RF Distribution - Receiver Equipment

Northern Utah WebSDR
Receiving equipment
RF Distribution and filtering

Figure 1:
Block diagram of the various RF signal paths at the Northern Utah WebSDR from the TCI-530
omnidirectional antenna and from the VLF antenna system.
The signal paths and receive equipment for the 6 and 2 meter bands is not shown.
See below for the signal path from the LP-1002 log periodic beam.
***Click on the image for a larger version.***

RF Distribution and Filtering from the TCI-530 Log Periodic omnidirectional antenna:

There are two antennas used for HF reception on site - and this portion describes that associated with the TCI-530 omnidirectional log periodic antenna which is designed for transmit coverage from 3 to 30 MHz, but is used for reception down to about 400 kHz: Below this frequency, a separate active antenna is used as noted below.

To achieve our goal, we decided from the outset that we should make the receive system capable of receiving on every HF band, but to do this we'd need a lot of outputs, as in:

2200 meters
630 meters
160 meters
80/75 meters *
60 meters
40 meters *
30 meters
20 meters *
17 meters
15 meters *
12 meters
10 meters *

* - Multiple outputs connections needed if narrow-band "Softrock" type receivers sound cards are used.

One way that we could have done this would have been to use conventional transformer-type splitters to divide the signal and the simplest way - to divide it by 16 - would have yielded about 20dB of insertion loss - and the above doesn't take into account that for many of the HF bands (those marked with an asterisk) we'd need several antenna connections to feed enough receivers to cover many of the bands if we use "high performance" receivers that can provide only up to 192 kHz of coverage.

With this method there are two other problems with which one must contend:

For good performance, there would need to be a band-pass filter specifically designed for the amateur band in question: While this is particularly true for RTL-SDR receivers, but it is also true for the SoftRock kits described above as their band-pass filters aren't "strong" enough to avoid some spurious responses (e.g. the 75 meter receiver will respond on its 3rd harmonic to strong signals in the 25 meter shortwave broadcast band).

As noted previously, the system noise figure will need to be lower as the frequency goes up which means that appropriate amplification (low noise, high dynamic range) must precede the at least some of the splitters in the signal path of the higher bands. Without filtering, these amplifiers must be able to handle everything that might be thrown at it!

While it is certainly possible to make this scheme work, there's another method: Use a "diplexer" type splitter.

Important note regarding kits available for some of the filter/splitter modules:

A kit is available to construct the filter module depicted in the upper-middle circuit of Figure 2 - the "Low HF Splitter". Please note that I have had nothing to do with the production of this kit.

Having said that, provided that the included parts are of reasonable quality and care is taken in its construction, reports indicate that it can work pretty well.

BE AWARE that in construction of circuits like this, having a NanoVNA or similar to "dial in" the inductance values (squeezing together, spreading and/or removing a turn) may be essential to obtain the best performance.

Signal distribution strategies:

A "diplexer" type splitter minimizes the insertion loss by selectively "picking" various bands from a common bus. By having a filter that pulls only narrow ranges of frequencies of individual amateur bands - but leaves the other frequencies alone - we can put several of these same filters on the same bus and instead of 15-20dB of insertion loss from cascaded splitters we can easily keep the loss down to single digits of dB. The idea is simple - but we decided early on that this wasn't going to be our only approach.

The receive signal path (from the antenna) was designed from the outset to be both versatile and high-performance with the following goals in mind:

Provide both a "Narrowband" and "Wideband" receiver signal branch.

The "Narrowband" branch includes filters designed to pass a single amateur band - such as the "Diplexer" type splitting.

The "Wideband" branch is for receivers that may cover signals outside the amateur bands, including RTL-SDR dongles and direct-sampling receivers (such as the KiwiSDR) that can cover from 0-30 MHz. Because the requirements of these receivers are fundamentally different than for the "narrowband" receivers it is easiest to have two entirely separate signal paths. Having this signal branch "future proofs" the RF distribution system of the WebSDR should a high-performance 0-30 MHz direct-sampling receive system capable of supporting many users at once become available.

To accomplish this several modules were built, depicted in Figure 1:

The "Splitter/AM BCB Reject/Amplifier module". See figure 2, top, for a schematic diagram. (For an article on this device, see reference #7, below.)

This module has a passive ferrite splitter that is designed to pass signals from 10 kHz to over 30 MHz from the antenna with one port going directly to the input of the "narrowband" branch of the RF system. This input is afforded a degree of lightning protection with the use of two gas discharge tubes in parallel.

