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efficiency of spark gaps vs tubes
---------- Forwarded message ----------
Date: Fri, 3 Oct 1997 08:57:18 -0700
From: Jim Lux <jimlux-at-earthlink-dot-net>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: efficiency of spark gaps vs tubes
Geoff Schecht made a comment to the effect that tubes were more efficient
than spark gaps. I made the same comment to James Corum and he pointed out
that really wasn't the case. With a Class C tube amplifier, the best
practical efficiency is around 65-70%, that is, the switch (i.e. the tube)
is dissipating 30% of the DC input power. In a spark gap, the switch is of
very low resistance, and dissipates very little power, and with a low duty
cycle to boot. For a short gap in air, the voltage drop is going to be
around 50 volts, so on a 15 kW coil with the primary at 15 kV, you are only
dissipating, at most, 50 Watts in the gap. This is 99+% efficiency. I
suspect that the inefficiency in a typical tesla coil (i.e. Power out/Power
in)( considering that the arcs and corona is desirable power out) is due to
the loss resistance of the windings (pri and secondary) (i.e. you have a
finite Q) and some heating of the primary tank cap.
----------
> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Re: And what of the FCC?
> Date: Thursday, October 02, 1997 8:44 PM
>
>
>
> ---------- Forwarded message ----------
> Date: Thu, 2 Oct 1997 20:11:13 -0500
> From: Geoff Schecht <geoffs-at-onr-dot-com>
> To: Tesla List <tesla-at-pupman-dot-com>
> Subject: Re: And what of the FCC?
>
> <snip>
> >
> > All of these posts regarding RFI etc have caused me to wonder:
> > What is the difference between a rotary gap TC and the prehistoric
> > rotary gap CW transmitters? Is it just a matter of tuning a TC to
> > deal with an antenna instead of a toroid or sphere as the "load"?
> > I have heard recordings of these signals from way back when, and I
> > can tell you that if the local birds sounded so melodious, we would
> > all spend our spare time outside blowing our feathered friends the
> > way of the Passenger Pigeon. If you have ever heard the Russian
> > Woodpecker on HF (shortwave, if you must), then I can tell you the
> > Woodpecker was much more agreeable of a sound. The Woodpecker was
either
> > Soviet OTH radar, or a Soviet mind control device, depending who you
> > want to believe.
> > I dont know much about sparkgap transmitters, but I am pretty sure they
> > contained no active device, if you will, such as a vacuum tube.... so
> > mustn't they have been pretty much a TC? They were about the same
freqs,
> > as I recall.
> > BTW, 500kHz is still very much an int'l distress frequency, the last I
> > heard. More of a ship freq than anything else, to my knowledge. So is
> > 2182 kHz. Correct me if I am wrong tho.
> > Randy
> >
>
> Randy:
>
> The idea behind a sparkgap (or any other radio transmitter) is to
> efficiently couple the RF energy from a source to an antenna. The antenna
> is then supposed to transfer this energy into free space. Sparkgap
> transmitters don't have the big secondaries (in terms of the number of
> turns) that are typical of a TC. Any transmitter must contain a
> tuning/matching network to allow optimal transfer of energy to the
> radiating system. The antenna itself usually has an input impedance
between
> 25 to around 1000 ohms, ideally with nothing but a real (resistive)
> component.
>
> I may raise some hackles with this comment, but a Tesla coil is really
just
> an easy way to generate high voltages at moderate frequencies (in
relation
> to the 2-30MHz, radio spectrum, that is). They have been used to power
> X-ray machines and they found limited use in early particle accelerator
> work but they have little other commercial value. Small, handheld TC's
are
> handy for finding pinhole leaks in glass vacuum systems, however, and a
> number of quack medical devices used those little units to energize UV
> generator tubes that were supposed to cure everything.
>
> Other than that, Tesla coils emit very impressive sparks and are fun to
> build and play with.
>
> The original Marconi radio station on Cape Cod, MA has a few of its
> artifacts on display in a museum on the site (which is now under the
> auspices of the US Parks Dept). It was a gargantuan affair and I believe
> that it may have used a rotary SG. There's a much smaller amateur SG
> transmitter on display at the ARRL headquaters in Newington, CT. The last
> time I was there, it was operable and they'd give you a demo (it isn't
> connected to an antenna, of course).
>
> An early SG transmitter advantage was that you didn't need a
> superheterodyne receiver with a BFO to receive the signals. Coherers and
> galena-based crystal sets were adequate for detection; that's all they
had
> until the early 1920's anyway. Tuning of those receivers was rudimentary,
> at best, so you'd hear just about everyone within range when they were
> key-down whether you wanted to or not.
>
> Stationary sparkgap transmitters had a very buzzy, ratty note as far as
> received tone goes. Rotaries have been described as being slightly
> "musical" in tone and skilled operators could frequently identify each
> other by both the sound of particular transmitters as well as by an
> operator's "fist" or sending cadence.
>
> The problem with the very broadband emissions from spark gap transmitters
> became intolerable as the world became more dependent on radio for both
> messaging and entertainment. SG transmitters were in use in the US until
> the 1930's. By that time, the good folks at RCA, EiMac and the various
> other tube manufacturing concerns had advanced the art far enough to
permit
> the construction of vacuum tubes capable of many kilowatts of (coherent)
> power output at really useful shortwave communication freqs (2-30MHz).
>
> I've read about SG's being used as the power supplies in some diathermy
> machines and induction hardening apparatus at least through the Second
> World War. Vacuum tubes were pretty much 100% war-critical materiel and
> sparkgap RF generators didn't tie up those valuable devices when they
were
> needed the most. Sparkgaps aren't particularly efficient but they're
cheap,
> easy to build and they don't produce that much interference as long as
> they're not attached to antennas.
>
> Anyway, sparkgaps and Tesla coils are inextricably linked. The old
Russian
> Woodpecker was a pulsed (I believe,spread-spectrum) OTH radar that was
used
> to detect missile launches in the Western Hemisphere. It certainly didn't
> employ a spark gap.
>
> Geoff Schecht
>