Nothern Utah WebSDR Logo - A skep with a Yagi Northern Utah WebSDR
Technical Info

This WebSDR server is located near the town of Corinne, Utah, U.S.A., about 60 miles (94km) north of Salt Lake City at an old HF research site.  The antenna for all HF and MF bands is a TCI model 530, an omnidirectional Log-Periodic antenna feeding a custom-built receiver multi-coupler array that, in turn, distributes RF to a number of receivers.

A quick overview of amateur and shortwave broadcast bands covered by this WebSDR system:

Here's more information about the Northern Utah WebSDR servers and how they are covered:

Server #1 (Yellow) covers the following frequency ranges.

Server #2 (Green) covers the following frequency ranges.
 Server #3 (Blue) covers the following frequency ranges.

S-Meter Calibration:

For all frequency bands other than the "AM-160M-120M" band the signal meter is calibrated to a nominal +/- 2dB of the actual signal level in dBm at the system's antenna input at the mid frequency of each amateur band.  The S-meter itself is calibrated to the IARU Region 1, Technical recommendation R.1 standard  where an S-9 signal is equal to 50 microvolts (-73dBm) in a 50 ohm system.  This calibration does not take into account the antenna gain or feedline losses nor does it currently take into account the slight roll-off that is present at the band-edges of the "SoftRock" type receiver+Sound card signal path.  At the input of the system is a -20dB tap where known signals are inserted when the system is active to allow calibration of the signal level metering without otherwise interrupting system operation.

On the "AM-160M-120M" band the s-meter calibration is accurate at approximately 1825 kHz and above due to a band-stop filter that significantly attenuates the signals between 540 and 1725 kHz to prevent overload of the RTL-SDR dongle and the KiwiSDRs on site:  Within the range from 530-1725 kHz the S-meter will read between 15 and 50 dB low, depending on the filter depth and whether the (local) AM station being listened to has a notch attenuator associated with it.

On the KiwiSDRs, the S-meter reads within a couple of dB of being correct above about 12 MHz, but due to some filtering to prevent IMD, the S-meter will read about 2 S-units (12 dB) low below 7 MHz with a gradual roll-off between the two frequencies.  Between 520 kHz and 1.8 MHz the readings are badly skewed owing to the (necessarily!) complicated filter (mentioned above) that attenuates that frequency range by about 20dB plus some individual notches on some of the stronger AM broadcast stations.

About the receive system:

The "Softrock" converters:

These are the "High Performance" receivers using "Softrock" converters based on the QSDs (Quadrature Sampling Detectors) that uses analog switching chips as mixers with the resulting baseband (audio) being fed to an analog-digital converter (e.g. computer sound card.)  While simple, these devices - when coupled with a good-quality sound card and properly configured in terms of gain balancing and RF filtering - can have excellent performance.  The front-end circuitry of these devices is very similar to that of several current radios, including the Elecraft KX2, KX3 and K3, and they can have better dynamic range and noise properties (e.g. "NPR" - Noise Power Ratio) superior to many "direct sampling" radios such as the Icom IC-7300 and the KiwiSDR while giving high-end radios such as the Icom 7610 a run for their money.

This system uses the SoftRock kits sold at the "FiveDash" web site (link) - both the synthesized "SoftRock II Ensemble" and the crystal-controlled "SoftRock Lite II".  As far as the signal path goes, both of these receiver types are pretty much identical, aside from differences in input RF filtering:  Because all receivers on this system are preceded by a bandpass-type multi-coupler, this difference in front-end filtering is largely irrelevant as the multi-coupler itself has much "stronger" filtering than the receiver modules - or even most amateur band radios.

Sound cards:

The "SoftRock" receivers (above) output baseband audio on two channels (e.g. I and Q) to produce a receive bandwidth that is about the same as the overall sample rate.  Because of this, it's desirable to have the highest sample rate practical and the most common, high sample rate sound cards that are available operate at 192 ksps permitting the simultaneous reception of a similar RF bandwidth.  Finding reasonably-priced sound cards that also "play nice" with the somewhat limited driver support on Linux is a bit of a challenge.

Sound cards that have been successfully on this system (which runs Ubuntu Linux with ALSA) are the Asus Xonar D1 (PCI bus), the Asus Xonar DX (PCIE bus) - both of which operate at a sampling rate of 192 kHz, as do the Asus Xonar U5 and Asus Xonar U7 USB-interfaced sound cards.  An Asus Xonar DS is available, as is the Intel sound card chipset on the computer's motherboard.  An Asus HDAV 1.3 (PIC bus) is on-hand and is supposedly capable of 96 kHz sample rate but its analog input is available only via its "front panel" header on the card itself:  It seemed to work in software, but we have not yet been able to get audio through it, but we haven't tried exhaustively to do so.  192ksps sound cards of various types are used for all of the bands on Server #1 (the yellow one) while a mixture of 192 kHz and 96 kHz sound cards are used on Server #2 (the green one) to provide coverage of 20 and 17 meters.

In general, "Sound Blaster"t cards are not well supported under Linux due to the lack of support from the manufacturer in the development of drivers:  We do not have information about other manufacturers/types of sound cards.  The computer being used as the WebSDR server has only a few expansion slots:  One PCIE and two PCI.  Expansion beyond three plug-in sound cards is done using USB sound devices such as the Asus U5 and U7.