The other port of the splitter goes to an AM broadcast band reject filter that is designed to attenuate signals between 540 and 1725 kHz by at least 30dB while minimally affecting signals just outside this range (e.g. the 630 meter band and the 160 Meter and higher bands) as well as the HF spectrum in general.

Included in the design of the AM broadcast band reject filter is a "bypass" adjustment that allows the amount of ultimate rejection of the filter to be reduced so that the signals across the AM band may be controlled in amplitude, allowing weaker signals to be received.

The design of the AM broadcast band reject filter also includes up to 7 tunable notch filters that can selectively reduce the signal strength of individual AM broadcast band stations.

By carefully adjusting the amount of "bypass" and setting the notches, the difference between the strongest local signals and the weaker signals is greatly reduced. By reducing the level of the AM broadcast band signals, front-end overload of connected receivers - such as RTL-SDR dongles or direct-sampling receivers - can be avoided. By "flattening" the signal level range between the strongest and weaker signals it is possible to reduce the amount of band-stop attenuation to permit the reception of even weak AM broadcast band signals through the band-stop filter while preventing overload of receivers with poor dynamic range (such as the RTL-SDR dongles with only 8 bits of A/D sampling).

It is this system that allows even the lowly RTL-SDR dongle to receive nighttime AM broadcast band without being overloaded by the strong daytime signals while still having microvolt-level sensitivity outside this band.

High dynamic range amplifiers are included in this module to allow down-stream splitting of the wideband signal and to accommodate losses of additional filtering, the intrinsic insensitivity of some types of receivers (e.g. RTL-SDR units operating in the "Direct" mode) and the need of variable attenuation for RTL-SDR receivers to allow the signal levels to be set to optimize for the limited dynamic range of these devices.

Figure 2:
Top: The schematic of the "Splitter/AM BCB Reject/Amplifier" module.
Upper Middle: The schematic diagram of the "Low HF Splitter" module.
Lower Middle: The diagram of the "High HF Splitter" module.
Bottom: The diagram of the splitter/low-pass/BPF module for RTL-SDR receivers.
PLEASE SEE THE COMMENT ABOUT KIT VERSIONS OF SOME OF THESE CIRCUITS, ABOVE.
Click on an image for a larger version.

The "Low HF Splitter Multi-Coupler Module". See figure 2, upper-middle, for a schematic diagram. Please see the comment about kit versions of this circuit, above.

This module gets its input from the "split" HF output of the previous module, but it could have been connected directly to the receive antenna. A gas-discharge tube is included on the input of this device to provide a degree of lightning protection.

Each band output is a result of the combination of high and low-pass filters. Because of the design, there is no need to incur the loss of individual splitters as the various bands' filters are simply paralleled on the same signal bus. Because the losses are minimized, there is no need for any amplification prior to each output filter port.

Because each "band" output is effectively band-pass limited, this limits the range of signals that each individual band's receiver will see, improving the overall signal handling capability, reduces possible image response and significantly attenuates local oscillator bleedthrough (an issue with "QSD" type mixers found on "SoftRock" receivers) while also providing a degree of lightning protection.

The most popular amateur bands are covered - namely 160, 80 and 40 meters. An additional output is low-pass filtered at 500 kHz to allow the possible future inclusion of the 630 and 2200 meter bands. The only band that is not covered is 60 meters, but this is be covered using a dedicated 60 meter module on the "wideband" module as shown.

This module includes a high-pass filtered output for the "high" HF bands (30 meters and above) as described below.

The "High HF Splitter Multi-coupler module". See figure 2, lower-middle, for a schematic diagram.

There is a high-pass filtered output from the "Low HF Splitter Multi-Coupler Module" that passes signals above approximately 9.5 MHz. This output is passed to a high dynamic range amplifier based on a GALI74+ MMIC. This amplifier is useful at this point in the signal path because the filters themselves incur 1-3dB of loss and this factor shouldn't be callously disregarded as one approaches the top end of the HF spectrum where noise figure can start to be an issue.

The output of the amplifier is passed to this "High HF" module where there are individual series-input band-pass filters for each of the amateur bands covering 30 through 10 meters. These method of "splitting" the signals is less lossy than transformer-type splitters and it provides a degree of isolation between the various receiver modules as well as providing additional band-pass filtering for each of the individual receivers - particularly important with receivers that use a QSD where a significant amount of local oscillator energy can find it way out of the antenna port.

