Nothern Utah WebSDR Logo - A skep with a Yagi Northern Utah WebSDR
Receiving equipment
RF Distribution and filtering

Figure 1:
Block diagram of the various RF signal paths at the Northern Utah WebSDR.
This diagram is slightly out of date and will be updated as soon as time permits - see notes.
Click on the image for a larger version.
Block diagram of the AM BCB splitter-amplifier and low HF splitter
RF Distribution and Filtering:


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:
  1. 630 meters
  2. 160 meters
  3. 80/75 meters *
  4. 60 meters 
  5. 40 meters *
  6. 30 meters
  7. 20 meters *
  8. 17 meters
  9. 15 meters *
  10. 12 meters
  11. 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:
While it is certainly possible to make this scheme work, there's another method:  Use a "diplexer" type splitter.

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:
To accomplish this several modules were built, depicted in Figure 1:
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.
Click on an image for a larger version.
The AM BCB filter/splitter module schematic
The low HF splitter schematic
The High HF splitter schematic
The BPF/LPF/Attenuator for RTL-SDR receivers
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.

Notes on updates to Figure 1:

Since Figure 1 was drawn, a few changes have been made to the signal path as noted below.  The diagram will be corrected as time permits:

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!



References:

  1. Youngblood, Gerald (July 2002), "A Software Defined Radio for the Masses, Part 1" (PDF), QEX, American Radio Relay League: 1–9
  2. Youngblood, Gerald (Sep–Oct 2002), "A Software Defined Radio for the Masses, Part 2" (PDF), QEX, American Radio Relay League: 10–18
  3. Youngblood, Gerald (Nov–Dec 2002), "A Software Defined Radio for the Masses, Part 3" (PDF), QEX, American Radio Relay League: 1–10
  4. Youngblood, Gerald (Mar–Apr 2003), "A Software Defined Radio for the Masses, Part 4" (PDF), QEX, American Radio Relay League: 20–31
  5. Johnson, Gary, "Measurements on a Multiband R2Pro Low-Noise Amplifier System, Part 2" (PDF)
  6. 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.
  7. Turner, Clint, (March, 2018), "Managing HF signal dynamics on an RTL-SDR receiver"
  8. Farson, Adam, "Antenna and Receiver Noise Figure"

Pages about other receive gear at the Northern Utah WebSDR:
Go to the main "RX Equipment page.


Additional information:
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