Loop in ground (LiG) – #4 – SDR for measurement?

A series of recent articles developed a Loop In Ground antenna system design.

To test the prototype, I thought it an interesting exercise to use a low end rx only SDR for the instrumentation, providing a graphic quantitative measure of performance that is within the reach of most hams.

The first device trialled was a RTL-SDR v3 dongle with Sdrsharp (SDR#) software under windows, a very low cost option ($40). I was unable to find meaningful NF specifications or end user measurements for the thing in direct sampling mode.

The v3 incorporates hardware for direct sampling, so allowing input up to ~30MHz. Well, it might have some of the hardware, but it lacks preselector filters which are essential to alias free behavior and the option was quickly dismissed as not suitable for my intended use from 0.5-15MHz.

So, that kicked off a review of ‘mid price’ rx only SDR, starting with a documentation review. Again, finding meaningful NF specifications is very difficult, but some state sensitivity or MDS in ARRL terms (an appeal to the ham market no doubt)… but a worry in its own right as these figures are almost always based on nominal 500Hz bandwidth, not the actual quivalent noise bandwidth of the receiver and the error can be significant (ARRL Test Procedures Manual (Rev L) – Noise Figure calculation).

I settled on an SDRPLAY RSP2 and did a more detailed search for issues, and lo and behold, SDRPLAY advice on their website of loss of several consignments of the things in the supply chain, and to protect someone (themselves mainly) they have “blacklisted” the stolen serial numbers to “render them worthless” (SDRPLAY) in the hands of the user. Their strategy is to attack the easily identified soft target, each buyer, irrespective of whether the buyer knew they were buying stolen goods (shades of the FTDI debacle here).

The next candidate is very new technology, very interesting, but a greater risk than more mature technology. That can wait for a little more user experience (this sounds like the young man who waited for SDR technologies to plateau).

So with a little delay low end rx only SDR are dismissed as a low risk low cost instrument for the project’s measurements, and more conventional devices will be used.

Work continues…