Also depicted in Figure 1 is another module, connected to the output of the Splitter/BCB filter module, that feeds two RTL-SDR dongles. As required for best performance, these devices should have their inputs filtered to pass only the frequency range of interest and the diagram shows this being done: A 3 MHz low-pass to accommodate the receiver that tunes 630 through 160 meters (including the AM broadcast band) and a 4.5-7 MHz band-pass filter for the receiver that tunes the 60 Meter SWBC and amateur frequencies and the 49 meter SWBC bands. This module also has adjustable attenuators that are set to the "sweet spot" - that is, just enough attenuation to prevent serious overload by strong signals and not so much attenuation that weak signals cannot be heard.

At first glance it might seem that placing a splitter at the input of the system and losing 3dB "off the top" would be a bad idea, but this ignores a fundamental truth about HF signal reception: As noted above, the HF frequency range is very noisy, which means that we can tolerate quite a bit of loss (and incur a rather high system noise figure) in front of our receivers without actually degrading overall system sensitivity. This simple fact can be demonstrated by connecting a highly-sensitive receiver to a full-size receive antenna and experimenting with a step attenuator and noting the amount of attenuation required to quash the atmospheric noise. Typically this value, on an antenna devoid of man made noise under normal "quiet", HF conditions, implies that an acceptable system noise figure ranges from about 45dB at 160 meters, decreasing to 24 dB at 20 meters and 15 dB at 10 meters. ⁵ What this means is that even if we end up with 6 dB of added loss in our HF signal path through splitters and filters, it is still possible to recover the natural noise floor on at 10 meters without requiring any sort of exotic, low-noise amplification.

Comments on Figure 1:

Broadband BCB filtered output #1 and the LF/HF Diplexer Unit:

This output goes to the "LF/HF Diplexer Unit" which removes signals below approximately 375 kHz from the signal path (from the TCI-530).
Another input to this module comes form an active antenna designed for LF reception. A low-pass filter removes signals below approximately 375 kHz which is then fed to an amplifier.
The high-pass filtered RF from the TCI-530 and the low-pass filtered output from the LF active antenna are combined to a single output.
This output goes to a "limited attenuation" high-pass filter for the KiwiSDR system (that device is described here) that reduces low frequency HF signals (below about 8 MHz) by 12dB but leaves higher-frequency signals (above approx. 12 MHz) unchanged. Because signals and noise at the lower frequencies are so much stronger than those at the higher frequencies (particularly during this portion of the sunspot cycle) feeding a "flat" response from an antenna would cause overload of the receiver by lower-frequency signals, particularly at night, if one attempted to add enough system gain to allow reception of background thermal/ionospheric noise at the high end of the HF spectrum (e.g. 12-10 meters).
Following the high-pass filter is another amplifier gain block, adding about 14dB to the signal level. Being done "post filter" reduces the likelihood of overload by the many strong lower-frequency signals.
The output of this amplifier goes into HF/LF combiner and then to the 4-way splitter as shown: These outputs feed KiwiSDR units that are capable of reception from nearly DC up to 30 MHz.

Broadband BCB filtered output #2a:

This output goes directly to the "AGC Filter Block" (described here) that provides both band-pass filtering for the connected RTL-SDR dongle receivers (those for "90/80", "60", "41/40" and "31/30" meters) as well as an AGC gain control mechanism that limits the average signal level to the RTL-SDR dongles to a "safe level" that prevents significant signal overloading. This allows more system gain to be placed in front of the RTL-SDRs so that they perform better when the band conditions are poor yet prevent strong signals from clobbering these receivers when the bands are open. Doing this maximizes both weak and strong signal performance of these (admittedly low-performance) receive devices on HF.
Because of the addition of the "AGC Filter Block" which contains filtering, the bandpass filters shown for "60" and "31/30" meters are no longer used.
Because of the improved usability of the RTL-SDR dongles when used in conjunction with the filtering and AGC, the "90/80" and "41/40" meter receivers were implemented to provide a lower-performance (but still generally usable) alternative to the heavily-used 80/75 and 40 meter band receivers should the WebSDR #1 server go offline.

Broadband BCB filtered output #2b:

This output still goes to a 4-way splitter of which two ports are used to feed the "AM-160-120M" receiver and the 25 and 19 meter SWBC receivers.

