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Airspy HF+


In the autumn of 2017, Airspy's website featured a new SDR to shake up the scene! The Airspy HF+. With its very good specifications and a competitive price of $ 199USD, the developer and Airspy company owner Youssef Touil sparked hype on various hobby forums. Much has been discussed and speculated. Because it was said in many places, the Airspy HF+ can offer the expensive Direct Sampling SDR's.
For me as a radio freak was clear, I had to go to the bottom of the matter myself. So I ordered in mid-November 2017, an Airspy HF+ directly from the manufacturer Itead. At the beginning of January 2018 I received a consignment from China. Expectantly, the very small package was opened. To the light comes a credit card sized device, about 1cm thick and unusually heavy. The gray case is made of cast aluminum and excellently processed. The device has a USB cable attached.



The most important key data of HF+

-- Frequency range: 9KHz - 31MHz, 60MHz - 260MHz
-- Tracking low-pass filter
-- Tracking bandpass filter
-- Sensitivity: 0.02uV
-- MDS (smallest receivable signal over the noise floor) at 500Hz bandwidth: -140dBm
-- Large signal behavior IP3 +: + 15dBm at HF
-- Large signal behavior IP3 +: + 13dBm at VHF
-- Dynamic range HF: 110dB
-- Dynamic range VHF: 95dB
-- DDC: 18bit
-- New Polyphase Harmonic Rejection Mixer
-- Frequency stability: 0.5ppm
-- Ultra Low Noise, Stabilized Oscillator (TCXO)
-- SMA antenna connections for HF & VHF
-- Micro USB port
-- Operating voltage 5V (via USB socket)
-- No Drivers required (Plug & Play)
-- Operating modes: AM, SAM, LSB, USB, CW, FM or software dependent
-- Bandwidths of the spectrum: 12KHz, 24KHz, 48KHZ, 96KHz, 192KHz, 384KHZ, 768KHz (SDR #) or software dependent
-- Dimensions: 91x50x12mm (WxDxH)
-- Weight: 190g

Working principle of the Airspy HF+

The HF+ is like the other Airspy devices from the same house, a tuner-based SDR. This means he works with mixer, PLL circuit and VCO. The signal passes through from the antenna, a tracking lowpass / bandpass filter. Thereafter, the signal is mixed in a special mixer and then digitized. It then passes through the DSP and is then passed on to the PC via the USB interface. There it can be used with various programs, such as SDR # and SDR Console demodulated and edited.

However, the operation of the new mixer differs from the usual mixers of RTL tuners. The HF+ works with a so-called "Polyphase Harmonic Redjection Mixer". This new Polyphase Harmonic Redjection Mixer allows the Airspy HF+ to stand out in the low-budget class.

Inside the Airspy HF+

From the outside, the HF+ cuts a fine figure with its elegant cast aluminum housing. How does it look inside? By unscrewing the four screws and removing the bottom cover, the view of a clean processed board is free. Important components are additionally shielded with a sheet metal housing. In the place a note; the HF + invites to modifications! But more below.


Compatible software for the HF+


The standard software for the HF+ is SDR #. This free software, which also supports some other SDR's, was developed like the HF+ by Youssef Touil. It is a flexible and responsive operating software that leaves almost nothing to be desired. The SDR # is stable and constantly evolving. It also allows you to remotely control other HF+ using the spy server. SDR# can be functionally expanded enormously with "plugins". For such complex programs, training is essential and time-consuming. Some settings and activating functions can only be changed by accessing the program files. For users without knowledge a difficult hurdle! Like any software, SDR# has its own peculiarities. A special feature is the reception level indicator. Normally the signal level is displayed in S-steps or dBm. Not so with SDR#, which uses "dBFS" as a level meter! A rather unknown and unfamiliar type of signal display for the normal listener. What's up with the dBFS? After clarification with Youssef and after some confusing discussions on the forum dx-u, here's the explanation:
The "dBFS" (dB Full Scale) is used to show the modulation of the SDR. At 0dBFS the full scale is reached. If the drive exceeds the 0dBFS limit, the SDR or certain modules will "clipping", which means overload. Thus, there is a danger that intermodulation or increased band noise will be noticeable. In such a case, a weakening of the signal by means of attenuator is required. The second display, which is displayed with a blue bar to the right of the spectrum, is the important SNR (Signal to Noise Ratio). This value is very important for the DX'er, because it allows conclusions about the signal quality. A good explanation for this I found at Wikipedia:

