Noise
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John
I started to look for ways to improve my spotting. I figure I should record baseline information. I thought I would calibrate the signal level with my
Elecraft XG3. I placed a 50 ohm load at the KIWI-PI. I looked at Grafana signal lever recordings. My on-and-off previous signals matched my 2/3 RX and 1/3 TX plots. Now most bands are down at less than -155 dbm. I then moved the 50 ohm load to the lightning arrester/Ground at the base of the antenna, Hy gain AV680 vertical. I am shocked by all the noise in the low bands shown on the Grafana plots. I did a screenshot of the KIWI display. The path of this cable is only from the KIWI to an arrester at the copper entry point- single ground point, 10 ft to the concrete block wall,40 feet at ground level to the antenna base. I hope that all the noise I see is caused by some error I have made and be corrected. What might be the cause and correction? You can see the jump at about 1600 11-26 in the Grafana charts and KIWI JPG down to 630 meters. John TI4JWC |
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Glenn Elmore
John, Your plot doesn't show the resolution/IF bandwidth. Turn off the labels with "yyy" and turn on the console the next time you share these. The waterfall detail doesn't add much time information except that the stuff below 10 MHz is not continuous. Can't tell the time scale without seeing the console either. The gradual bump up around 40 is, I think, the shaping due to jks preshaping of the DSP responose to correct for sin(x)/x. It's about 3 dB on a standard kiwi and you are at 2X that. The Kiwi should be as good as the XG3, I think knowing and properly normalizing noise with it will be fine. The bumpiness in LF-HF is a sign of unwanted ingress but almost certainly not from far-field radiation. It is likely either radiated near-field or common mode current induced. Almost certainly the latter from the shape and range of it. You probably have a common-mode noise problem to fix. See the draft document I recently shared to this group. Hopefully it will give you a plan for identifying and improving the situation. I'm not sure which Grafana plot dashboard you are using. Can you share this and also retake the Kiwi plot as mentioned above? Glenn
On 11/26/21 1:13 PM, John via groups.io
wrote:
I started to look for ways to improve my spotting. I figure I should record baseline information. I thought I would calibrate the signal level with my |
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John
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John
Glenn, what is the title of your post with the draft article?
John |
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Glenn Elmore
It's a v0 draft I'll try to clean it up as I have time and ideas https://groups.io/g/wsprdaemon/message/562
I still don't know for sure about absolute levels, a repeat Kiwi
plot showing console would let me be more confident. Let me just
mention
Between these two you can get both an overview and a good single frequency measurement. Grafana is showing a Kiwi system noise floor of around -157 which
means you aren't actually observing noise characteristics due to
everything else. There may be more steps and they may have larger
delta. Something doesn't add up here for a Kiwi hooked to almost
any antenna in a typical environment. At MW and low HF the
external noise from a quiet site to a matched antenna should
almost always be above the Kiwi floor. If yours isn't either you
have bad match or loss of some kind.
On 11/26/21 4:37 PM, John via groups.io
wrote:
Glenn, what is the title of your post with the draft article? |
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John
Glenn, you are correct. the Low reading of -155 on 11-26th was a 50-ohm termination at the KIWI connector, no cable, no antenna.
At this moment the noise recordings are to a 50-ohm load at the arrestor at the grounded base of the antenna, antenna disconnect, cable only. Looks like I have significant noise ingress in the low bands. I will do a new KIWI screen-shot after I finish my Saturday chores; wash the dog and dog beds. John TI4JWC |
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John
New KIWI jpg attached.
John |
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John
Glenn, I just read this from an article by W8JI.
" Measuring Common-Mode NoiseWe sometimes hear we can test or evaluate a system for unwanted noise or signal ingress by disconnecting and replacing the antenna with a dummy load. This idea actually has no theoretical foundation at all. Replacing an antenna with a small load significantly alters common mode impedance of the system, and removes the ingress point (the antenna's feedpoint) entirely. Dummy load substitution significantly changes system common-mode impedance. The only real test would come from a dummy load with the same connections and impedances (both differential and common mode) as the actual antenna. In other words the test load has to be the actual antenna to keep feed line common mode ingress the same. Obviously, casual dummy load substitution is a useless test! The best approach is to use preventative measures in initial system design and installation. Quite often the cost of being safe is less than a few percent of the initial system expense. " Tells me that my test to a dummy load has dubious value. John |
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Glenn Elmore
Agreed and I hope the draft article shows why. The MW stuff you are seeing is almost certainly coming in conducted. My only arguments with W8JI is one of scale and bandwidth. He's
correct about CM changing radically with that test but his
implication (maybe I'm unfairly reading that into what he wrote
) that mitigation over a broad (say 3-4 decade) bandwidth may be
done effectively compared to the need. I think most of us underestimate the distance between the level
of the propagated noise floor, even in a matched half-wave antenna
where it might be down near or below the Kiwi's floor at a quiet
site, and the level of CM that can easily/typically be introduced
by CM lines in and out of a Kiwi, through the ground and out the
other side. At HF I measure on the order of 80 dB rejection by the Kiwi,
which might sound like a lot but compared to, say, -154 dBm (20 dB
over KTB and maybe a reasonable upper HF number) that means the CM
better be causing less than -74 dBm equivalent injection. If one
considers the CM impedance pushing those currents to be a bit
higher than 50 ohms, then this means they better be smaller than
about 1 uA ! I bet for most of our (wired) installations this is a
fairly tall order. For antenna systems with lower levels these
numbers only get worse. I think what you have is a big CM source coming from LAN or PS (probably not GPS since it is shorter and smaller self-capacitance to the world) and going out through the SMA(s) through some path. What might help you a lot is a low inter-winding capacitance 1:1
transformer,maybe just a Mini-Circuits T1-1, isolating the grounds
of the SMAs for common mode. It's not perfect, still 1 pF but I
bet that and care that there's no return path through the GPS SMA
return will likely change things. As W8JI suggests, you can possibly see the effect by terminating the Kiwi with 50 ohms and then just touching the antenna system ground to the SMA ground at the termination to get a more representative comparison. You may see a huge jump in noise when you do that which would be confirmatory.
