The 630/2200 meter receivers:
The 2200 (135.7-137.8 kHz) and the 630 meter (472-479 kHz) bands are
the newest "LF" and "MF" band to U.S.
Amateurs - the other MF band being 160 meters. Like 160 meters,
these are mostly "winter time" bands when noise - a significant portion
which is lightning static - is lower and the nights are longer and
deeper, both of which are beneficial to reception at these
frequencies. Like 160 meters, there is a significant challenge
Full-sized antennas are out of the question which means that
overall transmit efficiency is quite poor meaning that signals are
generally weak. Because of the comparatively weak signals, the
high noise levels and the fact that the bands aren't very large (2.1kHz wide on 2200, 7 kHz wide on 630)
modes are rarely used with most operation on CW, WSPR, JT-9 and similar
The receiver is a modified "Softrock Lite II" - the same receiver used
for many other bands as described on the other pages. Designed primarily for
operation on HF, it was necessary to slightly modify the receiver depicted schematically in Figure 1,
Diagram of the 630 (and 2200) meter dual receiver system detailing the
modifications to the Softrock Lite II receiver modules. Even
though the lower receiver is marked as being intended for 2200 meter
use, the designed frequency coverage of the input filter is for the
range of about 125 kHz to 215 kHz, easily including the 1750 meter
"LowFER" band. As with other "Softrock" type rceivers, the local oscillator input must be
four times the actual receiver center frequency.
Click on the image for a larger version.
- Change coupling capacitor C12 to a larger value: I used 330pF, which was supplied in the kit, anyway.
- Design a new input band-pass filter. The original filters
for the Softrock Lite II use just two sections: Using the Elsie
program I designed a filter that has three sections and a significantly
sharper - and appropriately wide - band-pass.
In the end, the completed receiver looked very similar to those on the "Softrock Receiver" page (link) - with a few key differences:
- A high-dynamic range RF amplifier precedes the receiver.
While the signal levels are pretty high on 630 meters, the output
from the TCI 530 HF antenna drops as the frequency decreases requiring
that a bit of extra amplification (around 15dB) be brought to bear.
- The extra filter elements on the receiver. These extra components were mounted directly to the receiver board.
As can be seen in the block diagram of Figure 1 and the schematic in Figure 2 on the "RF Distribution" page (link),
the "Low-HF" signal splitter has a port labeled "<=500 kHz" that was
designed to accommodate precisely this type of receiver. At the
time that the splitter module was constructed it was known that the
antenna did work at least
somewhat at 630 meters, but the practical low end usable frequency was
unknown - but this has since been determined as mentioned below.
- There is room for another Softrock Lite II receiver, to be
installed later, which is earmarked for 2200-1750 meter coverage.
While this receiver will have its own RF input and audio outputs,
it will share the power feed and ProgRock synthesizer with the 630
meter receiver, using the second of its three outputs.
What about the 2200-1750 meter receiver that was mentioned?
There is also receiver that covers the 2200 meter amateur band (135.7-137.8 kHz)
and the so-called "1750 Meter" band (see FCC Part 15 §217)that
covers from 160 to
190 kHz - the two bands being comfortably covered using a receiver with
96 kHz of
bandwidth. Unfortunately, the main HF antenna at this site works
miserably below about 250 kHz which means that another antenna must be
used to cover these frequencies, details being described below.
Integrating an LF/MF antenna with the system:
Because there are "other" receivers at the WebSDR that can tune continuously from VLF through the top of HF (e.g. the KiwiSDRs) the signals from this separate VLF/LF/MF antenna should be avalable to these receivers and
the 2200-1750 meter receiver. To do this, another unit (depicted schematically in Figure 2) was
constructed that performed filtering and combining of the various
signal paths while also providing a separate LF output for the 2200
Diagram of the filter/combiner/splitter unit that processes
the HF/MF and MF/LF signals, making them available for the different
Click on the image for a larger version.
The "main" HF input (which includes MF signals down to approximately 350-400 kHz)
enteres via J101 where lightning protection is offered by SG101, an 80
volt gas-discharge tube. This signal then goes through a 350 kHz
high-pass filter which removes the low-frequency energy at this
point since the main HF antenna doesn't work very well at
frequencies below this. This HF/LF signal is applied to T101, a
hybrid combiner that mixes the high-pass filtered HF/MF signal with the
VLF/LF/MF signals from the low frequency signal path, described below.
Connector J201 is the input of the VLF, LF and MF signals from a
separate active antenna designed for this frequency range with a degree
of surge protection offered by gas discharge tube SG201. The
signal then flows through a low-pass filter that offers the inverse of
the high-pass filter in the previously-described section, blocking
signals above approximately 350 kHz. The low-pass output, now
devoid of strong signals from the AM broadcast band, is then amplified
by U201. The output of this amplifier is applied T201, another
hybrid device that operates as a 2-way splitter: One output goes
to T101 to provide the "combined" output for the KiwiSDRs while the
other output goes to J202 which feeds the dedicated 2200/1750 meter
In reality, neither the high or low pass filters are "brick-wall" in
their responses which means that the two overlap over a portion of
their frequency range: This overlap provides a smooth transition,
making the entire VLF, LF, MF and HF spectrum appear continuous at
Also at J201 is a regulated 18 volt DC supply inserted via L201 which
powers an active antenna that is used for this frequency range.
Because the low-pass filter is DC-coupled, diodes D201-D204
protect U201 from the a voltage impulse that occurs through C201 when
power is applied and/or if the antenna connection at J201 is
inadvertently shorted. (The active antenna may be described later.)
Pages about other receive gear at the Northern Utah WebSDR:
- Softrock Receivers
- This page describes the "High Performance" receivers that use
"Softrock" direct-conversion receivers and sound cards. These
receivers cover limited bandwidth (up to about 192 kHz) but have excellent weak and strong signal handling properties.
- RTL-SDR Dongle-based receivers
- Described here are the "not high performance" receivers using the
ubilquitous RTL-SDR dongles. These receivers cover up to 2 MHz of
bandwidth, but their limited A/D bit depth (only 8 bits)
means that they can suffer from too much and/or too little signal input
- often depending on band conditions. Included on this page is
information about how to make the most of these as well as helping to
manage when multiple RTL-SDR dongles are used on a Linux-based system.
- RF Downconverter for RTL-SDR receivers
- While there are RTL-SDR dongles that contain built-in upconverters to
allow reception across the entire HF spectrum, this may not be the best
way to do it. When receiving frequencies at or above the Nyquist
frequencie(s) on HF, one can downconvert to lower frequencies and get
good results, all described on this page.
- An AGC block for RTL-SDR receivers-
Because RTL-SDR dongles have only 8 bits of A/D, their dynamic range is
limited. While one can adjust the gain to fit their useful signal
range "window", HF band conditions change constantly, making it
impossible to keep one of these receivers's limited dynamics optimized.
Preceding an RTL-SDR dongle with proper filtering and an AGC
circuit can make the best of these devices.
Go to the main "RX Equipment page.
- RF Distribution and filter system
- Absolutely essential to any receive system is the means by which RF
is distributed - and filtered, the means by which this is done at the
Northern Utah WebSDR being described on this page.
- For general information about this WebSDR system -
including contact info - go to the about
- For the latest news about this system and current issues,
visit the latest news
- For more information about this server you may contact
Clint, KA7OEI using his callsign at arrl dot net.
Back to the Northern Utah WebSDR landing page
- For more information about the WebSDR project in general -
including information about other WebSDR servers worldwide and
additional technical information - go to http://www.websdr.org