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Receiving equipment

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 of 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 with transmitting:  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) voice modes are rarely used with most operation on CW, WSPR, JT-9 and similar weak-signal modes.

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 receivers, the local oscillator input must be
four times the actual receiver center frequency.
Click on the image for a larger version.
Schematic diagram of the 630 and 2200 meter modified Softrock Lite II receivers
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, including:
In the end, the completed receiver looked very similar to those on the "Softrock Receiver" page (link) - with a few key differences:
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.

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 available 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 narrowband receiver.
Figure 2:
Diagram of the filter/combiner/splitter unit that processes the HF/MF and MF/LF signals, making them available for the different receiver subsystems.
Click on the image for a larger version.
Schematic diagram VLF/LF/MF/HF combiner/filter/amplifier module

The "main" HF input (which includes MF signals down to approximately 350-400 kHz) enters 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 receiver.

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 J102.

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:
Go to the main "RX Equipment page.

Additional information:
 Back to the Northern Utah WebSDR landing page