus RTL dongles that receive at VHF & UHF frequencies. Presently, these upconverters are for receiving only.
Back in the April 2002 issue of QST, Frank Gentges (K0BRA) and Steve Ratzlaff, AA7U presented an article on the AMRAD Low Frequency Upconverter AMRAD_Low_Frequency_Upconverter (1). It uses a 74HC04 oscillator at 4 MHz, and a 74HC4053 as a mixer. There is also a significant amount of front-end filtering.
One of the less expensive options is the LF converter by WB9KZY. This unit sells for under $20, including shipping. It uses an NE-602 Gilbert Cell mixer chip, and therefore has some gain. Users may need a small attenuator between it and the attached receiver. The kit includes crystals for reception at both 4 MHz and 10 MHz. There is a $3 option for different front-end low-pass filtering components, based on Jack Smith’s (of Clifton Labs) review. This upconverter is designed to operate on 12 Volts. See the WB9KZY.com website for further information.
LF Engineering makes an LF upconverter that converts LF/MF to 4 Mhz, the L-111, which sells for $119. It uses a Mini-Circuits double-balanced mixer and 4MHz oscillator, so it has conversion loss instead of gain. It is powered by two 9 Volt batteries. LF Engineering claims to have better filtering than less expensive designs.
For those of you using an RTL-SDR dongle, NooElec sells the “Ham It Up” upconverter for about $45. This device converts all of the LF/MF/HF frequencies to either 100 MHz or 125 MHz. The front-end is wide open, with no filtering. It uses a Mini-Circuits double-balanced mixer. There is room for a nice noise generator circuit, with most of the components already on the board. They also sell these upconverters on ebay and Amazon. NooElec also sells the RTL-SDR dongles themselves, as well as other SDRs, cables, adaptors, and accesssories. The Ham It Up uses 5V for power, from a USB connector.
SV1AFN has a DC-55 MHz upconverter for use with RTL-SDR dongles, that sells for $55, plus $12 shipping. It has more sophisticated filtering on the input and outputs than the WB9KZY upconverter or the NooElec units. It uses a typical Mini-Circuits double-balanced mixer. The output is at 200-255 Mhz, and it operates from 5 Volts.
Some RTL-SDR dongles now come with two input connectors. One, often marked “VHF” passes the signal through the regular RF tuner chip, while the second, “HF” connector connects directly to the A/D converter, bypassing the tuner chip. These dongles can tune LF/MF/HF frequencies (to about 23MHz) directly, but the SDR software must be able to enable the “HF” port. On SDR#, and some other SDR programs, the user must select the Direct, Q channel samples instead of the normal tuner mode.
Note that RTL-SDR dongles use only eight-bit A/D converters, so their dynamic range is not great. Strong local signals from nearby broadcast stations (or lightning) can easily swamp these receivers.
There are also several other types of upconverters available on Amazon and ebay, most of which haven’t been tested yet by AMRAD or LCCC.
Some designs have been published for LF transverters. These units typically use an HF transceiver operating on an HF ham band (often 10 Meters), and upconvert for receiver, and downconvert for transmit from that band to LF/MF frequencies. The RSGB book “LF today” by Mike Dennison, G3XDV, has a typical design for use with an FT-817. This book is an excellent resource for LF information in general.