Band-pass filter/attenuator modules for the RTL-SDR dongles:

If you've been reading along you'll already know that it is imperative that RTL-SDR dongles used on HF (or anywhere else) MUST have filtering of some sort on their RF input: It's not just the signals in the frequency range of interest that are "seen" by the A/D converter when operating in "Direct" mode, but all signals at all frequencies. In order to maximize what (little) signal handling capability these devices have, it is required that effective filtering be used.

As mentioned previously, one must also provide a means of adjusting the RF single levels being applied to the input of an RTL-SDR dongle, trying to find the "sweet spot" where there is enough attenuation to prevent overload by strong signals yet there is enough overall system gain to receive weak signals. This balancing act can be quite tricky - particularly when one considers the number of signals and that the strength of those signals vary dramatically between day and night. At the Northern Utah WebSDR, we are "fortunate" in that there are no strong shortwave broadcast stations "nearby" that beam their signal in our direction - but you are in Europe and eastern North America, the story can be quite different, with multi-hundred kW stations being beamed in your direction and only one "hop" away!

The diagram of the filter module is shown in the bottom of Figure 2 and enough information is provided for several options. A two-way splitter is depicted on the diagram to allow the feeding of two separate RTL-SDR dongles and their filters while off to the side, a 3-way splitter is shown. If a 4-way splitter were required, one would cascade a pair of 2-way splitters after a single 2-way splitter (for a total of 3 splitters) - but as noted on the diagram, each set of splitters would incur a loss of about 3.5dB. If no splitting is required, these would simply be left off.

The upper portion of this diagram also depicts a filter suitable for use on the AM and 160 meter bands. The left-hand portion is a 500 kHz high-pass filter that removes potentially strong LF signals and noise while the right-hand portion cuts off signals above approximately 2.5 MHz. On the output of the filter is a very simple attenuator that is used to adjust the signal levels being fed to the RTL-SDR. Using a single potentiometer, this attenuator is not a "constant impedance" device, but it does provide an "approximate" load for the filters to preserve their general characteristics. In reality, the RTL-SDR really doesn't care about its input source impedance, and at HF frequencies with fairly short cables, it's not all that important, either!

Also depicted in the diagram is a band-pass filter along with the same attenuator seen in the low-pass portion. The design of this band-pass filter is one that is "borrowed" from the QRP Labs web site, from their "Band Pass" filter products (a link to that page is here). In the assembly manual, which may be found on that web page, you will find a technical description of the filters (along with some representative band-pass plots) that provide enough information for you to build your own filters. If you wish, you may buy these modules in kit form and I can recommend that any of the kits sold by QRP Labs are worth getting! If you plan to cover a frequency range that isn't shown - such as a shortwave broadcast band - these filters can be tuned/modified from the nearest amateur band.

As noted previously, these RTL-SDR modules are somewhat "deaf" so it is likely that some sort of RF amplifier will be required - particularly to provide the bit of "excess" signal that one would need to be able to adjust levels downward again: Any of the 2N5109-based amplifier modules described earlier in this page will fit the bill nicely.

Finally, remember that RTL-SDR dongles in the "direct" mode aren't really all that well-suited for covering the 20 or 10 meter bands owing to the Nyquist limitations - and reception on frequencies between these bands (e.g. 17, 15 and 12 meters) will suffer a bit owing to decreased sensitivity and the increased tendency for spurious signals to appear. On 20 through 10 meters one would be better off using a dongle that includes an "up converter" - or build a simple "down converter": In any case you will always want to use a band-pass filter in front of the RTL-SDR dongle's receive system to maximize its performance!

RF Distribution and Filtering from the LP-1002 Log Periodic Beam antenna:

**Figure 3:|**
Signal path diagram for the LP-1002 Log Periodic beam antenna.
Not shown is a lightning arrestor on the "HF Antenna" input.
Click on the image for a larger version.

Also on site is a U.S Antenna Products (formerly "Hy-Gain") LP-1002 antenna. This is a very large log-period beam antenna designed for coverage from 6 through 40 MHz that is pointed east on a non-rotatable mount and the signal path for this system may be seen in Figure 3. From the beam to the bottom of the tower is an approximately 80 foot run of 1/2" "Heliax" cable which has a loss of less than 0.4dB at 30 MHz.

At present, the feedline between the antenna tower and the building with the receivers is temporary, consisting of about 150 feet of RG-6 cable laying on the ground: A permanent feedline is awaiting the equipment needed to bury it underground.

