WebSDR #4 RF distribution and filtering

Northern Utah WebSDR
Receiving equipment
WebSDR #4 RF distribution and filtering

In early April, 2020, the Northern Utah WebSDR commissioned a new server - WebSDR #4 - that is connected to a different antenna - an LP-1002 (made by Hy-Gain, now U.S. Antenna Products) that is a 6-40 MHz log periodic direction antenna that is oriented on a heading of 87 degrees (relative to true north). To do this, new RF infrastructure needed to be designed and constructed to deal with the specific needs of this new subsystem.

Figure 1:
The signal path of the RF system used on WebSDR #4.
The "LF/HF Diplexer + HF Amp" unit is located at the base of the tower while the remaining equipment is located in the building with the server gear. As noted, the cable between the tower and the building carries LF signals and DC to power the LF active antenna and the outside RF amplifier.
***Click on the image for a larger version.***

This system consists of two major parts:

The "Outside" system - consisting of the antenna and the "LF/HF Diplexer + HF Amp":

This unit, depicted in the lower-left corner of Figure 1 is located at the base of the tower with the antenna and its main purpose is to provide some system gain prior to the loss of the cable that runs between the tower and the building with the servers - a distance of about 140 feet (43 meters).
At the moment, the "temporary" cable being used has a loss of between 2 and 5 dB, depending on the frequency, and placing gain at the tower, before the loss, assure that we don't incur the penalty of increased system noise figure that might degrade potential performance on the higher bands (e.g. 10 meters).
Also present is a 40 MHz low-pass filter. It was "discovered" that this antenna - although rated for only 40 MHz - was still capable of intercepting a tremendous amount of energy in the FM broadcast band. This filter reduces the signal levels at and above 87 MHz by at least 40dB, preventing it from reaching the RF amplifier and, by extension, the receiver stack in the building.
It was also "discovered" that this antenna was capable of intercepting a lot of energy from the "nearby" (>=20 miles/>=36 km distant) AM broadcast stations - the amount of which was capable of overloading the RF amplifiers and receivers. For this reason a "strong" low-pass filter was installed which will attenuate these signals enough to eliminate the possibility of overload and related intermodulation distortion.
This unit also multiplexes LF (<=400 kHz) reception from an active E-field whip that is on the same tower together with the amplified and filtered HF signals - more about this below.
On the input ports are 80 volt gas-discharge tubes to offer a degree of lightning protection.

The "LF/HF/DC Coupler/Splitter":

The LF active whip antenna on the tower preceded the installation of WebSDR #4 and a cable carrying RF and DC was already present - so it was "re-used" for HF coverage on WebSDR #4. This splitter unit - located inside the building - separates the LF and HF signals that had been multiplexed onto the cable.
Because the LF antenna was powered from 18 volts, this same voltage is used to power the RF amplifier in the HF portion of the "LF/HF Diplexer + HF Amp" box - and the "LF/HF/DC Coupler/Splitter" box passes DC from its source, through and onto cable.
On the input port is an 80 volt gas-discharge tube to offer a degree of lightning protection.

The "Input protection/Splitter/BPF Module":

Preceding this unit is a 20 dB directional coupler - a Mini-Circuits ZFDC-20-3 - that is used to allow the injection of signals to be used for receiver testing without needing to interrupt the signal path.
The HF signals from the HF antenna (coupled via the "LF/HF/DC Coupler/Splitter" unit) feed this unit. This unit is capable in injecting DC onto the coaxial cable, but this capability is not currently used as the DC power for the outside RF amplifier is provided from the LF antenna coupling unit.
The signals are, prior to any filtering, split two ways: An "Unfiltered RF Output" is made available to feed wideband receivers - both present and future - that are capable of receiving over a wide frequency range.
The "other" output of the splitter goes in several directions:

A 9.5 MHz high-pass filter provides a signal source for the 30-10 meter bands which are amplified/filtered externally.
7.6 MHz low-pass filter removes removes the higher-band signals without affecting the levels output by the 9.5 MHz high-pass filter.
Beyond the 7.6 MHz low-pass filter are two other filters:

A "strong" 6.8 MHz high pass filter which, in conjunction with the 7.6 MHz low-pass filter passes "only" the signals around the 40 meter band.
A 6.5 MHz low-pass filter provides a path for signals at and below this frequency. Even though the antenna is rated "only" to as low as 6 MHz, it is capable of receiving signals on lower HF bands, albeit at lower gain. Testing will be done to determine if this signal path offers any advantage in terms of signal-noise ratio and/or diversity when compared with the TCI-530 Omnidirectional antenna also being used.

On the input port is an 80 volt gas-discharge tube to offer a degree of lightning protection.

The "High HF Band-Split Module":

This unit provides relatively "strong" filtering for each of the higher HF bands, namely 30, 20, 17, 15, 12 and 10 meters and it is nearly identical to the same-function filter already in use for WebSDRs 1-3. Because this is a "diplexing" type filter, it does not employ conventional splitters, but rather each band is picked off from a common bus, offering much lower (less than 4 dB) insertion loss.
Preceding this filter is an amplifier module that is used to make up for the losses of the filters within this module: Whether or not this amplifier is really needed with the existence of the amplifier in the "LF/HF Diplexer + HF Amp" will require a bit more analysis.
On the input port is an 80 volt gas-discharge tube to offer a degree of lightning protection.

Dealing with large signals:

Those familiar with HF propagation are well aware that there are time during which certain shortwave broadcast signals can become extremely strong. During the "low" portion of the 11 year sunspot cycle, this is most likely to happen on the lower bands such as 60, 49, 41 and 31 meters.

While Utah is more distant from the "powerhouse" Shortwave Broadcast signals found in the Eastern U.S., there are certain times when such signals will peak - and this typically happens around the time of evening "grayline" propagation - that is, the hours during which sunset is moving across the North American continent.

