n6gn

Phil,

Addressing your noise issues and not the spectral display, I would encourage you not to immediately jump to whack-a-mole in seeking to improve Kiwi Performance. Rather, I have found it very productive to examine the coupling mechanism rather than simply trying to stay on top of suppressing interference sources.

For electrically small structures, which almost everything at LF and below is, the radiation resistance is minuscule. For virtually all situations we encounter actual inverse-square radiation from any sources/antennas is far below the interference levels the Kiwi reports. Thus I think it worthwhile to look at near-field and, particularly common mode (CM) coupling mechanisms.

In my experience, the dominant undesired coupling mechanism into the Kiwi is CM current over the path between wired LAN connection and the SMA-end of the Kiwi PCB. This includes the BB ground plane, cape connections and ground plane current paths on the Kiwi PCB. If one uses an isolated source having low self-capacitance (and then perhaps further reduces the potential for CM with a low inter-turn capacitance 1:1 transformer to create a test current source), -10 dBm on 15 MHz injected between the BB RJ45 shell and the Kiwi Antenna SMA results in about -85 dBm displayed on the Kiwi. This is more than 70 dB above the Kiwi noise floor in 1 Hz. At 100 kHz it's only down another 22 dB or so, still far above what the Kiwi can easily detect. It's for this reason that for each of my four Kiwis at the home QTH I have gone to WiFi interface, BBG/Kiwi's as described elsewhere on this forum and BBAIs with their native WiFi interface.

At LF, the noise floor of interest will depend upon the antenna system. Broadband electrically small antennas (they all are at LF and below) have antenna factors rising at 20 dB/decade while the ITU propagated noise, though all over the map with diurnal and seasonal variations, generally falls at about 25 dB/decade of frequency. Thus the noise limit of interest may be quite a bit above the Kiwi's native floor of ~ -157 dB/1-Hz but it still may be well below the kinds of levels that CM current injected via the LAN, PS or even GPS lines might be able to produce within the Kiwi.

For the Kiwi, and really every receive system we use "ground isn't ground" except by definition. But care in examining coupling mechanisms, reducing current in CM paths and symmetry (passive and active baluns) can really pay off and gains made here tend to apply no matter what new SMPS or other noise source pops up in the environs.

As a proof-of-performance it's also very useful to use symmetric antenna structures rather than single ended ones, e.g. monopoles, because the intended antenna, e.g. dipole, can be shorted to observe and confirm that the residue CM is not significantly limiting system performance. Considering the many-10s of dB of symmetry/CM_rejection we may need broadband RF baluns really are insufficient over the 3-4 decades the Kiwi covers.

When CM has been removed, there can still be the issue of near-field coupling to deal with, but I digress...

Glenn n6gn

I designed and built an APRS/solar_charger for monitoring the PS at a remote KiWi site. It's not quite what you are asking for but since the Kiwi data is IP over microwave, the use of out-of-band monitoring means I can see the power situation and termperature even if/when the microwave link is down.

You can see the present condition of N6GN/K2 this way

If you are interested, write me.

Glenn n6gn

I have found it very worthwhile to use toroidal 'choking' in the form of a Guanella balun simultaneously on all lines going in/out of my KiwiSDRs.

as shown in the bottom portion of the figure.

Like the situation so nicely described by G3TXQ's graphic, suitable cores need to be used, possibly several of them to cover the broad frequency range of the Kiwi. By winding equal turns in the same direction with each line on a single core, the effective current that can flow through the receiver is reduced by the "transformer action" of the cores. Current that would otherwise flow into and through the SDR creates common flux within the core that causes an opposite current to come out, thus pushing total current toward zero and effectively creating a much higher impedance than can be the case when each line has only a dedicated core.

From my experience, CM currents through the entire KiwiSDR structure produce IZ drop that appears across the effective ADC input, which is a complex function of PCB layout and conductivities - the "attenuator problem". For this reason, I believe, treating all lines as common mode lines, in addition of course to also eliminating differential noise across pairs of individual conductors of those lines, e.g. power line or inter-pair CAT5 noise, can be very effective in improving the KiwiSDR noise floor.

Glenn n6gn

I tested Master Electronics by ordering only a single enclosure. The process went flawlessly and the enclosure arrived as promised after 8 days.

They seem to be fine.

Glenn n6gn

I know very little about the TDoA algorithm but I suspect both @jks and @Christoph are correct. Between the limited bandwidth and especially ionospheric propagation, typical Kiwi clock imperfection probably does not become an issue.

For an appreciation of this, you are welcome to examine the phase of one of NIST's transmitters received via a visual line-of-sight 20km path and displayed on a Kiwi having a ~.1 ppb (1e-10) GPS-disciplined external clock:

(1) N6GN External Clock WWV15

and by a different 'stock' GPS-corrected Kiwi at the same distance and also not receiving via the ionosphere:

(2) N0EMP Kiwi GPS WWV15

Then have a look at a time/frequency signal via the ionosphere, CHU on 14670 kHz

(3) N6GN CHU 14670

or if conditions don't permit, perhaps 7850 kHz

N6GN CHU 7850

Whether or not the the phase wander from the standard Kiwi in (2) causes significantly extra error compared to the bandwidth and sample-length restrictions and ionospheric variations would need to be examined more closely but I rather doubt it. Thus, improving the Kiwi's local clock probably wouldn't make much difference in the TDoA accuracy or resolution.

