AirSpyHF+ vs netSDR Shootout

Here’s my comparison of the new AirSpyHF+ plus vs the RF Space netSDR. The netSDR is one of the higher end SDRs, often considered the “gold standard”. So how does the AirSpyHF+ match up?

The AirSpyHF+ has two Sigma Delta ADCs with a 36 MSPS rate, an 18 bit DDC (Digital Down Converter) and (near as I can tell) always produces a 768 kHz I/Q stream. The frequency range is 9 kHz to 31 MHz, then 60 to 260 MHz. The selling price is about $199.

The netSDR has a 16 bit A/D, sampling at 80 MHz. The frequency range is 10 kHz to 32 MHz, which can be extended to over 1 GHz with the Downconverter option. I/Q stream rates are up to 2000 kHz. The list price was $1449.

I fed both receivers with the same antenna, my Crossed Parallel Loop, through a splitter. The AirSpyHF+ always samples at a 768 kHz rate, I set the netSDR to a 625 kHz rate, the closest. SdrDx software was used in both cases to make the I/Q recordings. In the case of the AirSpyHF+ I used my own server app to feed the I/Q data to SdrDx. I then recorded the 25 and 19 meter bands, and selected several transmissions to compare. In both cases, mySdrPlayback (an app I wrote) was used to playback the I/Q recordings and convert to WAVE audio, which was then converted to mp3 at a 64 kbit rate. I tried to start each record at about the same time and used the same IF filter width, for fairness.

I also made one set of recordings of a relatively low power pirate radio station that plays Christmas music. I think it’s pushing the Part 15 limits, but still is not very powerful, and is probably about ten miles away.

Many of these recordings are of weak signals. There’s no doubt that most any modern SDR is going to do well with strong stations. A more important question is, how to do they work with weaker signals?

So how do you think each receiver performed? Let me know in the comments.

All India Radio 11560 kHz 1443 UTC


China Radio International 11785 kHz 1143 UTC


Yemen 11860 kHz 1444 UTC


Radio Liberty 11890 kHz 1444 UTC


China Radio International 11920 kHz 1144 UTC


Radio Liberty 15265 kHz 1452 UTC


VOA 15580 kHz 1452 UTC


China Radio International 15160 kHz 1144 UTC


Pirate radio station (about 10 miles away) on 1620 kHz 1536 UTC


SSTV (Slow Scan TV) Images Received From The ISS (International Space Station)

The International Space Station is sending SSTV (Slow Scan TV) images on 145.8 MHz today through Friday. Here are two images I just copied from the 1938 UTC (2:48 PM EST) pass.

They were transmitted in PD120 mode, and copied using MultiMode Cocoa software (which I happened to write), on a netSDR receiver connected to a discone antenna (which is many years old, and missing several elements).

SSTV is a method of transmitting a picture using audio tones. It takes about two minutes to send an image. Hence the Slow part of Slow Scan TV.

What does SSTV sound like? Here’s a recording of the audio that produced the second image:

Crossed Parallel Loop Antenna Build

This is the antenna I was planning on building when I ordered the LZ1AQ amplifier (and then built the hula hoop antenna as a quick test of the amp).

There is some technical information on the antenna here, which I won’t repeat, but I’ll summarize the design:

The antenna is made of four square loops. These loops are connected to the LZ1AQ amplifier. So there are eight wires feeding the amplifier, with four pairs of shorted connections, so four wires in total attached to the amp.

The amp is mounted inside of a plastic box, the type used as a junction box for runs of conduit.

The output of the amplifier is shielded ethernet cable, which runs to a control board in the shack. One pair is the signal from the amp, the other wires are used for power, as well as controlling the amp, as it can be switched remotely to use all or some of the loops. I still need to apply silicone sealant to the eight openings where the wires run in, and seal the ethernet cable entry hole with some tape.

I was debating between building a wood or PVC pipe frame. I went with the PVC because I did not want to deal with cutting and gluing the wood, or using nails and brackets and add additional nearby metal objects. I used 1″ PVC pipe.

I first cut the 10 ft PVC pipe pieces into 5 ft lengths and built the loop frame that size. I stood it up, and realized that no, this was not going to last long. So I cut the pipe down from 60″ sides to 40″ sides (close to one meter) and ended up with a much more mechanically stable design.

The wire that forms the loops is attached to the PVC pipe with plastic wire ties. the wire is white, so difficult to see in the photos. It is #10 stranded wire. Ideally you want to use as large a diameter conductor as possible, to reduce the inductance. But you quickly run into two issues: cost, and ease of use. Larger diameter cable was much more expensive. And it was going to be difficult to work with. Copper or aluminum tubing could be used, but they were also more expensive.

The loop is mounted on an old rotor (the one I used with my large resonant loop antenna project, which has been moved to a back burner for now). This antenna is indeed directional, at least on MW. I am able to hear a nearby pirate station on 1620 with it aimed in that direction, while an orthogonal bearing drops it down to just a weak carrier. Meanwhile, on my 670 ft sky loop antenna, I only have a very weak signal.

I’m quite impressed with the performance of the antenna so far. On HF, it does quite well, usually close to the big 670 ft sky loop (my main HF antenna) and sometimes better. Two places it always beats the sky loop are LW and the lower end of the MW band (where the sky loop is too short) and 11 meters (where the sky loop is way too large). It will take some more time to fully test it on a variety of signals.

Woofferton Transmission Test eQSL

I received this eQSL and email for the transmitter test I heard from Woofferton this morning on 9650 kHz, starting at 1237 UTC on 16 November 2017:

Hi Chris,
Thank you for your report and I confirm the details are correct. These transmissions were to fault-find on a 250/300 kW sender at the UK HF transmitter station at Woofferton.
These duration of these tests can be variable as the engineers can sometimes need a long time to establish a fault or they may interrupt the test, make an adjustment and resume. This is particularly so if the fault is of an intermittent nature.
Babcock, Woofferton is the only remaining UK HF sender broadcast station and also is the only one with this transmission test audio and email address.
The audio is contained in a file play-out system and incorporates non-copyright music and voice announcements from one of the engineers, Martin 2E1EKX at the transmitter site.
Thanks for your interest.
Dave G4OYX,
(Retired) Senior Transmitter Engineer Woofferton 1982-2012.