[TangerineSDR] [HamSCI] VLF Antenna Site Isolation

Jonathan emuman100 at gmail.com
Fri Apr 29 16:03:10 EDT 2022

Hi All,

I wanted to give an update to this issue. Nathaniel and I were able to pull
up the ground wire run alongside the conduit. Since the soil wasn’t to
settled, it ended up being easy to pull out. After it was pulled out, the
difference was quite remarkable!

This is a diurnal plot of the 180 Hz mains harmonic for that day. Around
17:30 UT, Nathaniel and I removed the ground wire. The spike was when I
momentarily disconnected and moved the earth ground connection (the other
antenna electrode) from a ground spike to a ground clamp attached to the
mast. After that, the 180 Hz harmonic is greatly reduced!

The difference is quite remarkable in the spectrogram too! Here is the
spectrogram before the ground wire was removed:

And here is the spectrogram after the ground the ground wire was removed:

These were spectrograms were generated right before and after the ground
wire was removed. As you can see, the reduction in mains harmonics is very
noticeable! Aurally, it sounds so much better too!

A side benefit is reliable reception of the amateur bands of 8720 Hz and
5170 Hz. Here are spectrograms centered at each of those frequencies. With
detecting GPS referenced phase and frequency locked carriers and coherent
BPSK, the reduction in mains harmonics will make QSOs possible now. Here is
the spectrogram of 8720 Hz:
On the left is before the ground wire was removed, on the middle and right
is after. A faint line can be observed 4/24 from 12:00-18:00UT. This is a
harmonic due cross talk from the PPS signal inside the Raspberry Pi box.

Here is the spectrogram of 5170 Hz. The same PPS harmonic can be observed.
This can easily be remedied, however.

Here is Paul’s comment on the difference:

> The difference is quite remarkable!

I'm measuring almost exactly 20dB reduction of the noise floor,
much more like a properly isolated VLF receiver should be.
Still a little bit of scratchy interference from somewhere
which I haven't identified yet.  It was there before but
weaker now I think.

I've added vlf44 to the fbins page and the extra isolation
makes a big difference,

We can see faint lines at 8270 and 5170, cross-talk from the PPS
channel.  It's a pity VO1NA beacon is on 8270.000000, best to
avoid integer Hz because of PPS harmonics.

Now we just need some more activity, some chorus and whistlers
would do nicely and I think there's a good chance that you
can detect DL3JMM (6506km) next time he is on 8270.03.

I would expect the aluminium mast, set in the soil as it is,
would be a good enough ground itself, without any extra stakes.
But the extra won't do any harm.

> Starlink internet service was installed

You can see your uplink logs, and some other info


> I removed the ground wire completely.

That must have been quite a job, but well worth the effort!


On Wed, Jan 5, 2022 at 1:09 PM Dr. Nathaniel A. Frissell Ph.D. <
nathaniel.frissell at scranton.edu> wrote:

> Hi Mike,
> Like Jonathan said, no worries at all! Thank you for all of the
> suggestions. This is part of the process.
> 73 Nathaniel W2NAF
> > On Jan 4, 2022, at 9:10 PM, MNaruta GMail <mnaruta at gmail.com> wrote:
> >
> >
> > My apologies Nathaniel and Jonathan, for my bad advice on burying a
> ground wire near the conduit.
> >
> > In my broadcast and LMR experience, we wanted to bond everything
> together.
> >
> >
> > Michael Naruta - AA8K
> >
> >
> >
> > On 1/3/22 11:20 AM, Jonathan wrote:
> >> Hi All,
> >> As was seen in the SAQ spectrum plot, there were prominent mains
> harmonics in that section of spectrum. Normally that isn't the case, as
> mains harmonics typically tail off at around 6-8 kHz. I was showing my
> spectrum plot to Paul, the author of vlfrx-tools and a VLF enthusiast for
> over 20 years with many advancements for VLF reception at home to the VLF
> community. His website is abelian.org <
> https://nam10.safelinks.protection.outlook.com/?url=http%3A%2F%2Fabelian.org%2F&data=04%7C01%7Cnathaniel.frissell%40scranton.edu%7C25f541ffb7594d1fd0f208d9cff0a522%7Ca8edc49a41f14c699768a7f6d7c3b8c3%7C0%7C0%7C637769455817903205%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=eHmiZBafB7uUiwDvG8VB7YJq%2BUpC5icKICQZ%2Bd7nAR4%3D&reserved=0>
> and runs and publishes data from a network of VLF receivers for both
> natural radio events and VLF amateur transmissions in the Dreamer’s band.
> >> Paul and I had a long discussion about these harmonics and the VLF
> system site installation. He asked the initial question:
> >> /Impressed that you're getting mains harmonics so high, have you got
> something clipping somewhere?/
> >> /
> >> /
> >> Because I was using his VLF preamp design and he added a lot of
> headroom for strong mains hum and sferics in the front end, there
> definitely wasn’t any clipping. The audio level was quite reasonable and
> even the strongest sferics would be no greater than 45% of the soundcard’s
> input range. Initially, he wanted me to perform an experiment by wrapping
> grounded foil on the antenna, but after looking at some captures of the VLF
> spectrum, he decided it wasn’t necessary to perform the experiment. The
> experiment was to attempted to eliminate a potential source of harmonics,
> any clipping in the VLF preamp /feedline chain. Paul then talked about the
> issues that heavy mains harmonics can cause for a VLF receiver:
> >> /You'll find that with a mains harmonic spectrum like this, your system
> noise floor is also significantly raised. The mains cross-modulates with
> the VLF noise to raise the floor, and you can hear that, as a sizzling
> frying sound to the noise rather than a pure hiss./
> >> This was exactly what was going on. It can be heard very clearly in the
> mains filtered stream and the raised noise floor, with mains harmonics
> extending beyond 20 kHz, can be seen in the spectrum display. The problem
> is, the increased noise floor makes weak signals from natural radio events
> and amateurs difficult to detect. This was clearly seen in the SAQ spectrum
> plot, as the mains harmonics were much more stronger than the weak SAQ
> transmission. I described the site installation and showed him pictures
> that included the installed active VLF antenna, conduit with ground wire,
> and ground magnetometer. Paul then went on:
> >> /Looks like a fine installation in an enviable location and just about
> far enough from the trees, the VLF reception looks good./
> >> /You have three good reference signals coming in: NAA, NAU, NML./
> >> /No sign at all of any clipping and the incoming 60Hz and harmonics are
> nowhere near strong enough to overload the pre-amp or SP70./
> >> /Wrapping the antenna doesn't just hinder the E-field pick-up, it also
> shunts the ground pick-up too: The pre-amp amplifies the potential
> difference between local ground and E-field and the wrap just puts a
> capacitive shunt across its input, thus reducing the response to both
> 'sources', a non-invasive way to temporarily drop the pre-amp input level.
> But it seems you're not overloading so no need to do this test./
> >> Paul makes a good point here which I wanted to emphasize that the earth
> ground connection is part of the antenna, so shunting the antenna element
> would also shut any pickup from the earth ground connection, as the
> potential difference between the antenna element and earth ground is what
> the input of the front end is seeing. Paul, continuing, comments about the
> source of the heavy mains harmonics:
> >> /We are probably seeing noise on the domestic ground getting into the
> rx ground circuit somehow. The spectrum and waveform looks very typical of
> switching PSU interference, UPS, chargers, etc, typical domestic ground
> noise./
> >> /I see a ground wire dropping down from the pre-amp into earth, that
> should be the only ground connection out at the site. The cat6 screen and
> the extra conductor in the conduit are both potential sources of trouble,
> providing channels to bypass the isolation and couple the domestic ground
> and the noise it carries, out to the receiver site ground./
> >> /Where/what does the conduit ground wire connect to at each end?/
> >> /I see two conduits, a grey with box which is presumably the cat6
> downlink/power, and a white with a single cable, is that the head of a
> ground stake/?
> >> Here, Paul reflected my initial fears with adding any grounds and low
> impedance paths between the antenna site and residence. On the VLF preamp,
> both power and audio lines are magnetically isolated as well as at the
> residence inside the Raspberry Pi box. This isolation is critical to
> keeping mains hum and harmonics out of the VLF spectrum. I mentioned the
> magnetometer’s cable shield was isolated, enclosed in the “white conduit”
> he mentions. I told him the shield on the cat6 VLF receiver feedline is
> also floating. I also mentioned the ground wire is just coiled up and
> laying on the ground at both ends. Paul goes on:
> >> /The likely culprit is that ground wire, outside the conduit, it will
> be galvanically grounded all the way along./
> >> /By connecting the two earth zones you're bringing all the muck on the
> domestic ground out to the rx site, bypassing all your DC and signal
> isolation efforts, you may as well move all the antennas back to the
> residence./
> >> I seconded his sentiments. The ground wire is the lowest impedance
> path. I do believe there is a high likelihood of these harmonics are
> primarily coming from this ground wire, seeing as the site is fairly remote
> with low hum levels and domestic interference, but even so, without some
> isolation from domestic sources, even the quietest of radio-quiet areas can
> furnish interference in the form of ground currents. Finally, Paul proposes
> an excellent solution, not only for this case, but for future PSWS users
> who want to co-site a VLF receiver with the magnetometer and HF antennas:
> >> /That ground wire might also be a good way to channel lightning
> currents back to the residence, at least for the few microseconds before
> the thin wire vapourises. A better solution would be a good solid earthing
> arrangement at the rx site, establishing a firm and clean local ground for
> all the electronics and a short low inductance sink for lightning currents.
> This, combined with isolation on all DC and signal circuits in the conduit,
> and suitable surge protection at key points, ought to protect the
> residence./
> >> /You're going to co-site things like HF loops and magnetometer and
> their power/signal links would need to be isolated too, to both offer an
> un-inviting path to lightning currents and to keep at bay the residence
> ground noise. The I2C link could be a challenge there. At some point it
> gets easier to digitise at the remote site and have a single isolated power
> and data link./
> >> I feel Paul’s argument here is a good one, and can be used for the
> benefit of the PSWS. Good, solid grounding at the local antenna site with
> isolation on all feedline a will not only keep domestic mains harmonics
> from polluting the antenna site ground, but offer good lightning protection
> for the residence. This also brings up a case for both power and signal
> isolation of the magnetometer, on both ends, that could add a lot of
> benefit for the magnetometer’s performance. It has been shown by Bill that
> the Pi host does create some interference in the HF bands, and isolation
> would be a great mitigation measure. TI makes an IC with both I2C and power
> isolation that could work quite well in a future revision.
> >> Jonathan
> >> KC3EEY
> >
> > --
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