It has been observed that some of these signals will peak to well over -20dBm at the antenna terminals - a signal level that correlates to "50 over S-9" or higher - and there are often several of these very strong signals - and the total RF power of these signals - plus the modulation peaks - combines to instantaneous levels that can significantly exceed that of any, single signal. As you can imagine, this amount of total RF power can cause issues with even "strong" receivers by several mechanisms:

Front-end overload. Many receivers - particularly older/portable radios - simply cannot deal with a signal of this level on nearby amateur band. An example of this are the (sometimes) extremely strong 41 meter SWBC signals just a few 10s/100s of kHz above the 40 meter amateur bands.

Overload of direct-sampling receivers. Modern direct-sampling SDRs aren't immune to this issue. The recent crop of direct-sampling receivers (e.g. Icom IC-7300, IC-7600) must have very strong RF band-pass filtering in front of their analog-digital (A/D) converters to prevent overload - but these filters aren't "sharp" enough to effectively filter the very strong 41 meter SWBC signals while passing the 40 meter amateur signals - so these receivers will simply decrease the gain in front of the converters. While this prevents overload, this gain reduction can (and does) reduce the absolute sensitivity of the receiver during these conditions and very weak signals (near the noise floor) can suffer. In this situation, more "bits" of A/D conversion can help - but even the 16 bits of the IC-7600 does not make this "high end" receiver completely immune to this issue.

As implied above, there are several ways to deal with these issues:

"Strong" filtering: With a single-band receiver, it is imperative that one not pass any more frequencies to the receiver than are necessary for that band. For example, the 40 meter receiver's filtering depicted in Figure 1 strongly blocks signal below about 6..8 MHz and above 7.6 MHz, effectively eliminating 49 and 31 meter signals. Unfortunately, the 41 meter SWBC band abuts the 40 meter amateur band making it more-or-less impossible to filter some of these very strong signals.

Only as much gain as necessary: The "ideal" situation is to feed the receiver with only enough signal to do the job. In this case, one would add enough gain to the signal path so that the receiver was just able to comfortably "hear" the background HF noise floor on a given band during quiet conditions - but no more. By elevating the background noise floor by 3-6 dB above that of the receive noise floor one will be able to "hear" any signal that is strong enough to overcome mother nature's background noise and/or your local noise floor. Having more signal than this robs the receiver from the ability of handling strong signals.

Add AGC as necessary: In some cases, even if one has added just enough gain, there will still be too signal present under some conditions - so some means of automatically reducing the gain under strong-signal conditions may be necessary.

An extreme example of this is the use of RTL-SDR dongles on the lower HF bands.

With only 8 bits of A/D conversion, adding enough gain to "hear" the noise floor with enough bits of A/D conversion to prevent quantization distortion (which can lead to intermod/distortion on very weak signals) during quiet conditions will guarantee that the RTL-SDR dongle will overload when even moderately-strong signals appear.
With strong filtering and AGC, one can maximize the usability of the limited dynamics of this device: When a strong signal is present, the gain in the signal path is automatically reduced to a point just below the "clipping" level of the A/D converter.
While this can (and probably will) affect weaker signals represented by few "bits" of A/D conversion, this is far preferable to the terrible distortion effects that will occur with severe A/D clipping.
As noted above, an example of such signal processing is described on the page "An AGC block for RTL-SDR receivers".

Receive equipment on WebSDR #4:

The receive equipment used on WebSDR #4 is similar to that used on WebSDRs 1-3:

40 Meters: With the adoption of the FiFiSDR receivers, the original 40 meter SoftRock-based dual receive module previously used on WebSDR #1 was made available - and re-deployed here: A pair of 192 ksps, 16 bit sound cards are used to digitize the I/Q (audio) channels from these receivers.
30 Meters: Via a circuitous route, we obtained one of the original receiver chassis that had been used years ago at the KFS WebSDR. This unit contains four "Softrock Ensemble III" receiver modules that are capable of tuning anywhere from 160 meters through 10 meters - and one of these four receivers is used on this band, fed to a 96 ksps sound card.
20 Meters: A pair of FiFiSDRs is used provide coverage of 20 meters in two overlapping 192 kHz segments.
17 Meters: Coverage of this band is via one of the other "Softrock Ensemble III" receiver modules in the box that is also used for 30 meter reception. With only two of the four receive modules currently being used, two more are available as "spares".
15 Meters: A frequency downconverter is used along with an RTL-SDR dongle to entirely cover this band. This unit is very similar to that described on the RTL Downconverter page (link) except that it has a built-in AGC (Automatic Gain Control) circuit in the signal path which allows much greater dynamic range capability than would otherwise be possible using a typical RTL-SDR.
12 Meters: This band is not covered at this time owing to the eight receiver limit of the WebSDR software. Even though 40-10 meters implies only seven amateur bands, 40 and 20 meters are covered with two receivers each, leaving us one receiver short - so 12 meters was left off for the time being.
10 Meters: This also uses a frequency downconverter with an RTL-SDR dongle and it, too, is similar to the that described on the RTL Downconverter page (link) - and it, too, incorporates and AGC circuit.

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.

LF/MF reception using the Softrock receivers - The Northern Utah WebSDR's coverage includes the "new" 2200 and 630 meter amateur bands and these are received via modified softrock receivers.

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.

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.

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 receiver'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.

Additional information:

For general information about this WebSDR system - including contact info - go to the about page (link).

For the latest news about this system and current issues, visit the latest news page (link).

For more information about this server you may contact Clint, KA7OEI using his callsign at arrl dot net or to sdrinfo@sdrutah.org

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

Back to the Northern Utah WebSDR landing page