Bob, Thanks for the feedback, sounds like you have corona well-managed on the coil. For future reference, in case someone in the group has an outdoor HV application in a particularly humid environment wherein operation without maintenance is particularly desirable: There are inexpensive lightweight encapsulation materials and encapsulation methods that will allow long-term maintenance-free HV outdoor operation in high humidity. The humidity resistance of most high-humidity HV insulation materials degrades with time and temperature cycling; a small subset of these hold up over time and temperature cycling in humid environments; an even smaller subset is also lightweight and inexpensive. Unfortunately this small subset of inexpensive long-term robust outdoor high-humidity solutions is difficult to repair or rework if such becomes necessary. This is why, for low-cost lightweight outdoor HV equipment, we liked to develop outdoor-application HV electronics in Fluorinert until turns ratios, inductances, winding losses, spacings, coil form size/weight and other factors were just right (with Flourinert one can make winding changes and test in minutes), and then apply the long-term robust encapsulant. The above doesn’t matter for your application, since in your application the issues are at the HV output end and beyond, and those localized problem sites can be managed with spheres. Nor does it matter for Stefan’s application, since Stephan’s coil does not remain outdoors between tests. The above is useful for equipment that remains outdoors in high humidity environments. For most low-cost lightweight HV insulation materials, the humidity threshold of corona inception declines over weeks and months of diurnal temperature cycling in high humidity environments; the solutions impervious to this degradation don’t accommodate quick rework, and Fluorinert makes this rework easy until the bugs are out and optimization is complete, at which time an inexpensive lightweight high-humidity outdoor HV encapsulant with 10-year life can be applied. Apologies for the digression from your theme; I thought this might come in handy for future generations of unattended/remote VLF equipment, and thought I’d better write it down while it was in mind. Looking forward to your next step in LF! 73, Jim AA5BW Jim;
I see it's a liquid that is really a coolant. I use clear lacquer and corona dope in a spray can and spray coils with it. I use Stefan's round brass balls that I have soldered to any point that shows signs of corona arcing. It's at the high voltage end of the coil that starts causing trouble and from that point onward.
Fluorinert would be great for an item that had high voltages and large amounts of heat present to submerge the item in it-Bob
From: [email protected]
To: [email protected]
Date: Thu, 26 Dec 2013 15:14:36 -0700
Subject: RE: VLF: 8969.99 Bob and Stefan, Any thought of using Fluorinert to prevent coil breakdown? In indoor applications, if we didn’t want a high-voltage (including 100kV and higher) coil (or transformer or connector or circuit board) to arc (including during wide-temperature range testing), we would simply drop the whole device (including batteries, power supply and modulator, if that made things more convenient) into fluorinert, without any insulation, encapsulation or coating on the circuit boards, coil or other components. We re-used the fluorinert without complications, and after removing items from fluorinert did not need to clean the previously-immersed items in any way. High dielectric strength: >160kV/cm High resistivity: > 10^15 ohm-cm (1E15 ohm-cm) Inert Wide temperature range: -70C < T < 150C (various formulations available down to -100C and up to 215C) Thermal conductivity: 0.07 W/m*K (ROM similar to water) Color: colorless, clear (transparency ROM same as purified water) Toxicity: non-toxic for practical purposes, non-hazardous according to 3M. Experimental deep-water breathing fluid; inert; but as with all products observe safety data sheet (3M: “… not recommended for implantation in humans” “… Eye Contact: … flush, rinse”); the following lends a perspective: (3M) “…skin/eye irritation: no significant irritation”; “if swallowed: no need for first aid is anticipated”. Downside: ~ $200 USD per gallon from manufacturer; but reclaimed fluorinert is available at lower prices. The evaporation rate is probably dependent on which formulation you select; I’ve heard some folks mention quick evaporation at > 100C temperatures, but I do not remember needing to add fluorinert during environmental temperature testing; a wide range of formulations with different boiling points is available. 73, Jim AA5BW Stefan;
I understand your setup. It is not that hard to move.
