Tag Archives: Software Defined Radio

Airspy Youloop and Homebrew Passive Loop Antenna designs

Almost two weeks ago, at the 2020 Winter SWL Fest, I gave a presentation called “A New Era in Portable SDR DXing.

The presentation was essentially an in-depth version of an article I published in the January 2020 issue of The Spectrum Monitor magazine (see cover above).

I devoted a good portion of the presentation describing how to build a passive loop antenna design by Airspy’s engineer and president, Youssef Touil. This passive mag loop takes advantage of the Airspy HF+ Discovery‘s exceptionally high dynamic range and is an impressive performer.

The homebrew loop on the balcony of a hotel.

You may recall, I posted a short article about this loop in November after enjoying a little coastal DXing.

In short? This passive loop antenna pairs beautifully with the Airspy HF+ Discovery. I’ve also been very pleased with results using the new SDRplay RSPdx on the mediumwave band where the receiver now sports a high dynamic range mode.

Overdue corrections…

After returning from the Winter SWL Fest last week, I was hit with an upper respiratory bug. No doubt, a souvenir of my travels!  It wasn’t the flu (I was tested), nor COVID-19, but it did knock me off my feet for a few days with fever, coughing, and headaches. You might have noticed a lot less posts last week and almost no replies from me via email. I’m only now feeling totally human again and trying to catch up with my backlog.

Shortly after my SWL Fest presentation, I realized I made (at least!) two mistakes. I had planned to post corrections here on the SWLing Post last week, but the bug delayed all of that, so here you go:

#1 Schematic of my homebrew passive loop antenna

When Youssef started experimenting with passive loop antenna designs, he posted a few schematics of at least three build options.

Although I described how to build my passive loop antenna, I grabbed the wrong schematic for my presentation slides. Many thanks to those attendees who noticed this.

Here is the schematic I should have shared:

Note that the transformer has four turns on both sides (the one in the presentation had 4:2).

Again, apologies for any confusion.

#2 The Airspy Youloop passive loop antenna

If you’re not inclined to build your own passive loop antenna per the diagram above, Airspy is planning to manufacture and sell a lightweight, high-performance loop of a similar design.

Prototype of the Airspy Youloop in the field (note bright blue cable jacket)

During the presentation, I called the future AirSpy antenna, the “Spytenna.” I was incorrect. (Turns out, I got this name from an early antenna schematic and somehow it stuck in my head!)

Airspy is calling their passive loop antenna the Youloop. Youssef posted the following note in the Airspy email discussion group:

We are currently arranging the shipping of the affordable passive version to Airspy.us and RTLSDR Blog.

Btw, It’s called “Youloop”

Many thanks to Richard Langley and a number of other readers who pointed this out last week.

I’ve had a prototype of the Youloop since November and brought it to the SWL Fest and presentation. It’s a quality antenna and incredibly compact when disassembled and rolled up.

When the Youloop is available to order, we’ll post links here on the SWLing Post.

More to come!

Once I catch up here at SWLing Post HQ, I plan to publish detailed construction photos of the homebrew loop antenna.

Many of you have questions about how to tap into the center conductor at the mid-point of the loop. These photos should help guide you.

Stay tuned!

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KiwiSDR: A brilliant new map portal!

Last month, we noted that the popular SDR.hu KiwiSDR portal now requires registration and an amateur radio callsign to use the site.  While SDR.hu is still online, we certainly get the impression all site development has been halted. This is not the end of the world because SDR.hu is only one of several KiwiSDR portals–we linked to others in our January article.

Personally, I only used the SDR.hu map view to keep track of KiwiSDR sites and found it quite useful because I typically select sites based on geographic location.

A better KiwiSDR map portal

I’ve just learned via the KiwiSDR Twitter account that Priyom.org has updated their KiwiSDR map portal using Dyatlov maps. The results are brilliant and, in my opinion, even better than the SDR.hu’s map.

