DRAFT HF+ REVIEW

Over the last few months we’ve been posting and getting excited about the Airspy HF+, an upcoming high dynamic range HF/VHF receiver designed for DXing. The Airspy team were kind enough to supply us with an early pre-production unit for review.

Long story short, the Airspy HF+ is probably one of the best low cost SDRs we’ve seen for DXing or weak signal reception out there. So far few details on the availability of the HF+ have been released, but we’re aware that preorders are due to start soon, and the target price is expected to be $149 USD from iTead Studio in China. 

What follows is the full review and comparisons against other similarly priced SDRs. The Airspy team want us and readers to understand that our review unit is a pre-production model, and apparently already the matching and thus SNR has already been improved by about 1-3 dBs.

Disclaimer: We received the HF+ for free in exchange for an honest review, but are not affiliated with Airspy. We’ve been in contact with the Airspy team who have helped clarify some points about the architecture and technology used in the design.

Introduction

The Airspy HF+ is designed to be a HF/VHF specialist receiver with a frequency range of DC to 31 MHz, and then 60 to 260 MHz. It has a maximum bandwidth of 768 kHz. So the question is then, why would you consider buying this over something like the regular Airspy R2/Mini or an SDRplay RSP2 which both have larger frequency ranges and bandwidths? You would buy the Airspy HF+ because has been designed with DXing and weak signal reception in mind. Basically the main idea behind the HF+ is to design it so that it will never overload when in the presence of really strong signals. Combined with it’s high sensitivity, weak or DX signals should come in much clearer than on the other radios especially if you have strong blocking signals like broadcast AM/FM around.

What is overloading, intermodulation and dynamic range?

Basically strong signals can cause weak signals to be drowned out, making them not receivable, even though they’re there at your antenna. A simple analogy is to think about what happens when you are trying to drive, but there is sunstrike. The road is very hard to see because the sun is so bright and right in your eyes. The human eye does not have enough dynamic range to handle the situation of sunstrike. The same analogy applies to radios which can struggle to ‘see’ weak signals if there is a very strong signal nearby on the frequency spectrum. There are a few ways to solve this:

• Filtering: Block the strong signals that you don’t want using LC filters.
  • Light analogy: using your sun visor to block the sun.
• Attenuation: Reduce the strength of all signals.
  • Light analogy: using sunglasses or squint.
• Increase dynamic range: Get a better SDR with better design/technology and more bits in the ADC.
  • Light analogy: upgrade your eyes.

Technology

The HF+ uses a typical Filter->Tuner ->ADC architecture. So it is not a direct sampling receiver like most of the more expensive SDRs. Direct sampling receivers directly sample the analogue spectrum, without the need for a tuner so they avoid losses and the intermodulation problems that usually come from the mixing stages. But there are some major cutting edge technology differences in the HF+ architecture that should make it’s performance even better than direct sampling receivers.

Tuner: The tuner on the HF+ is one of the first to use a “Polyphase Harmonic Rejection” architecture. Essentially this means that harmonics produced in the mixing stages are naturally rejected, making the front end filtering requirements much more relaxed. So unlike the tuners used in other SDRs, this one is extremely unlikely overload in the mixing stage.

An additional benefit to this architecture is that the mixer is very low loss, so the LNA in the tuner only needs to use low gain, giving it a very high IIP3 value. So the first LNA which is typically another point of saturation and imermodulation, is very unlikely to saturate in the HF+ design. Most of the amplification only occurs after the mixing stage with the filtered narrowband output of the tuner.

Analogue to Digital Converter (ADC): The ADC is 16-bits and uses a “Sigma Delta” (ΣΔ) design. Basically a Sigma Delta ADC has a natural filtering ability due to its narrowband nature. Instead of seeing say a 30 MHz signal, it only sees 1 – 2 MHz, thus increasing dynamic range and reducing the likelihood of out of band overload.

Digital Down-Converter (DDC): Then after the ADC is a DDC which decimates the output from the ADC, increasing the effective number of bits. The more bits the larger the resolution of the digitized RF signal, so weak signals are less likely to be lost when converted from analogue to digital.

So the block diagram flow goes like this:

A weakly filtered signal enters the tuner, is weakly amplified by the tuner LNA, mixed down to baseband and filtered to 1-2 MHz. It is then amplified and sampled with the sigma delta ADC into 16-bits. The DDC decimates the output into 18-bits which is then sent to the microcontroller and PC via USB.

The Airspy team also compiled this comparison chart for us to understand the differences in architecture between the current SDRs on the market.

