On Sun, 15 Jun 2014 19:40:40 -0500, "Tim Williams" <tmoranwms@charter.net> wrote:>"Jeff Liebermann" <jeffl@cruzio.com> wrote in message >news:jptrp958ju1opod0ftdendvvan9p31bde2@4ax.com... >> For air dielectric: >> Z = 138*log(D/d) = 138*log(5) = 96 ohms >> I'll call it 100 ohms. > >Sounds about right.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. <http://www.microwaves101.com/encyclopedia/coax_power.cfm> <http://www.microwaves101.com/encyclopedia/why50ohms.cfm>>Any guess how much the open vs. solid construction changes it, if at all?That depends on the frequency, but at 225 KHz, my guess is that 12 wires is sufficient.>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.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.>> VOA made use of open wire transmission lines at high power levels: >> <http://cryptome.org/eyeball/voa/voa-antennas.htm> >> <http://www.lbagroup.com/blog/wp-content/uploads/2010/08/open-wire-antenna-transmission-lines.jpg>>I can't even tell what's going on here, what a mess!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.>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?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: <http://hawkins.pair.com/voaohio/voab5.jpg> <http://hawkins.pair.com/voaohio.html> According to the Wikipedia article <http://en.wikipedia.org/wiki/Voice_of_America> VOA had stations in California, Hawaii, Okinawa, Liberia, Costa Rica, and Belize. Today, only Greenville, North Carolina remains.>> 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. > >Seems unlikely that the line is anything below 100 ohms, so that sounds >like a good bet.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
PCB transmission line transformer
Started by ●June 13, 2014
Reply by ●June 15, 20142014-06-15
Reply by ●June 16, 20142014-06-16
On Sun, 15 Jun 2014 13:58:42 -0500, "Tim Williams" <tmoranwms@charter.net> wrote:>John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message >news:99tpp9h3ivho86avm4ddgge98acl3ds1n6@4ax.com... >>>Or if you can find SiC diodes in that range, too. The junction (not >>>schottky) types are just as awful as silicon, for instance, the >>>performance >>>of SiC BJTs, or MOSFET body diodes. AFAIK, they are constructed exactly >>>the >>>same as their Si counterparts, just in different material. >> >> Schottky diodes don't do the step-recovery thing; diffused silicon PNs >> seem to have the best doping profiles. > >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.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.> >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.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.> >> What I may wind up doing is talking the customer down to something >> more reasonable, 2-4 ns. The sine-squared transfer function of a >> Pockels cell speeds up the optical risetime a bit. >> >> I guess we may as well get some SiC transistors and bang the gates >> until we blow them up. Then we'll know. The Cree, Rohm, and Ixys parts >> seem to be remarkably similar. > >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.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
Reply by ●June 16, 20142014-06-16
"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message news:86uup9t3i34j599ifnsbldv0b7jrlfhp7f@4ax.com...> 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.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
Reply by ●June 16, 20142014-06-16
On Sun, 15 Jun 2014 09:49:42 -0700, Joerg <invalid@invalid.invalid> wrote:>John Larkin wrote: >> On 14 Jun 2014 04:11:16 GMT, Jasen Betts <jasen@xnet.co.nz> wrote: >> >>> On 2014-06-13, John Larkin <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote: >>>> >>>> We've used transmission-line transformers to drive mosfet gates. We've wound >>>> them from micro-coax on ferrite toroids, with the shield being the primary and >>>> the inner conductor the secondary. Sub-ns speed and low leakage inductance. But >>>> it's labor intensive. >>>> >>>> So I was thinking about doing it on a multilayer PCB, like a 6-layer. Layers >>>> 1/3/5 could be one to three layers of spiral trace, primary, and 2/4/6 ditto, >>>> secondary. I'm not sure how to think about the impedances, but it ought to have >>>> wide traces and thin dielectrics, I guess. >>>> >>>> Maybe one layer, one turn, per winding? That takes no vias. >>> one layer will be better, with 6 interleaved layers you have the L2-L3 and the >>> L4-L5 parasitic capacitance working against your goal, >>> >>>> It needs a ferrite core. Who makes the sorts of cores that work for PCB >>>> inductors? >>> with the coax transformer how much coax does it take before you don't >>> need a core? >> >> The coax would have to be really long to sustain wide pulses, and the >> the prop delay becomes a problem. I don't entirely understand this, >> but it seems to me that the output impedance becomes the Zo of the >> coax when the coax is long relative to the pulse rise time. >> >> Here's a version we did, but only for 200 ns 5-volt pulses. >> >> https://dl.dropboxusercontent.com/u/53724080/Parts/Inductors/Xfmrs.JPG >> > >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. > >[...]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
Reply by ●June 16, 20142014-06-16
On Sun, 15 Jun 2014 14:04:31 -0700 (PDT), Klaus Kragelund <klauskvik@hotmail.com> wrote:>On Friday, June 13, 2014 6:33:20 AM UTC+2, John Larkin wrote: >> We've used transmission-line transformers to drive mosfet gates. We've wound >> >> them from micro-coax on ferrite toroids, with the shield being the primary and >> >> the inner conductor the secondary. Sub-ns speed and low leakage inductance. But >> >> it's labor intensive. >> >> >> >> So I was thinking about doing it on a multilayer PCB, like a 6-layer. Layers >> >> 1/3/5 could be one to three layers of spiral trace, primary, and 2/4/6 ditto, >> >> secondary. I'm not sure how to think about the impedances, but it ought to have >> >> wide traces and thin dielectrics, I guess. >> >> >> >> Maybe one layer, one turn, per winding? That takes no vias. >> >> >> >> It needs a ferrite core. Who makes the sorts of cores that work for PCB >> >> inductors? >> >> >> >> Anybody done this? We might be up for some consulting, if the project gets >> >> serious. >> > >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 > >KlausRather 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. https://dl.dropboxusercontent.com/u/53724080/Circuits/Power/Floating_Gate_1.JPG 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