The Priyom.org map uses the full window, is uncluttered and easier to navigate.

If you click on a KiwiSDR site, you’ll see a pop-up window with basic site information. If you hover the cursor over that site info, another window will pop up with current details about the receiver, number of users, antenna, SNR, and GPS clock (see above).

This is now my favorite way to geographically surf KiwiSDR sites.

Click here to check out the new map portal.

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The Malahit-DSP: A potential Holy Grail portable SDR?

(Image via Fenu Duarte)

Many thanks to SWLing Post contributor, Dan Robinson, who provides this update to his article on the Belka DSP receiver. Dan writes:

One of the other DSP radios shown being demonstrated on YouTube is this one by Georgy Yatsuk, presumably from somewhere in Russia. In his comments posted on January 14th, Fernando Duarte who runs the FENU site says:

“This little gem makes a name for itself. Everyone wants it. But it is still difficult to get. Georgy Yatsuk (RX9CIM) developed this small portable SDR with two of his colleagues. What this little guy offers is simply phenomenal! -Frequency range: 50KHz-2GHz -All important types of operation -160KHz wide waterfall & spectrum display zoomable -Noise reduction adjustable -Noiseblanker adjustable -Equalizer for adjusting the timbre – Controllable via PC -Control via CAT -etc, etc … Because the firmware is still in full development, certain functions will definitely be added. A test and a detailed presentation will be available on my website in the near future. Stay tuned !!”


As of this moment there is no additional information as to whether this seemingly excellent receiver will ever become available and in what numbers.

Many thanks for this update, Dan! We published a post about the Malahit-DSP in November on a tip from H. Garcia (PU3HAG). I have put in an inquiry to purchase one to evaluate here on the SWLing Post as well.  This does look like a fascinating portable SDR!

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New ELAD FDM-S3 Direct Sampling Wideband Receiver – Specifications and Photos

Many thanks to Paul Jones with ELAD who recently shared the following photos and specifications of the upcoming FDM-S3 SDR:

ELAD FDM-S3 Direct Sampling Wideband Receiver


2 switchable HF Antenna inputs direct sampling

1 VHF Antenna input direct sampling

Works with FDM-SW2 ELAD Software & SDR Console

Optional: Antenna RF input downconversion (50MHz – 2GHz preview)

Real Time I/Q Stream Bandwidth 192kHz, 384kHz, 1536kHz, 12880kHz, 24576kHz

122.88 MSPS – 98.304 MSPS 16bit A/D converter

Clock synchronized to GNSS Global Navigation Satellite System or 10MHz Ext Ref


Auxiliary USB used to monitor GPS status or for clock firmware updates

10MHz Clock reference Output

10MHz internal standard TCXO 100ppb referenced, optional 3ppb OCXO referenced

Paul notes that the price will be 949 Euro (roughly $1040 USD). No delivery updates were mentioned.

I’m a bit in awe of the maximum working bandwidth: 24.576 MHz–!

No word on availability yet, but I will post it when ELAD has a firm date. I do plan to review the FDM-S3 once it’s released. Follow updates by bookmarking the following tag: ELAD FDM-S3

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A compact homebrew Si5351-based SDR

(Image source: Circuit Salad)

Many thanks to SWLing Post contributor, Paul Evans, who writes:

I see this receiver as a remarkable break through. Using audio processor to emulate modes from IQ is very, very clever. This is perhaps the article of the year!:

(Source: Circuit Salad)