<< The signal-to-noise ratio serves as an evaluation number for judging the quality of an (analog) communication path. To be able to extract the information safely from the signal, the useful signal must stand out clearly from the background noise, so the SNR must be sufficiently large. If the SNR drops, the error rate increases with digital transmissions. As a characteristic of a receiver, the SNR characterizes when the receiver can distinguish noise from the signal. For a human being, at least a SNR of about 6 dB is required in a noisy signal in order to be able to hear the language contained therein >>

SDR# does not have to be installed, which greatly simplifies the commissioning of the HF+. Connect the HF+ to the PC via USB cable, start SDR# and select the Airspy HF+ in the upper left corner under "Source" and click Start.

SDR Console V3

The well-known and popular SDR-Console V3 fully supports the HF+. Long before the HF+ officially came on the market, Simon Brown had received a pre-production device to integrate it into the SDR-Console V3. It was worth it! Although the SDR-Console V3 is still in the "Preview" phase and still has various bugs, it still makes it fun to work with. If set correctly, you will get the best performance from the HF+. With the V3 you can also access the spy server network.

The spy server network

With SDR# and SDR-Console V3 you have the possibility to access online Airspy HF+. The website https://airspy.com/spy-servers/ lists some HF+ but also other Airspy or RTL SDRs that allow remote access. To access HF+ online, you only have to go to "Source" in the menu "Spy-Server" and enter the dialing address, which can be found on the Airspy website.

As an example the address of my HF+: sdr: //fenu-airspy.ddns.net: 5555

For comparisons and first steps with the software, this is a very good option. As long as you are accessing a device on its own and it is unlocked to receive the entire frequency spectrum, you can test the device almost without any restrictions. Once a second user accesses the same device, you can only move in the 660KHz wide spectrum that is currently visible. Unfortunately, this had to be done because the data become too big and the transmission would collapse.

On a wave hunt with the HF +

After reading so much about the HF+, I was naturally very curious about the reception behavior! Before I crashed into the DX-turmoil, I first heard in the stronger KW and MW channels purely. First, the low background noise was noticeable! I would not have expected that. Quickly turned on the Winradio G33DDC Excalibur Pro to compare the situation. Indeed! The small HF+ offered the same low noise as the 10x more expensive Winradio. The calibrated display of the Winradio showed -135dBm in the 25m band as a noise floor. Since the SDR # software uses "dBFS" as a level meter, this could not be converted. But sonically, both were alike. Weak signals were no problem at all for HF+. The reception quality was in the first rough overview as good as the Winradio.
In the evening, the signals are known to be stronger and present many receivers with challenges. It happens that ghost signals appear on frequencies that should actually be free. So did the HF+. There were a lot of unwanted signals in the 19m band. These had their origin in the 49m band and in the medium wave. With the Winradio G33DDC, not a single ghost station could be heard or seen on the spectrum, even though the preselector was off. To rule out a software bug, I took both programs to check the situation in the 19m. The ghost stations were received on both programs. A software error was thus excluded.
The pictures below show in a 400KHz wide spectrum how many ghost signals the HF + produces in the 19m band. Compared to the Winradio G33DDC the absolutely clean reception offers. The transmitter at 15580KHz is not a ghost station.


