Caerful when soldering. When the solder wicks up that far and into the coax, especially in a bend like this, it can partially melt the insulation. Even PTFE can be nicked a bit.
[...]-- Regards, Joerg http://www.analogconsultants.com/
Just keep in mind that the longer your test the more compromised your barrier becomes. Hipot tests are partially sacrificial, they eat away at the insulation properties.
However, there is a huge difference between a continuous duty working voltage on the isolation versus a single-even safety function. Easily 10:1.
In the ham radio world there are some.
-- Regards, Joerg http://www.analogconsultants.com/
I said "not schottky".
Though the lower voltage ones have so much nonlinear capacitance, they are sometimes worse than true recovery.
I once built a circuit to try and demonstrate step recovery; I got the best result from a schottky that couldn't possibly be giving me true step recovery. It was only a ~10ns blip. More like parametric sharpening. Broadly the same idea, in any case.
Suppose it's worth wondering if a chain of SiC schottky junctions could do the same. Maybe a row of SMC or DPAK devices would have low enough stray inductance to do the job.
And there's always toobs. I recall reading research papers from the 80s-90s where they were using planar transmitter triodes in transmission line structures for driving nonlinear sharpening networks (magnetic saturation in that case, I think). Plenty of voltage, you'll have to see if the Vg, Qg, etc. is reasonable enough to work with.
Tim
-- Seven Transistor Labs Electrical Engineering Consultation Website: http://seventransistorlabs.com
Well, it would presumably work up to the cutoff frequency, limited by competing coaxial waveguide modes. And possibly degenerate modes involving wire-to-wire coupling, which shouldn't couple strongly to differential mode energy, but are nevertheless present. Guessing from the picture, it might be good up to... 10 to 100MHz?
I think the downside of ladder line would be the prohibitive wire diameter, even for a high impedance (widely spaced wires) line. And too wide a spacing and you start having to worry about unwanted radiation. (They could go one further and twist it every half wave, so the far field is nulled.) But yeah, the less balun you need at those power levels, the better.
Interesting to note, the voltage on the outer cage (with respect to ground) is probably nonzero. Or if it is, it's because they did indeed add a common mode choke, or something along those lines. Thought being: the ground impedance is nonzero, so the "ground" at the antenna feedpoint will have nonzero voltage.
Hmm, not obvious if they used litz -- suppose that's a possibility. Probably not worth it though.
I would guess the impedance is high enough not to be a big problem (obviously, they would've thought of that; question is, we want to know why). Seems like it would be around 50 ohms if it were solid coax, but since it's not solid, it's probably higher. But by how much? 100 ohms?
300 ohms?Still quite a few amperes at that power level, either way.
If the load is a standard 1/4 wave tower, it would match near 50 ohms, which would require a matching network or transformer, which is again undesirable. Given the potential for ground currents, they may well have a transformer out there anyway, in which case it doesn't matter much what the line or load are; they just tap them off as needed.
And lightning isolation, might be a plus.
Tim
-- Seven Transistor Labs Electrical Engineering Consultation Website: http://seventransistorlabs.com
I have done a safety approved transformer this way:
Layer 1 and 2, primary winding, about 12 turns. Blind via from Layer 1 to 2 Layer 3 and 4, secondary winding, same turns, same stuff with a via
It ran resonant at about 8MHz with added capacitance to trim it. (you could do a SW controlled sweep to make the trimming)
I got an overall input to output efficiency of 70%, which I thougth was good for a air core transformer and 0.5mm distance from layer 2 to 3
Cheers
Klaus
I like order of magnitude guesses. I don't think I can do better. Let's see if I can still function after eating my own cooking.
Looking at the photo at: there's not much in the way of known references from which to extract dimensions. The best I can do is the roadway, which I would guess(tm) is about 2.5 meters wide. Translating the track width to the square metal frame and adjust for parallax, perspective, and indigestion, I get 2.0 meters for the shield diameter and 0.4 meters for the center conductor. (5:1 diameter ratio extracted from my monitor with a ruler, which I think is fairly accurate). For air dielectric: Z = 138*log(D/d) = 138*log(5) = 96 ohms I'll call it 100 ohms.
The original photo at: indicates that it's 225 KHz at 1200 kW. Power = E^2 /R E = sqrt(Power*R) = sqrt(1.2*10^6 * 100) = 11 kV volts RMS. Peak voltage (which is what makes the arc) will be 1.4*11kV = 15kV. At 3x10^6 V/meter, a 0.8 meter gap will flash over at 2.4 kV. So either my guesswork and math suck (a likely possibility), or this thing isn't going to work at rated power. (I really hate it when reality doesn't agree with my reverse engineering).
Here's the station and tower: More photos including better pictures of the open wire coax arrangement: Google street view works, but doesn't get close enough to the towers to show details:
Agreed. If we accept my 100 ohm impedance guess(tm), the current in the coax would be: P = I^2 * R I = sqrt(P/R) = sqrt(1.2*10^6 / 100) = 110 amps However, that's distributed equally among the individual wires. I count 12 wires each in the shield and center conductor or about 9 amps per wire. No problem there.
That's what the old telegraph wires did to minimize crosstalk.
Yep, that's what's nice about using coax cable. If the antenna and transmitter impedances were fairly close to my speculated 100 ohms, the baluns are not needed.
I beg to differ. If you look at the photo: the coax shield wires appear to be directly connected to the metal frame. If they were isolated, insulators would have been needed. Methinks the outer cage is at ground potential.
Litz wire would certainly have helped a little, but I suspect the cost and maintenance problems might have been prohibitive. Besides, if you've got 1.2 Megawatts to play with, what's a few extra watts of loss?
See my previous guesswork. I would say 100 ohms.