On 11/27/21 12:12 PM, John via
groups.io wrote:
Glenn, I just read this from an article by W8JI. |
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Jim Lill
a quick disconnect of the gps antenna will check if that is a
source and the unit will remain operational for that short period. On 11/27/21 2:43 PM, Glenn Elmore
wrote:
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John
I am back to my normal RX configuration. T1-1, AM filter, FM filterr.
I will do the GPS disconnect test and touch Antenna ground while KIWI is terminated to a dummy load test, Monday, Sunday is our Thanksgiving dinner with the locals. John TI4JWC |
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John
Today at 1900z I changed the 5 v power supply. From a 20 amp 5.25 v AC powered to an inline 12/5.2 buck converter.
10M to 40 m noise dropped 2--4 dbm, 60m to 630 noise went up. 160 up by 10 dbm. John |
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John
At 2031z I removed GPS.
Waiting to see. John |
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Rob Robinett
I think you are introducing too many variables. Can you battery power your kiwis and somehow get to it over wifi? Starting with such a configuration, then add the coax to your arrestor with a terminator at the arrentor. then add the coax to the antenna feed point with the terminator there, etc... As soon as you DC power you Kiwi, you have introduced a giant loop antenna in your receive chain which extends from the Kiwi's DC ground through the DC power supply, then through who know what house wiring and finally the earth back to your arrestor. My guess is the major source of your RFI is in that ground loop and it will be difficult to suppress. Perhaps if the SC power supply gets is ground from the same point the arrestor and the DC and RF follow each other from there to the Kiwi At 2031z I removed GPS. --
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Glenn Elmore
Do you have a low interwinding capacitance isolation transformer
located at the KiWi SMA or can you at least temporarily try one?
Wind a couple of turns on primary and secondary of a high-mu
core if you don't. Even if it doesn't have sufficient low-end for
your needs it will test the hypothesis. and is probably an easier
way than first changing PS and LAN connections.
On 11/29/21 2:14 PM, Rob Robinett
wrote:
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Rob Robinett
I believe I sent John an assembled version of one of your T1-1 PCBs which he can use to break the ground loop as you have suggested. On Mon, Nov 29, 2021 at 1:53 PM Glenn Elmore <n6gn@...> wrote:
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Glenn Elmore
John,
If you can't find that PCB you (or anyone reading) can easily make something from an inexpensive TV/FM 4:1 transformer that will serve purpose:  The picture shows examples of a Mini-Circuits T1-1 along with a homebrew transformer with 2turns:2turns wound on a core from one of the TV baluns at the top. With typical core materials it won't have quite as low insertion loss in HF as the T1-1 but to it will have pretty low inter-winding-capacitance and good common-mode suppression for testing. Here's a quick example I just wound and measured. It has about 2.5 dB insertion loss at 15 MHz compared to the T1-1 which is well under 1 dB but is plenty adequate to identify the large amount of CM that we're discussing. This is at 5 dB/division, green is T1-1 Yellow is TV balun wound with wire size small enough to get through the holes in the core:  If anyone wants the SMA-connectored PCB shown in the top photo, drop the attached xfrmr.brd file onto Osh Park's web page, pay them a small fee and receive it postpaid in the US within about 10 days. Connectors are normal SMA or (preferrably PCB mount SMAs available on eBay. Don't be tempted to use a Mini-Circuits T6-1, perhaps because the specifications show a lower low end. In fact that part is not a conventional flux-coupled transformer, it's a transmission line and has too much coupling between input and output for our purpose. If inserting one of these at the Kiwi's SMA connector doesn't make any difference then you've just proven that this isn't your problem and need to look elsewhere, but many times it is. Glenn n6gn |
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John
Glenn,
Rob provided me with a T1-1 assembly months ago. Other than the first test with a 50-ohn load at the KIWI, I always use the T1-1. John |
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Glenn Elmore
John
That's helpful. It probably means that the conversion from CM to differential on your feedline is occuring on the antenna side of the T1-1. If you have a symmetric antenna, one on which you can short a desired differential signal to examine an undesired CM component, you can verify this. If OTOH you have an asymmetric antenna like a monopole this won't be possible since shorting the connections shorts both modes. Earth currents and actual near-field sources are still possibilities too. There may be others. |
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John
Glenn,
The antenna is a Hy-Gain AV-680, 9-BAND HF VERT ANT, 80/40/30/20/17/15/12/10/6M. I am guessing that the"shorting method" does not apply. Can I learn anything by disconnecting at the antenna feed point and applying a short or 50-ohn load? John |
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