RTL-SDR dongles:

RTL-SDR dongles and similar devices are attractive in that they are inexpensive ($5-$30 for the basic device - more for those like the "Fun Cube Dongle" with additional filtering/amplification) and with their built-in frequency synthesizers and A/D converters, they do not need to use a sound card, they have very broad frequency coverage (typically a few hundred kHz in "direct sampling" mode to hundreds of MHz in normal "I/Q" mode - with a few gaps) and can reliably cover up to 2048 kHz of RF bandwidth via a USB 2.0 port - but the down side is that their dynamic range and noise properties are limited by the fact that they use A/D converters with only 8 bits of resolution.  What this means is that when used in situations where signal dynamics can vary widely (e.g. both very weak and very strong signals are present within the passband) their performance can be rather mediocre.  Despite this, they can be made to work "pretty well" over fairly wide bandwidths if appropriate gain balancing and filtering techniques (e.g. appropriate attenuation/gain, the notching of frequencies with very strong signals, judicious selection of sample rate, etc.) are used.

The RTL SDR dongles used on this system for wide-bandwidth (>2MHz) coverage are the "Version 3, Batch 2" units designed and sold by the "RTL-SDR Blog" folks.  While still subject to the same dynamic range limitations intrinsic to all devices based on RTL2832 chip, this particular design has been optimized as much as is practical to reduce internally-generated spurious signals.  This design also includes a 1 PPM TCXO frequency reference and an amplified and filtered "Direct sampling" signal path to allow continuous tuning from at below 500 kHz to 24 MHz (with a few caveats) in addition to the range from about 25 MHz to above 1700 MHz when its onboard frequency converter is used - all from a device costs only U.S.$20 or so.  For more information about where to get one of these device go to the RTL-SDR Blog "where to buy page" or, for a data sheet, go here.

It should be noted that if you wish to use any RTL-SDR type dongle of HF and you have some nearby transmitters on HF or in the AM broadcast band, care should be taken to prevent front-end overload.  If signals are very strong this overload can result from the front-end protection diodes on these devices going into conduction and causing intermodulation distortion - even at frequencies far removed where one is receiving.  What this means is that in many cases it will be required that a high pass filter designed to remove AM broadcast band signals (or better, a band-pass filter designed for the frequencies involved) should be used between the dongle and antenna.

Filtering on the RTL-SDR dongle RF inputs:

To optimize performance, a custom-built, adjustable AM broadcast band reject filter is used in front of the RTL-SDR dongles to reduce very strong signals in that area to prevent signal overload and make the most of their limited 8-bit A/D converter dynamic range.

This filter has an adjustable amount of "bypass" so that AM broadcast band signals aren't completely excluded, allowing reception of many signals within this frequency range as well as having several adjustable "notch" filters to reduce some of the very strong local signals down to levels that are in line with weaker, local stations.  By "flattening" the signal levels in the AM broadcast band to a narrower range of amplitudes best use may be made of these receivers' dynamics to allow the reception of a mix of the various signals within.  Signals outside the range of 500-1775 kHz are minimally affected, allowing sub-microvolt level signals outside the AM broadcast band to be received.  This filter is described in more detail on the "RX Equipment" page.

While higher performance QSD-based devices such as the SoftRock receivers will (generally) be used for the busiest and most popular amateur bands covered by this WebSDR, RTL-SDR dongles may be employed on "new" bands to evaluate their popularity and usefulness prior to the (possible) addition of that amateur band using more expensive, higher-performance hardware - or as a cheap way to add some extra frequency coverage!

Do you want even more information about the receive system and its bits and pieces?

For more information than you probably wanted to know about the various components that make up the RF and receiver sub-systems, visit the RX Equipment page (Link).

The omnidirectional log periodic antenna at the Northern Utah WebSDR, at sunset in early April.  This is a view to the southeast with "Vees" of honking geese being visible in the sky.
Click on the image for a larger version.
The Omnidirectional log periodic antenna at the Northern Utah WebSDR

About the antenna(s):

The antenna being used for reception is a TCI Model 530 Omnidirectional Log Periodic antenna.  Centered about a 94 foot tower, this antenna consists of two separate broad-band (3-30 MHz) log-periodic arrays arranged in quadrature.  The result of this element arrangement is a pattern that is nearly omnidirectional toward the horizon with a main lobe providing up to 6dBi gain at relatively high take-off angles.  This antenna is more-or-less circularly-polarized over its design frequency range which means that it is generally agnostic to the polarization of the signal being received.

This antenna is designed for "short-to-medium" range HF communications, but as a receive antenna it can still function well at greater distances, over a wider frequency range than its design specifications:  It is, in fact, being used for reception on the 160 Meter (1800-2000 kHz) and 630 Meter (471-479 kHz) amateur bands with excellent results.   More information about this antenna may be found here (.PDF, 1.8 Meg.)

There is another antenna on-site:  A large, non-steerable, log-periodic beam (Hy-Gain/U.S. Antenna Products LP-1002 - data here) that is pointed due east.  We are not using this antenna at the moment and due to issues with the antenna and the tower that are gradually being addressed, there are no immediate plans to do so - although this may change in the future.  This antenna can be easily seen from aerial photos as well as from the nearby highway, but the TCI 530 is almost invisible by comparison.

For 2 meter reception, a 5 element, vertically-polarized Yagi is used.  This antenna is pointed (mostly) south toward the Salt Lake Metro area as that is the direction of most of the repeaters.  For 6 meters, a full-sized J-pole is used.

Additional information:
 Back to the Northern Utah WebSDR