[…]This is a revised version of my FV-1 based SDR. I replaced the CS2100 clk generator with the Si5351 clk generator. The Si5351 has some advantages over the CS2100, namely you can generate quadrature clks directly. This simplifies the hardware design and improves the quadrature accuracy. The sideband rejection in LSB/USB modes is impressive..somewhere around 60 db as best I can measure. The DSP processing is accomplished by the use of a FV-1 audio processor. The device makes the base band signal processing a snap. It requires some code to be loaded on a EEprom but the circuitry is simple and allows for up to 8 selectable programs. I created three: AM/USB/LSB . The FV-1 provides for three analog POT inputs to control any parameters you choose. Gain, variable filter bandwidth and depth, AGC are some examples of adjustable parameters if you desire. I kept it simple and created fixed band pass filters to taste. I did use one of the controls for AF gain. The design has no tuned circuits or band pass filters but they could easily be added.  It works just fine without them. Occasionally, I come across a ghost signal from harmonic mixing, when tuning, but not enough to matter. The design uses an OLED display and a rotary encoder for tuning. The frequency coverage is from 2.7 Mhz to 25Mhz. The bottom limit is created by the inability of the Si5351 to support quadrature below this frequency. Although I have improved my DSP programs for the FV-1 and have developed new display drivers and the new code for the Si5351, useful detail about using the Fv-1 can be found in my original design from a few years ago: https://circuitsalad.com/2015/06/19/comming-soon-stand-alone-software-defined-radio-baseband-demodulator-no-computer-required/

The design uses a LTC6252 low noise op amp as an RF input with gain. It provides a constant and reliable resistive Rf termination for the sampling detector.  This allows for random antennas to be used without adversely affecting the input termination to the detector. All the code to operate the main processor(display/clk generator/tuning, band select and receive mode) was written in MikroC which is a C compiler for PIC and AVR processors. The generation of quadrature signals out of the Si5351 is not difficult to implement once you know how but..figuring that out took me a couple weeks of experimentation! You can connect switches, the encoder, volume pot and display directly to the main board for operation but I created a secondary board to mount the display and encoders. Instead of an analog pot and selection momentary switches, I used another microcontroller and two encoders(with one built in momentary push switch each) to create all of the switching signals, gain control, etc. This allowed me to have just two controls for all features.  The controls include: tuning, audio gain, mode, and tuning step. Tuning resolution is from 1Hz to 100KHz . For fun, I made the output of the FV-1 differential into the audio amp. This is not necessary.

Here is a link to all the files used to build this radio in a zip file(updated 1/18/20):


Demo video

Click here to read the full article, download all design notes/files and watch videos at Circuit Salad.

Wow–that is fascinating! Thanks for sharing, Paul. I’m curious if any SWLing Post readers have experimented with the Si5351.

Interestingly, SWLing Post friend Dave Richards (AA7EE), also recently shared this video of an amazing Si5351-based VFO built by JF3HZB:

This must be one of the best analog emulations I’ve seen on a display. Marry the SDR receiver above to this VFO and you could have a top-shelf homebrewed receiver!

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Mike Ladd’s primer on decoding NAVTEX using an SDRplay SDR with SDRuno

Many thanks to SWLing Post contributor, Mike Ladd with SDRplay, who shares the following PDF primer on decoding NAVTEX with an RSP series SDR.

Click here to download “Basics to decoding NAVTEX using an RSP and SDRuno” (PDF).

Thanks for sharing this excellent guide, Mike.  Without a doubt, SDRplay has some of the best documentation and primers in the world of radio. Click here to check out more.

Also, check out Mario’s post, from our archives, which discusses decoding NAVTEX, RTTY, and Sitor B.

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Mike compares the SDRplay RSPdx on mediumwave and longwave

Many thanks to SWLing Post contributor, Mike Ladd with SDRplay, who shares the following videos comparing the new RSPdx with a number of benchmark SDRs:

SDRplay RSPdx and ELAD FDM-S2 weak NDB station

SDRplay RSPdx and ELAD FDM-S2 medium wave selectivity

SDRplay RSPdx and Airspy HF+ Discovery medium wave selectivity

SDRplay RSPdx and Airspy HF+ Discovery weak NDB station

SDRplay RSPdx and Microtelecom Perseus medium wave selectivity

SDRplay RSPdx and Microtelecom Perseus weak NDB station

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