Re: HB9VQQ testing Kiwi SDR

Rob Robinett

Hi Bruce,

In case you and others in this group missed my posts' to Roland's WSPR Beacon Project Telegram group on this subject,  I'll copy those remarks here,  but the subject of how to construct high performance (and what that means needs to be defined), easy to deploy and cost effective WSPR receive site is far more complex than can be covered in one post, but here is my perspective:

Roland wants to deploy a wspr rx system which mirrors his tx system and feels that to do that the cost of each site needs to be $200 or less.  At $400 for a Kiwi + Pi4, on cost alone that system doesn't qualify, so he could have stopped there and settled on the Airspy + Pi and compared it against other single channel SDRs.  However he also wants to know how well his low cost system compares in performance to the Kiwi + Pi used by many of the top spotting sites.  But in making that comparison he put the Kiwi system at a severe and undocumented disadvantage by not adding the AM and LNA requried to properly utilize the Kiwi which is used at those top sites.  

As one of many examples of the disadvantages potentially suffered by the Kiwi in such a comparison, without an AM blocking filter the Kiwi (and and wide band SDR) is likely to be frequently going into overload.  The Kiwi GUI displays those events with the flash of a red OV next to the S-meter, but if you are comparing over a 24 hour period there is no place to find out on the Kiwi the number and severity of those events.  In order to document this problem, wsprdaemon records the frequency and severity of OV events to a log file which the operator should examine to see if there is sufficient filtering to minimize overload events.

As a second example, if there is no blocking filter and no overload events, there is evidence that the Kiwi system is not receiving a strong enough RF signal and it needs a  LNA ahead of it.

While a single channel SDR probably doesn't need blocking filter(s), it too can suffer from impairments introduced by its installation environment, and those are much harder to identify than when using a wideband SDR.

People write whole books on the subject of building a good receive station and I worry that the effort to deploy low-cost plug-and-play rx systems will result in many 'deaf' stations.  Certainly by listening only to the bands and wspr cycles when/where WSPR Beacon stations are transmitting, many potentially interesting spots from non-Project stations will be missed.  So I think it is a shame to invest a lot of time and money in rx stations which could be far more useful to the wider community.

But Roland deserves credit for funding this and I hope his rx stations do make a contribution to our understanding of radio propagation.  But his tests don't prove the Airspy is a 'better' WSPR receiver than a Kiwi.  The Airspy just works better in the way he has been testing and the way he wants to deploy his systems.




"I understand your application requirements are different, but for many users the Kiwis are cheaper and better solutions when they are properly deployed.  My point here is to clarify to others that your comparison tests are for your specific application:  band hopping with no external filters or amplifiers.

Rob Robinett AI6VN, [Oct 10, 2021 at 1:35:29 PM]:

Given your cost and single band at a time requirements, then the Airspy seems the better choice.  But the Kiwi is not 'bad', it has different features and testing by many knowledgeable hams has shown how to optimize its performance to match that of other top quality receivers like the Airspy.  So I don't think it is appropriate to characterize the $40/rx channel Kiwi as 'worse' than' than the $170/channel Airspy when your test criteria precludes optimizing the Kiwi's RF feed.   I think you have identified the Airspy as the right SDR for your application, but that doesn't mean it is the 'best' SDR for all applications"

On Wed, Oct 13, 2021 at 1:49 PM Bruce KX4AZ <bruce@...> wrote:
I wouldn't be at all surprised if the Airspy HF+ Discovery outperforms the KiwiSDR for WSPR spot counts, give the different architecture and narrower bandwidth on the Airspy.  In my informal comparisons of the two (attached to a simple y-splitter), the Airspy had a measurably higher S/N ratio on the weakest signals.  It appears that modest signal amplification (very carefully applied) can compensate for the difference.  I'll leave it to the experts on this forum to (like KA7OEI) to detail the pluses/minuses of amplification.

I just started setting up a new Pi4 today and hope to get the wsprdaemon running with the KiwiSDR one of these days.

I also wanted thank you for your post, calling my attention to yet another that I'll need to join!  Might be a good place to discuss my current 10 microwatt WSPR experiments.
Bruce KX4AZ

Rob Robinett
mobile: +1 650 218 8896

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