I am not going to use such low VLF freq but as I mentioned am going for license to try get 28-30 kHz. Then I can run power up with less troubles of 8.9 kHz freq. It won't be till next season but am planning now for coil cover-coil size-etc. I have already 2' piece 18 inch white PVC pipe. Will wind 1000 meters #16 wire to start. I want to use my present 74 kHz coil inside as it is and add the extra coil outside then I can trim inside to hit exact resonance.
Date: Thu, 26 Dec 2013 19:44:09 +0100
From: [email protected]
To: [email protected]
Subject: Re: VLF: 8969.99
Hi Bob,
Oh, you're planning to become QRV on VLF, great!
What inductance will you need for your system? What coil diameter and wire diameter will you use? It is important to calculate the dimension of that coil before you start to wind it. A diameter of at least 60cm is to be recommended, better 80cm. But of course it depends on the wire diameter and isolation (or insulation?). If you just use a coil body that is on hand and start winding, you may find the residual L is much to low and then you come out with 3 coils in series which will be a huge construction in very inefficient. You should also leave some distance to ground, to prevent losses into the soil. A distance equal to the coil diameter is fine.
How will you manage the high voltages on the antenna? You will have to manage 50 kV rms or so.
For me, the situation is not as easy. I must not leave the coil its position permanenty. I have to remove it after the experiment. But that's not so dramatic. I can leave it into the building and we have an elevator...
Keep us informed by some photos :-)
73, Stefan/DK7FC
Am 26.12.2013 18:58, schrieb Bob Raide: Stefan;
If I go to VLF I am going to add to my existing inductance outside. I plan to make a box using "plexiglass" quarter inch thick with glued seams so is weather proof. It will be clear [no color] and is very good dielectric. I was going to ask you how much clearance I need and 30 cm is about 1 foot or 12 inches clearance around the coil. Leave air gap around bottom so air can circulate so you don't build up moisture. Small muffin fan with shroud to pull fresh air up and around coil and out.
Could even use clear vinyl material .025 thick and a clear vinyl box over a plastic pipe frame would be cheaper. Clear vinyl used for side curtains on boats-etc.
I am in sewing business [make boat and truck covers] so could make you cover and you build the frame-I ship clear vinyl cover UPS to you or you could have local canvas company make it. Use "RTV" over stitches to make water tight-Bob
Date: Thu, 26 Dec 2013 18:32:43 +0100
From: [email protected]
To: [email protected]
Subject: Re: VLF: 8969.99
Hi Bob,
Of course, if the coil is protected against rain, then it is no problem. But the coil is quite large and due to the high voltage, the housing should ideally have a distance of 30cm or more to the coil. So the housing must be quite large, even more. And that makes it dangerous because of wind blasts on the to of the building in 35m AGL...
Anyway it would be nice and i also thought about using a simple foil across the coil, without additional distance.
However on LF, there are partial discharges on the top of the coil which burns holes into the protecting foil. And the VLF winding is 2000m long. It is a lot of effort to repair it after a short cut between some turns due to arcing. So it is a high risk.
I think i will make a test in the near future and see if 9 kHz partial discharges are as agressive as 137 kHz partial discharges :-)
73, Stefan/DK7FC
Am 26.12.2013 16:32, schrieb Bob Raide: Stefan;
Even if you have your coils in weather proof housing rain is no good to transmit on 9 kHz? Bob
Date: Thu, 26 Dec 2013 13:55:11 +0100
From: [email protected]
To: [email protected]
Subject: Re: VLF: 8969.99
Hi Uwe, Tom, VLF,
Best signal ever from Tom, capture attached. Is it due to the propagation or did it improve your signal?
What about some tests in OPDS32 or maybe JT9-30? Unfortunately there is permanent rain here so i have to wait some days longer.
73, Stefan/DK7FC |
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