[TangerineSDR] [HamSCI] VLF Antenna Site Isolation

Jonathan emuman100 at gmail.com
Tue Jan 4 23:13:10 EST 2022


I think that both ICs (with and without power) would work pretty well.
I know the magnetometer works perfectly fine without isolation, but I
do believe that it adds a lot of benefit in general, including any
situation where unwanted signals can be fed back, like what Bill
experienced, and for VLF receivers. It would be a good addition to a
new layout revision, and may help a lot of people in the future
because everyone has different installations/local conditions.

I had planned on using a small SLA battery at the site with high
current capability for VLF receivers. It'll power the receiver for a
few months before it'll need to be charged, and you can have one
inside, trickle charging and ready to change. Plus, it gives you a
good excuse to go out and work on the antenna. Do you find the need to
clean the solar panels often? A benefit to an outdoor charging
arrangement would be continuous power with no interruptions, but the
same can be achieved with parallel battery connections. For the lower
voltage magnetometer, a DC-DC would be required if you use a SLA.


No worries! Your advice was not bad at all. In fact, it was excellent
advice because it is an effective way of channeling lightning currents
away from an antenna site. We still don't know exactly is channeling
these mains harmonics and it could be from other sources as well. The
ground wire is likely, but some more troubleshooting is required.
Thank you for your advice that you've given me on many different
topics! I appreciate it all and learned so much from you.

And the same is true for you Jules! Thank you so much for all of your
advice and all that I've learned from you!


On 1/4/22, 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 <http://abelian.org> 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
> --
> Please follow the HamSCI Community Participation Guidelines at
> http://hamsci.org/hamsci-community-participation-guidelines.
> ---
> You received this message because you are subscribed to the Google Groups
> "HamSCI" group.
> To unsubscribe from this group and stop receiving emails from it, send an
> email to hamsci+unsubscribe at googlegroups.com.
> To view this discussion on the web visit
> https://groups.google.com/d/msgid/hamsci/7a1f58b5-214c-65b0-6549-e7a0b20df5c0%40gmail.com.

More information about the TangerineSDR mailing list