I collected the above experiences in the first few hours. That was sobering and somewhat dampened the initial euphoria. But that did not stop me from testing and comparing the HF+ as always. So we start the test on the lowest receivable frequency.
In the VLF range up to about 130KHz, the HF+ was quite deaf. Other testers reported good performance in this area. The reason for this was the coupling capacitor in the RF antenna input. This pretty much dampened the low frequencies. In the pre-serie specimens this was not yet in it. In place of the capacitor was a 0ohm resistor which allowed full frequency pass. It was not long before resourceful specialists found a way around this shortcoming. Some made a bridge over this capacitor, others were cleverer and tested various capacitor values ​​to find the best result. But why a capacitor? This coupling capacitor prevents the penetration of harmful DC voltage into the electronics and protects the sensitive receiver assemblies from damage.
Because of this fact, I first let the VLF range test until my HF+ was also modified. The long wave range worked well to very well. Here very low background noise and acceptable sensitivity. Presumably, the effect of the coupling capacitor was also noticeable on long wave. Small level differences to Winradio were detectable. On the medium wave and shortwave the HF+ played very well! Sensitivity and noise were just great! In these disciplines, the HF+ could actually beat the much more expensive Winradio!
Now the DX camp in the Holzerbachtal near Solingen was on the program. A colleague who also participated, modified the HF+, so that the VLF range was also receivable. On the whole we had ten antennas at the antenna distributor. These include Loops, Active Dipoles, Mini Whip and a 80m longwire. Especially the long wire delivered extreme levels. Fortunately, the disturbances were weak at this camp. Noise levels of -140dBm were almost normal. The Airspy HF+ coped with most antennas without oversteering. Most of the larger and high-power antennas were in the evening oversteer effects on VLF and KW noticeable. Apart from these manageable slips, the HF+ delivered very good reception results and proved its worth.
To return to VLF again; After the modification, the HF+ played really well in this low frequency range. We had five HF+ at this camp. All modified on the coupling capacitor. With the small MiniWhip we managed a rare reception. A time signal at 40KHz from Japan. Easy to receive with the HF +.
Back at home, the VHF range was scrutinized. With the existing antennas, which are mainly designed for HC, the VHF range was only tested. After all, FM broadcasting was in stereo receivable. Even radio signals were received here and there.
Modification of the coupling capacitor in the HF input.
A major topic on some Internet forums was the modification of the coupling capacitor for maximum sensitivity of the HF+ in the VLF range. Many simply bypassed this, others swapped out for capacitors with different values. Well proven has a value between 100nF to 150nF. The original has a value of 330pF. The colleague has soldered at my three-piece parallel connected with a total value of 130nF. The result can be heard. The HF+ now receives from about 10KHz. This modification has no effect on the upper frequency ranges. The shift change should only be made if you put a lot of emphasis on the VLF range, or if you want full sensitivity on the long wave. Airspy informed me that the next big batch of HF+ has already installed this modification. However, with a little different values.

An additional modification is a firmware update of the HF+. A few days after receiving the HF+, I got the firmware update from Airspy for testing. This update changed the AGC control limit by -6dB, which made a good impression. The reception became quieter and more pleasant. In particular, the gain control of very strong signals has been improved. Interestingly, there were fewer intermodulations after the update.

The latest firmware updates can be downloaded here. At the bottom of the website. https://airspy.com/airspy-hf-plus/

The latest SDR# Software can be downloaded here. https://airspy.com/download/

Picture 1: Original 330pF SMD capacitor
Figure 2: Modification with three parallel-connected capacitors. Total value 130nF
Note: The label "R3" on the board is wrong. That would have to be called "C3", because a capacitor is installed.

Audio comparisons between Airspy HF+ and Winradio G33DDC Excalibur Pro

Granted, a somewhat unusual comparison! But given what the HF + is said, I still find it fitting to compare a low-cost SDR with a premium SDR.
Comparison conditions: Both devices were connected via the proven antenna switch ELAD ASA-62 either to the Kreuzloop/RLA4-2E or the Datong AD370. The control programs were set as equal as possible. No noise reduction (NR) was used. Bandwidths are always the same for both. Noiseblanker switched on, no additional amplification or attenuation of the signal.

When listening to the audio comparisons necessarily use a headphone!

Second 0-10 >> Winradio G33DDC Excalibur Pro
Second 10-20 >> Airspy HF+


Time Signal
Taldom Severnyj
Radiodifusora Roraima
Radio Rebelde
RAF Volmet
Radio Marti
CHU Canada
RAF Volmet
Radio Liberty
Radio Farda
Radio Saudi


After the intensive test phase of about 4 weeks I got to know the HF+ well. With SDR# both with the SDR console V3 was able to convince the small HF+. Not in all respects, his behavior on good amateur antennas revealed his weakness. The large signal behavior was not always good enough. During the day the HF + played great and without problems. In the evening, however, ghost stations appeared in the VLF area and on the shortwave. Apart from that, the HF+ delivers excellent reception performance in all areas. In the low-budget class a blast!

The HF + is a highlight in the SDR scene! Best price performance ratio.

Posted 09.02.2018


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