Software and User Experience

The Airspy HF+ runs on the standard SDRSharp software. The first thing you notice when selecting the HF+ on the SDRSharp menu is how simple the controls are. There is no gain control – the AGC algorithm automatically adjusts the internal gain for maximum SNR, whilst ensuring zero overloading. The only control is the bandwidth selector where you can select from 768 kHz, 384 kHz, 192 kHz, 96 kHz and 48 kHz. Browsing the spectrum without having to adjust the gain slider is quite a liberating experience and the AGC always seemed to optimize the reception nicely.

The HF+ is also compatible with the SpyServer software, which allows you to stream the data radio IQ data over a network. SpyServer saves network bandwidth by sending only the currently actively tuned IQ signal plus the waterfall data. This is in contrast to other SDRs like KiwiSDR which send only compressed audio, or rtl_tcp which sends the full IQ data. Sending the IQ data rather than compressed audio allows you to perform various DSP algorithms to the signal on the host side, such as noise filtering. Sending the IQ data (even if only a slice of it) still uses significantly more bandwidth compared to sending compressed audio however, so internet connections and wideband signals such as BCFM may not work well together over long distances and slow internet connections.

External Design/Photos

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Comparison SDRs

In this review we are doing side by side comparisons of the HF+ against similarly priced SDRs, including the ColibriNANO, Airspy Mini + SpyVerter Upconverter and the SDRplay RSP2.

Name	Freq. Rage	Bandwidth	Technology	Price
Airspy HF+	DC to 31 MHz 60 to 260 MHz	768 kHz – 18 bit	Polyphase Harmonic Rejection	$TBA – “Under $200”. Airspy team note “expected ~$149 USD at iTead Studio”
ColibriNANO	100 kHz – 55 MHz Up to 500 MHz undersampling.	768 kHz – 24 bit 3 MHz – 16 bit	Direct sampling + Filters.	$350 USD
Airspy Mini	24 – 1800 MHz Down to DC with SpyVerter Upconverter	6 MHz – 12 bit	Upconverter + Direct Convesion	$99 USD $148 USD (incl. SpyVerter)
SDRplay RSP2	1 kHz – 2 GHz	10 MHz – 12 bit	Upconverter + Direct Convesion	$169.95 USD

Comparison Tests

In these tests we compare each SDR on a real world signal. SDRs are cycled through, taking screenshots and recording audio as fast as possible to ensure that conditions don’t change. To verify conditions didn’t change part way through we go through our loop twice to confirm that similar results are recorded.

The HF and below tests use a Wellbrook Loop antenna. VHF Tests use a discone or dipole tuned for the tested band. The RF environment is one with strong broadcast AM and FM stations. The location is 10km away from an AM tower, and LOS to the FM/TV transmitter tower.

In all cases the signal of interest is optimized for best SNR. For each SDR we used the officially recommended software package. For the Airspy devices this was SDRSharp, for the ColibriNANO this was ExpertSDR and for the RSP2 this was SDRUno.

With the RSP2 we used the recommended HiZ port for all LF – HF signals and also flipped between then Zero and Low-IF mode choosing the best one. Just to be sure, we tested the A and B ports on the RSP2 as well, but experienced heavy broadcast AM overload (with the filters turned off) and weaker signals with the filters on, so did not continue to use these ports.

On the ColibriNANO we used bandwidths at or below 768 kHz to get the 24-bit output.

A modern 2016 Core-i7 laptop is used for all tests, but all SDRs were confirmed to run smoothly on an older model core-i5.

LF (Low Frequency 40 kHz Time Signal)

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This signal is a 40 kHz time signal originating from Japan. It is know as the Ohtakadoya-yama LF Standard Time and Frequency Transmission Station (NICT).

From the screenshots we can see that the only SDRs successful at receiving this station where the HF+ and the Airspy Mini + SV.

• The HF+ comes in with a very clear copy and there is no sign of overloading from broadcast AM.  VLF signals down to 20 kHz are also visible and copyable.
• The Mini + SV receives the signal too, but there is significant overloading from broadcast AM stations present all around the signal. 
• The ColibriNANO cannot receive the signal at all. According to the advertised specifications the ColibriNANO starts receiving at around 100 kHz so this is expected. From the screenshot we start to see a response at around 70 kHz.
• The RSP2 just barely receives the signal (a very faint line is visible in the waterfall), but no audio was copyable. There is some minor signs of overload from broadcast AM as well.

NDB’s (~325 kHz)

NDB’s or Non-Directional Beacons are beacons used to aide with aircraft navigation. In this test all SDRs were able to receive NDBs with good performance and it was difficult to notice a difference.

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• The HF+
• The Mini + SV 
• The ColibriNANO 
• The RSP2 

Broadcast AM

Here we tuned to the broadcast AM band and tested reception with one of the weaker signals.

