Presence of a permanent central spur on v2.3 using different SDRs and an active antenna...


Cormac, EI4HQ
 

Hi,


So I've recently built an active antenna for wideband VHF/UHF & higher listening (in effect a Dressler ARA2000 from the 1980's for those of you old enough to remember them... see https://apollo.open-resource.org/mission:log:2012:08:02:active-wideband-receiver-antenna-for-sdr). The system I've built utilises the LNA4ALL wideband low noise amplifier (http://lna4all.blogspot.com/). This provides circa 20dB of gain depending on frequency. This setup is useful from circa 50MHz to as far as I can "hear" with my current SDRs - 3.8GHz via the excellent BladeRF. As an aside, with the exception of a tendency to overload as a result of strong FM broadcast signals (easily cured with a good notch or bandstop filter that is now a permanent fixture in the above setup), it's a surprisingly effective wideband receive antenna solution .


In addition to the BladeRF, I also have an SDRPlay (that goes to circa 2GHz). I'd appreciate some advice/insight on the presence of a central spur with both the SDRPlay and the BladeRF when used with SDR Console v2.3, a result I think of the fact that with the above active antenna setup, in general I need very little gain at the receiver for good signal copy. With both SDRs I use very little receiver gain (if any at all on some bands e.g. airband). With little receiver gain however, I'm stuck with a permanent central spur on the SDR spectrum display. If I increase the receiver gain on either SDR, I end up very quickly with an overload problem on all but the weakest of signals. Yes, the spur drops away in SDR Console but the resulting RF mush is not something pretty! In the old SDR Console there was a manual spur reduction, I understand spur reduction is now automated however with the above setup I seem to be stuck between a rock and a hard place...


I'd be grateful if someone (a) was able to point me to information on the relationship between the spur and gain (I've not had much success googling that one - probably I've not used appropriate search terms) - I'm not adverse to some theory btw, I'm still on a steep SDR learning curve and have alot to learn, and (b) had some advice/insight about what I can do to square the low receiver gain with no spur circle (presumably lower the LNA4ALL gain so some receiver gain is added to the chain?)


Many thanks

Cormac, EI4HQ

[Cork Harbour / UTC+1 NNNN++++]

 

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