Many thanks, Ivan, for sharing these comparison tests! I agree with you that it’s most helpful to look closely at the peak to valley ratios on the spectrum display rather than using the audio levels as a measure.
Many thanks to SWLing Post contributor, Tudor Vedeanu, who has kindly shared details about his portable Raspberry Pi system which now can run the AirSpy HF+ SDR.
I bought the RPi to use it as a Spyserver for my Airspy HF+ SDR.
My main radio listening location is a small house located on a hill outside the city and there is no power grid there (it’s a radio heaven!), so everything has to run on batteries and consume as little power as possible.
My first tests showed that the Raspberry Pi works very well as a Spyserver: the CPU usage stays below 40% and the power consumption is low enough to allow it to run for several hours on a regular USB power bank. If I add a 4G internet connection there I could leave the Spyserver running and connect to it remotely from home.
Then I wondered if the Raspberry Pi would be powerful enough to run a SDR client app. All I needed was a portable screen so I bought the official 7” touchscreen for the RPi.
I installed Gqrx, which offers support for the Airspy HF+. I’m happy to say it works better than I expected, even though Gqrx wasn’t designed to work on such a small screen. The CPU usage is higher than in Spyserver mode (70-80%) but the performance is good. Using a 13000 mAh power bank I get about 3.5 hours of radio listening.
This is fantastic, Tudor. Thanks for taking the time to put together a video for us. I’ve just ordered the latest Raspberry Pi 3 (Model B+). It has slightly more horsepower than the previous Pi3. Tudor, you’ve inspired me to grab the 7″ touch display as well and try my hand at running the AirSpy HF+ portable.
I’m not sure if the Raspberry Pi 3 will be able to record spectrum without hiccups, but it’s certainly worth a try.
As you tweak your system, please keep us in the loop!
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.
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!
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.
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