Results: AirSpy HF+ vs Elad FDM-S2 Weak Signal Comparisions

Looking north toward Cape Lookout, Oregon, near the site of my SDR receiver recordings. Photo courtesy of Wikimedia Commons.
In my original article 10 days ago, I set up a SWLing Post reader poll to let you give your opinion on which shortwave recordings within four pairs of audio files provided the most intelligible result. The recordings were intentionally noisy, low-level signals to help us discover–through critical listening to the files–if there is a clear favorite between the AirSpy HF+ or the Elad FDM-S2 receivers. Of course, there were only four pairs of recordings…not a very large sample size.

However, 34 readers of the original article took the time to listen and respond, so let’s get to the numbers, shown in these graphs:

Interestingly, the responses above seem to point to:

  • Two recording pairs tied in the results (50% / 50%) or were very close (HF+ 52.9% / FDM-S2 47.1%)
  • The FDM-S2 led one recording pair by a large margin (67.6% / 32.4%)
  • The HF+ led another recording pair by an equally large margin (67.6% / 32.4%)

Taken as a whole, no obvious winner emerged, although one might conclude the HF+ has a slight edge due to its lead in the “very close” recording pair of 7.230 MHz.

One thing is clear–the AirSpy HF+ is a surprisingly good performer for its price of $199 USD! For many enthusiasts this will be all the SDR they need.

As a final note, I’ll mention that the AirSpy HF+ used for the tests was totally stock. I have not yet performed the “R3 Bypass” mod nor the firmware update to my HF+ units. The simple R3 Bypass, discussed at length on the AirSpy forum, significantly boosts sensitivity of the HF+ from longwave up to about 15 MHz, without any noted overload issues. For more on this modification from a MW DXer’s perspective, read Bjarne Mjelde’s insightful article at his Arctic DX Blog.

Thank you to all the readers who took the time to listen to the SDR recordings in this comparision and register your opinions.

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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8 thoughts on “Results: AirSpy HF+ vs Elad FDM-S2 Weak Signal Comparisions

  1. Mark

    Hi Guy, thanks for the reply, yes I just read the part where the firmware was removed on the Airspy site.

    Does the R3 mod have to be performed with the firmware update or can the Firmware update be performed on it’s own ?

    Thanks again.

    1. Guy Atkins

      Hi Mark, the firmware update and R3 mod are totally separate and can be applied individually. For a DXer of HF frequencies and below, I’m sure that using both would be the best. Even though the radio covers some VHF range, it is after all still an HF+… dedicated to best performance on shortwave on down.

  2. Mark

    Thanks for taking the time to do the review Guy, I was one of those who took part in the test. I’ve a HF+ ordered.

    Regarding the mod, where do you get the firmware , I haven’t seen it anywhere ?

    I can’t wait to try it out with the Bonito MA305 antenna I recently purchased, I got the larger 44 cm antenna, it’s absolutely amazing for the size of it, truly , amazing !

    I can’t say I’m happy at the thoughts of having to open it up to do a mod that should have been done in the first place ?

    1. Guy Atkins Post author

      Hi Mark, the “R3 bypass” mod is NOT a modification in the sense that there is a problem with the radio that needs correction. The AirSpy designer said that the R3 component helps level out the sensitivity throughout the radio’s reception range, and they fully expected some owners to bypass or adjust the capacitor’s value in order to improve response at frequencies of interest to them. Bypassing R3 improves reception from 15 MHz down through longwave. So, it is more of an option, not a design problem to be circumvented. It is simple to perform and can be reversed if needed.

      A search on “R3 bypass” in the AirSpy’s forum will come up with a lot of messages and some photos and screen shots relating to this subject. Note that you need to go to the “Main” AirSpy forum. The one for the HF+ is just for general announcements from AirSpy, not for user discussions.

      The firmware was taken offline for a while while AirSpy sorts out an incompatibility with Windows 7, 64-bit systems. A message by a user stated “The firmware update is also extremely simple to perform, but the initial file posted was found not to work on Win7x64 and was pulled until an ‘all situations’ installation routine is completed. If you have a Win7x32 or Win10 PC then there is no problem with it”.

      AirSpy found a way to increase AGC range by an additional 6dB, so that is the improvement in the firmware update. Keep an eye on the forum; it sounds like the file will be back shortly,

  3. Thomas

    Excellent! Thank you, Guy. I believe I listened to each sample pairing three times before I submitted my selection. It’s amazing that technology has taken us to a place where the differences between a $200 SDR and a $500 SDR are so very close! And I do think the HF+ must have a slight edge overall.

    It was big fun for me, too, to participate in this sort of listening survey. Typically, I’m the guy putting it together and receiving the results!

    Additionally, I was not familiar with the R3 Bypass mod. I’ll check this out as well.

    Thanks, Guy!

  4. Andy

    Thanks for running the tests, Guy. Real-world performance is often a much better indicator than any number of lab tests. Would there be any chance of running a similar comparison at LF (ie below 500 KHz) to see which of the two performs best? LF is an area neglected by most reviewers but there are plenty of enthusiasts out there. Some SDR’s are not happy at the low frequency end of the spectrum, either having poor sensitivity or lots of spurious responses or both. My own RSP2 works well at LF only if I use suitable LPFs. Without LPFs it is messed up with MW images. Many thanks.

    1. Guy Atkins

      Hi Andy, I am not very knowledgeable about LF DXing, but there has been a lot of discussion about the HF+ on longwave and below in the forum for the HF+. I would recommend you sign up for the group and check out the archived messages. (Be sure to go to the AirSpy “Main” forum; the one labeled “HF+” is just for general HF+ announcements from the factory.) There has been some very positive results reported by noted LW DXer Roeloff Bakker PA0RDT (designer of the original mini-whip antenna). This message from the *Perseus* forum (not AirSpy) should give you a lot to consider:


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