I think this is the primary reason that long-distance HF standard frequency transmissions tend to be only useful to .1 ppm. 1e-7, or so. Even though as-transmitted error may be 1e-12 the ionospheric path length is varying too much, particularly near the MUF for better accuracy.

Presuming I have the correct K7GFH-associated Kiwi, at 1450 UTC you really aren't doing too badly. While I do hear a mains-related family of QRN from LF half way into HF, the levels aren't that terrible. Using the 10 kHz AM detector to allow hearing it (the individual lines tend to correlate and 'pop out' much more than in a narrower SSB bandwidth, they are found to almost completely disappear by 10 MHz leaving the Kiwi -154 dBm noise floor. This is assuming that the calibration is still about -16 and thus close to correct and no significant gain/attenuation on the way to the antenna.

The largest MWBC signal is around -30 dBm but there's no other station close to cause too much worry and there's no indication of OV.

Even at VLF you are hearing many stations pretty well, HI and Australia included. There's line related noise but I've seen far worse on many other systems. You're hearing well on 160m with several stations showing a lot of SNR.

Because this family of QRN starts so low rather than as modulation superimposed on a family of SMPS rate beginning higher up, I tend to suspect that it is rectification noise getting either back into the mains wiring or ground currents.

Here's a crude analysis of a little IQ/10kHz done by Audacity:

It does have 'ragged' edges around full-wave rectification (120 Hz) which may be what's responsible. I can't account for these.

OTOH, maybe the problem you are asking about has chosen this time to be absent.

Here's an 8-way one for around US$30 if you order two.

Martin

The differentiating point I was trying to make is about 'local environment' You wrote that further improvement can only be improved by using a better antenna. I was trying to make clear that there can be and usually are multiple causes for increased noise in a local environment. Some of these are actually arriving by way of the perceived antenna due to near-field coupling but a second type come in after that in the form of CM currents and other kinds of systemic ingress.

A preamplifier at the antenna may actually help reduce this second type and create an overall improvement. This improvement would not be due to an antenna change, at least not what was thought to be the antenna. In fact, in that case "the antenna was not the antenna" ! The system had multiple but separable sources of unwanted ingress.

It is a widely held belief that the shield on single ended/coaxial cable prevents ingress from occurring there. This is only true for TEM systems where there is zero common mode current, unwanted current flowing on the outside of the coax. When such current does flow on the outside, then imbalance at either end converts some of that current to differential current in the desired TEM component.

Because the systems we use often deal with such small levels - down near the thermal noise limit of KTB, it can require far more system balance to sufficiently reject unwanted currents than is achievable with passive hardware. So-called 'broadband baluns' are often only able to achieve 20-30 dB of balance at best. This can be many ten's of dB less than is necessary to force the currents due to unwanted conversion below the incoming noise floor.

Unwanted current traveling through the groundplane of the KiwiSDR, for example, is converted "only" 80 dB down to current through the center conductor of the coax and injected into the ~50 ohm input impedance of the preamp. That CM ==> differential conversion can and does easily damage reception SNR, often without the knowledge of the user who may think "I hooked up the coax and the noise floor came up, I guess it came in from my antenna" when in fact the majority of it did not.

Our goal is to transfer the DX (far-field) propagated SNR present within the radiation resistance of our antennas to the detector of our receiving system. Without understanding and identifying the multiple mechanisms by which this can be degraded and which are active in a particular system, there really are no simple answers to improving a [poorly behaving] receiving system. As you suggest, blindly adding a preamplifier may make no improvement at all and may only make things worse due to IMD, internal noise or additional conversion from CM ==> differential QRN.

Without understanding that "our antennas are not what we may think they are" and "coax shield does not prevent unwanted ingress" the techniques we might use to improve our results likely will not deliver the improvement that is possible.

If you use a broadband display, particularly one with min/max hold you can appreciate where the QRM is by the ham-band notch depths as well as how much it is hurting your SNR.

Here is capture of reverse channel, user ==> Internet, Internet-over-coax, QRM leaking from a neighbor of N6GN received on an RX888 and recorded with ka9q-web from ka9q-radio. Some of the ham band notches are IDed. The depth of the notch indicates how badly the SNR is being hurt by this interference.

This is being received by a 6m dipole SAS which has quite flat field response over these marked frequency range.

Because of the notches, ham band performance isn't being hurt but SWL and other listening at other frequency certainly is.

Don't confuse lower levels with lower SNR, which can sometimes happen. A properly operating loop should have ultimate performance very similar to a similar sized dipole probe, at least up through mid HF, modulo antenna pattern in az&el which can favor one or the other depending upon what you consider 'better'.

As LZ1AQ has written, making a small lowZ system reach propagated noise limits in the quietest locations may be impossible at the high end using components known-to-mankind as it is for small dipoles. But in general if/when all the other system noise ingress, noise floor, and IMD are pushed below the propagated noise produced within either antenna's radiation resistance the limits are very similar.