Tag Archives: Airspy HF+

The AirSpy HF+ R3 bypass modification

After SWling Post contributor, Guy Atkins, posted the survey results of his excellent Elad FDM-S2  vs AirSpy HF+ weak signal comparison, I received a few questions about the AirSpy HF+  “R3 Bypass” modification Guy mentioned in his post.

Guy has not yet performed the modification on his HF+–neither have I–but he points out that others have noted it: “significantly boosts sensitivity of the HF+ from longwave up to about 15 MHz, without any noted overload issues.”

I reached out to AirSpy president, Youssef Touil, for a little more insight about this modification. Youssef replied:

During the early phases of the design R3 was a place holder for a 0 ohms resistor that allows experimenters to customize the input impedance. For example:

  • A 300 pF capacitor will naturally filter the LW/MW bands for better performance in the HAM bands
  • A 10µH inductor would allow the use of electrically short antennas (E-Field probes) for MW and LW
  • A short (or high value capacitor) would get you the nominal 50 ohms impedance over the entire band, but then it’s the responsibility of the user to make sure his antenna has the right gain at the right band
  • A custom filter can also be inserted between the SMA and the tuner block if so desired.

Click to enlarge. (Photo source: RTL-S1DR.com)

R3 and the nearby resistors have been intentionally left outside of the RF shield, and their size was picked to be big enough to allow anyone to play with them. You will notice the size difference with the rest of the components.

In general, unless one knows what he’s doing, it’s not recommended to alter a working system. “If it’s working, don’t fix it”. But, we are hobbyists, and not doing so leaves an uncomfortable feeling of something unachieved. Most brands addressing the hobby market leave some tweaks and even label them in the PCB.

The main purpose of the HF+ is the best possible performance on HF at an affordable price. This is to incite HAMs to get started with this wonderful technology while using an SDR that isn’t worse than their existing analog rig.

The MW/LW/VLF crowd may have slightly different requirements, but that can be addressed by shorting a resistor.


Youssef Touil

Thank you, Youssef, for replying to my inquiry so quickly and thoroughly.

No doubt, I too will eventually modify R3–it’s very difficult not to experiment, especially when a product was designed with the experimenter in mind.

I really feel like AirSpy has knocked it out of the ballpark with the HF+. For those of us primarily concerned with HF performance, this SDR is very hard to beat–especially at its $199 price point!

Video: Comparing the SDRplay RSP1A and Airspy HF+ on HF & MW

Many thanks to SWLing Post contributor, Ivan Cholakov, who shares the following video where he compares the SDRplay RSP1A and the AirSpy HF+ software defined radios on shortwave and mediumwave:

Click here to watch on YouTube.

Digging in the Noise: Weak Signal Audio Recovery with the AirSpy HF+ and Elad FDM-S2

I’m currently spending the better part of a week at Cape Lookout State Park on the Oregon coast, with a great view of the ocean through tall evergreen trees. This is one of my favorite parks in the Pacific Northwest, especially when DXing during the blustery winters from one of the nice cabins at Cape Lookout.

The view from the beach near my cabin; the turbulent waves were a precursor to the gale force winds at the park during the night of the 23rd!

Although I’m at the park for trans-Pacific medium wave DXing, I’m also comparing receivers, both SDRs and portables. This morning I sought out a few weak shortwave signals, pitting the Elad FDM-S2 SDR ($529 USD) against the AirSpy HF+ ($199 USD). I have a pair of the HF+ receivers to cover all of medium wave (as the FDM-S2 easily does). Many SWLing Post readers already know that the upstart HF+ trades bandwidth to gain high performance in order to keep the price reasonable.

My antenna used for the following recordings was a small “Flag” antenna using a Wellbrook Communications FLG100LN module and a 2K ohm variable potentiometer for termination. The design uses crossed tent poles in an “X” formation to support the wire loop. This design travels easily in a compact package; I have Dave Aichelman of Grants Pass, Oregon to thank for this very useful “tent pole loop” implementation of the Wellbrook FLG100LN.

The Wellbrook-based antenna functions superbly, and its low-noise design helps hold down QRM from the nearby cabins (which unfortunately have been “upgraded” recently with noisy cold fluorescent [CFL] light bulbs). The area around the Cape Lookout cabins used to be superbly low noise and suitable for radio listening, but now it is more of a challenge than before. The Wellbrook FLG100LN is perfect for the situation though; Wellbrook ALA1530LN  Pro and ALA1530S+ 1-meter loop antennas work commendably at the park too.

The Wellbrook FLG100LN module with a home brew RFI choke in-line

A 2K ohm variable potentiometer is protected from the elements in a small plastic bag. The “pot” is adjusted for the best nulling of medium wave stations off the back side of the antenna’s reception pattern.