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• The HF+ receives this weak station clearly and fading is minor.
• The ColibriNANO 
• The RSP2 also receives the station clearly, but there is a bit more static noticeable in the background.

2.6 MHz FAX

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• The HF+ received the fax cleanly.
• The Mini + SV was not tested as the fax finished before we could get to it.
• The ColibriNANO received the fax cleanly. 
• The RSP2 received the fax as well as the other receivers, but on the waterfall are some broadcast AM station images. Reducing the gain reduced the fax signal strength, but the images remained as well.

7 MHz Shortwave

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• The HF+ receives weak shortwave stations well with pretty clear audio.
• The Mini + SV has similar performance to the HF+
• The ColibriNANO seems to be a bit noisier.
• The RSP2 experienced quite a bit of fading which wasn’t as intense on the other SDRs. It didn’t seem to be related to changing conditions as switching back to another SDR after the RSP2 didn’t show the effect.

96.03 MHz FM

This is a low power DX station, very closely spaced on the frequency spectrum to a powerful station.

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• The HF+ quite clearly has a slight sensitivity edge on the broadcast FM spectrum as can be heard in the audio examples. The HF+ audio seems to be noticeably clearer.
• The AS Mini receives the station too, but it noticeably noisier.
• The ColibriNANO cannot receive the station at all. As it is working in undersampling mode, it is possibly overloaded. To use undersampling mode successfully filters are required.
• The RSP2 works similarly to the AS Mini.

96.2 MHz FM

Regular non-E’s FM station about 120km away.

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• The HF+ was able to receive the FM station fairly clearly
• The AS Mini could also receive the station, but the audio was barely audible. Turning up the gain further caused overload, a rise in the noise floor and a weakening of the SNR.
• The ColibriNANO could not receive this station, so we did not record any audio.
• The RSP2 was also able to receive the station, but like the AS Mini the audio was barely audible.

Pagers

In this location we have some very strong pagers at 158 MHz and most SDRs show signs of overloading near to the pager frequency. The HF+ seemed to handle them quite well, however when two transmitted at once there was a about a 100ms period of overload before the AGC kicked in to reduce the gain.

Here we tested a weak signal about 2.5 MHz below the pagers.

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• The HF+ was able to clearly receive this data station without any sign of overload from the pagers.
• The Airspy Mini could also receive the station, but much weaker. Turning up the gain any further caused overload, and caused pager + WFM noise to appear over the frequency whenever the pager transmitted. Even at a gain setting of 10 there was some mild interference noticeable in the screenshot when the pager transmitted.
• The RSP2 could also receive the station with fairly good strength, but overload was severe whenever the pager transmitted, causing a loss of signal. Turning down the gain did not help with the interference, and only reduced the signal of interests’ strength further. Enabling the MW/FM filter did not help as the pager interferer is outside the notch range.
• The ColibriNANO could not receive this station.

Conclusions

The Airspy HF+ is an exceptional SDR and will truly please any DXers or people wishing to listen to weak stations. It is a relatively narrowband SDR (in comparison to say the Mini/R2 and RSP2) that can only tune up to 260 MHz, so don’t expect to be able to use it as a wideband scanner for trunked radios for example. But on VHF it would perform very well on FM DX, airband voice scanning and for 137 MHz WX satellites.  The reception on the HF+ is almost entirely unaffected by extremely strong pagers in the 157 MHz region.

Below 30 MHz the HF+ also shines. VLF to MW is the best we’ve seen on any sub $300 SDR. Overload is non-existent on broadcast AM, and no effects from the strong AM signals can be seen further up on the spectrum.

The closest competing unit to the HF+ in terms of price and and use cases is probably the ColibriNANO. But the ColibriNANO commands a decently higher price at $350 USD. Performance on HF seems similar, but an edge does go to the HF+. But the ColibriNANO has the downside of poor LF/VLF reception (advertised response starts at 100 kHz), and heavily aliased VHF/UHF due to undersampling. A filter is needed for proper operation on VHF/UHF. That said the ColibriNANO itself is a very good SDR, but the HF+ certainly wins out in terms of value and general performance.

The Airspy Mini/R2 and SDRplay RSP2 also generally perform well for the majority of signals, but will struggle when it comes to really strong signals. Comparing against these SDRs really shows where the HF+ shines. But when compared against regular (non-weak) signals it is pretty much impossible to determine which SDR is better.

So there is obviously some brand new cutting edge technology going on in this receiver which possibly even puts it on top of the very expensive direct sampling SDRs. On the HF+ weak and DX signals are noticeably more accessible. Performance for the price (expected $200 USD) is phenomenal. This is a highly recommended SDR.