LF/HF Diplexer + HF amp:

At the base of the tower for this antenna is a module that takes the signals from the beam, removes signals above 40 MHz (to remove FM broadcast signals) and filters signals below 1.8 MHz (to remove AM broadcast signals). This filtered signal path is then amplified using a 2N5109-based amplifier to provide gain to minimize noise figure degradation along the rest of the HF signal path. The output of the amplifier is then passed through a 1.4 MHz high-pass filter which forms part of a combiner network, mentioned below.

Also provided by this box is an input from the active LF antenna that is mounted on this tower. The signals from this antenna are low-pass filtered at 1.4 MHz and combined with the HF signals from the beam. The DC power fed via the coax is used not only to power the LF active antenna, but also the aforementioned HF amplifier.

All inputs contain gas-discharge tubes

LF/HF/DC Coupler/Splitter:

This device is located at the "receiver" end of the feedline from the antenna and it contains a 1.4 MHz high-pass filter to separate the HF signals and a 1.4 MHz low-pass filter to separate the LF signals: The former are sent to the Input protection/Splitter/BPF Module (described below) and the latter is sent to the LF input of the LF/HF Diplexer unit mentioned above.

Input protection/Splitter/BPF Module:

The RF input to this unit passes through a Mini-Circuits ZFDC-20-3 coupler which, like that used on the TCI-530 system, allows the injection of test signals into the receive system while causing well under 1dB of loss: This allows the testing and calibration of the receive gear without interrupting the signal path. Via prior measurement, the amount of overall amplification provided by the amplifier at the antenna is known, allowing the absolute signal level at the antenna terminals to be known.

The input to this module has a 60 volt gas discharge tube to protect against lightning and like the similar unit on the TCI-530, the signal path is immediately split two ways - one going to the narrowband filter system for the individual receivers of the WebSDR and a "wideband" signal path to feed receivers like such as the KiwiSDRs.

The "narrowband" path goes to a diplexer that splits the signals two ways: A high-pass filter that allows signals above 9.5 MHz to go through, which is then fed to the "High HF Band-Split Module" for 30 meters and up and a 7.6 MHz low-pass filter. Beyond the 7.6 MHz low-pass filter, a "strong" 6.8 MHz high-pass filter provides signals for the 40 meter band, the high-pass filter offering about 20dB attenuation of the extremely strong 49 meter (6 MHz) shortwave broadcast signals. The output from 6.5 MHz low-pass filter is for experimental use: It allows lower-frequency signals that might be intercepted by the beam antenna - even those below the 6 MHz lower limit - to be passed to receivers that might be used in the future.

High HF Band-Split module:

This module is nearly identical to the same-named module used on the TCI-530 signal path described above. This unit consists of individual band-pass filters that pick their respective amateur bands' frequencies off a common bus: This approach is taken instead of a signal splitters as it provides lower loss than a simple signal splitter plus it intrinsically provides the needed band-pass filtering for each, individual amateur band. Because overall system gain is provided at the antenna, there is no amplifier immediately preceding this module as there is in the TCI-530 signal path.

References:

Youngblood, Gerald (July 2002), "A Software Defined Radio for the Masses, Part 1" (PDF), QEX, American Radio Relay League: 1–9
Youngblood, Gerald (Sep–Oct 2002), "A Software Defined Radio for the Masses, Part 2" (PDF), QEX, American Radio Relay League: 10–18
Youngblood, Gerald (Nov–Dec 2002), "A Software Defined Radio for the Masses, Part 3" (PDF), QEX, American Radio Relay League: 1–10
Youngblood, Gerald (Mar–Apr 2003), "A Software Defined Radio for the Masses, Part 4" (PDF), QEX, American Radio Relay League: 20–31
Johnson, Gary, "Measurements on a Multiband R2Pro Low-Noise Amplifier System, Part 2" (PDF)
Reisert, Joe, (November, 1984), "High Dynamic Range Receivers, Ham Radio. An English translation of part of this article from a Dutch web site may be found here.
Turner, Clint, (March, 2018), "Managing HF signal dynamics on an RTL-SDR receiver"
Farson, Adam, "Antenna and Receiver Noise Figure"

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.

RTL-SDR Dongle-based receivers - Described here are the "not high performance" receivers using the ubiquitous 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 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 ka7oei dot com.

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