The “tent pole loop” antenna is strapped to a fence railing with ultra-strong Gorilla Tape to keep the 7-ft. square loop vertical.

On with the recordings…

For the FDM-S2 and HF+ comparisons I used the SDR-Console V3 software. Every parameter was identical for the receivers–sampling bandwidth, filter bandwidth, AGC, mode and so on.

Take a critical listen to the weak signals recorded with the SDR receivers, identified as only “Radio A” and “Radio B”. A link to a poll is at the end of this article; please indicate which recording of each pair has the most intelligible audio in your opinion, and submit your choices when you’ve made up your mind on each audio clip. After a week or so I’ll post the results of the voting, and identify the receivers.

9.615 MHz, LSB, Radio A

9.615 MHz, LSB, 
Radio B (note: the same male announcer heard in clip “A” begins at 00:14 in this “B” clip)


9.730 MHz, USB, Radio A

9.730 MHz, USB, 
Radio B


7.230 MHz, S-AM, Radio A

7.230 MHz, S-AM, 
Radio B


9.860 MHz, S-AM, Radio A

9.860 MHz, S-AM, 
Radio  B


Note on 7.230 MHz recording: this was an interesting frequency, as the signal was tightly surrounded by a very strong local 40m ham radio LSB station as well as a strong China Radio International signal. There were other strong amateur and broadcast stations within 30-50 kHz of 7.230 MHz, also. This A-B test more than the others may indicate receiver performance in a strong RF environment on a crowded band.

Ready for the poll? Register your votes at the Google Docs form below:


In a week to 10 days I’ll post the results in another article. NOTE: I haven’t provided a “both sound the same” choice in the poll to encourage you to ‘dig deep’ into the audio and listen critically–to find something that stands out in one clip versus the other.

Guy Atkins is a Sr. Graphic Designer for T-Mobile and lives near Seattle, Washington.  He’s a regular contributor to the SWLing Post.

Ivan compares the AirSpy HF+ to the KiwiSDR

Many thanks to SWLing Post contributor, Ivan (NO2CW), who writes:

I have been running a public Kiwisdr server for a while and yesterday decided to plug in the new Airspy HF + into the same antenna for a side to side comparison. The antenna is an 80m dipole and the test was done during local afternoon, around 3 PM. I did not use any of the many new noise reduction features that are incorporated into both SDR Console 3 and the SDR web server. The 11 minute video is located here:

Click here to view on YouTube.

When I have the time I will run a similar test in nighttime conditions and also test the Airspy HF+ against a few other radios sitting on my desk.

Thank you for sharing this, Ivan!

The Airspy HF+ SDR: First impressions

Yesterday, I received a package in the mail containing the new Airspy HF+ software defined radio receiver.

It came as a bit of a surprise.

I’ve been busy lately with humanitarian work, the radio spectrum archive,  product evaluations and travels–not to mention an active family life. I had completely forgotten that about a month ago, I received a message from Airspy telling me that they had dispatched their latest SDR to me for evaluation.

This morning, I unpacked the box to find that the HF+ is a very compact, relatively dense little SDR in a metal alloy case/enclosure. I’m simply floored by the size. The case feels incredibly durable and of excellent quality.

The Airspy HF+ under my business card

In terms of footprint, dimensions are nearly identical to a business card. It’s about as thick as the typical USB memory stick.

The HF+ is so thin and compact compared with the Elad FDM-S2 (which is quite a compact SDR!)

Other than my RTL-SDR dongle, it’s by far the smallest SDR I’ve ever tested.

Though compact, it sports two SMA antenna inputs: one for HF, one for VHF.

The HF port is labeled as “H” and the VHF port as “V”

The Airspy HF+ uses a common micro USB for both data and power

The HF+ is the first AirSpy product I’ve ever put on the air.

I read the HF+ product description this morning while downloading SDR Sharp–it claims the HF+ is “plug and play”.

Turns out, that is no exaggeration.

The HF+ on top of the Elad FDM-S2 and WinRadio Excalibur

In fact, the entire SDR Sharp package downloaded in seconds, installed in seconds and the only thing I had to do after opening the SDR Sharp application was select “AirSpy HF+” from the product drop down menu.

I started the application and *boom* signals all over the place!

SDR applications, in general, have become so much easier to install over the years but I believe SDR Sharp may be the  quickest install I’ve ever experienced. About as easy and lightweight as the WinRadio Excalibur application. True plug and play!

I’ve had no time to properly evaluate performance–I just put this little SDR on the air.

Fortunately, propagation is cooperating this morning–I’m hearing both WWV Fort Collins and WWV Hawaii on 10 MHz. I’m really enjoying playing with the Airspy HF+ and re-familiarizing myself with SDR#. I had forgotten how responsive and intuitive the interface is–great user design.

My first impression?  I’m impressed. More to come…

Click here to read about the HF+ on the Airspy website