110 A RMS total or 9 A per wire.VOA made use of open wire transmission lines at high power levels:
1/4 wave at 225 KHz is 333 meters (1100 ft). If the tower had a capacitive hat, it could be made considerably shorter. The web site says 289 and 330 meters.A 1/4 wave monopole over a decent ground works out about 35 ohms impedance. That's not going to match very well to a 100 ohm feedline. I couldn't find much techy detail on the station, so I have no clue how their doing the matching. A clue is that there are small brick building at the base of both towers, which suggest a matching network.
There are going to be ground currents anyway. A monopole requires a buried counterpoise in the form of ground radials in order to form the other half of the dipole.
The antenna is in Poland (and my family is from Poland). I suspect that using cheap electrical wire and just replacing the wire when it gets hit by lightning, might be all that's required.
I once designed an antenna that included weight loading measured in standard bird loads, which I vaguely recall was based on the number of Gooney Birds (albatross) that could be lined up on the antenna radials times 10Kg per bird. I took exception to the spec claiming that it was an unlikely possibility, and was presented with a photo showing a rather large number of birds neatly lined up on a seriously sagging antenna element. I didn't protest any further.
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
Why do I only see my math errors AFTER I post them? Argh...
At 3x10^6 V/meter, a 0.8 meter gap will flash over at 2,400 kV so coax air gap is sufficient to prevent flash over.
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
Gate drive rise time should be ~~1 ns. Max pulse width might be a few hundred us.
That will make the gate drive 50 ohms, which would radically slow things down. That same thing, but short and with a ferrite, would be lower impedance.
Yes. It might be that it's a lot easier to parallel a few low-voltage GaN fets to simply muscle though that, it solves the MOSFET driving problem to boot, and placates Mr. Miller.
Could be. It would take actual parts & numbers to figure out what's better.
Cheers, James
Sounds about right.
Any guess how much the open vs. solid construction changes it, if at all?
Without measurements of similar structures, I really don't know what to guess... more than double or triple seems rather unlikely, but I can't see it being equal to the theoretical (solid wall) coax figure either.
I can't even tell what's going on here, what a mess! I don't remember if VOA was just one massive station or several frequencies; would that be for improved propagation in, ahem, certain target regions, or just for bandwidth?
Seems unlikely that the line is anything below 100 ohms, so that sounds like a good bet.
Needs a periodic surge of high power to scare them away. 'Ey boys, meat's back on the menu!...
They'll screw up your tuning, too... what's an albatross, 20pF? Wings down or outstretched? (African or European Swallow?...)
Tim
-- Seven Transistor Labs Electrical Engineering Consultation Website: http://seventransistorlabs.com
I double checked my guesswork and again found a diameter ratio of 5 which results in about 100 ohms. For a 1 Megawatt transmitter into a
35 ohm monopole, I would think it would use a much lower coax impedance, perhaps 50 ohms. I'll ask.That depends on the frequency, but at 225 KHz, my guess is that 12 wires is sufficient.
Well, if you look at cage dipoles, which use a similar principle, the rule of thumb is that 6 to 8 wires is good enough. However, this thing is pushing 1 Megawatt, where current sharing among the wires is important. I would guess more is better up to the limit of the power handling abilities of the wires.
Sorry, but it was the best I could find.
I visited a short wave station in the distant past and had some difficulty finding the coax feeds to the antenna farm. I asked if the coax cables were buried and got a good laugh from the engineers. They pointed to what looked like ordinary wooden power poles with wires on the cross arms on top of insulators. I had thought they supplied power to the station. They were open parallel wire feed lines.
VOA targeted specific areas using big curtain (phased) antenna arrays to obtain directionality. I don't know how they handled transmitter and antenna switching. Here's photo of the manually operated antenna switch at the former Bethany, Ohio site:
According to the Wikipedia article VOA had stations in California, Hawaii, Okinawa, Liberia, Costa Rica, and Belize. Today, only Greenville, North Carolina remains.
I'm fairly sure it's 100 ohms from the photos. What I don't understand is why 100 ohms when 50 or 75 ohms would make more sense if the criteria was maximum flash over voltage, or minimum copper losses.
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
I once built a 120 amp polyphase buck switcher, with schottly diodes, that had grotesque reverse recovery currents and spikes. Turns out that those "schottky" diodes had PN "guard rings" in parallel with the schottky junctions.
Shock lines (NLTLs) use diode c/v curves to sharpen edges, down to a few ps. There's a saturating magnetics version, too, for making high power, fast edges.
I'm going to need ca 50 amps peak, hard to do with vacuum tubes. The reprates will be high for gas tubes, too. And besides, my customer would think I was crazy.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation
"John Larkin" wrote in message news: snipped-for-privacy@4ax.com...
You were driving them too hard. :)
Incidentally, you'll see the same thing on SiC schottky, though probably more noticeable due to the high RS. One example, I saw an Infineon datasheet that had a bizarre temp-dependent second knee in the Vf graph. Supposedly, that's the guard ring kicking in.
Tim
-- Seven Transistor Labs Electrical Engineering Consultation Website: http://seventransistorlabs.com
Pshaw, it only needs to isolate 4 kilovolts.
(which is one reason to consider making it inside a PCB)
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation
Rather than trying to get tens of amps of mosfet gate drive through a transformer, I'm considering using a floating gate driver, so the transformer would be low-level.
A 1G123 one-shot will fire on a 2 ns pulse, so maybe I can use single-loop windings, no ferrite or vias.
Of course, now I need a floating power supply per fet.
Gotta try that. A 1 cm diameter loop will be roughly 20 nh, so a step of 250 mA/ns makes 5 volts drop. 500 ma in 2 ns does it. Scary but not completely insane.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation
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