VT Fuse

"Proximity" means "near." The signal that fires the thyratron is
mathematically the doppler frequency. In as much as the shells
detonated several wavelengths (up to 100 feet stated) away from an
airplane or the ground, it's far-field RF, not some capacitive thing.

http://en.wikipedia.org/wiki/Proximity_fuze#Radio_frequency_sensing

John


As I understood it, an RF wave was sent out and the reflection phase was
detcted. The phase changed as the target got ever closer, and an
increasing low frequency signal was generated by the changing phase.It was
this LF that detonated the device by way of comparison to a preset
frequency. Doppler or not, who cares.... the subject is very interesting.

It is detecting the amplitude of the frequency difference between the signal
and Doppler shifted reflected wave. The range of the device indicates that
it operates in the far field (more than a couple of wavelengths by the
classic definition of far field). But the classic definition of far field
is where the target's absorption or reflection of the signal has no effect
on the transmitter. This is clearly not the case with the VT fuse.

True Doppler radar (weather radar, for example) doesn't look at the radiated
power change. It simply compares the generated and detected frequencies. In
fact, in some forms of radar, transmitted the pulse (or chirp) is turned
off by the time the return signal arrives (so as not to swamp the receive
stage).

--
Paul Hovnanian mailto:Paul_at_Hovnanian.com

The whole 'false modulation' issue was pretty interesting to read in
Baldwin's book. They did have some problems with microphonics which they
discovered by whistling at a bench test article.

I can't get over the bit where a shell containing3 tubes is fired out of a
gun that can fire the shell more than 20,000' straight up!

"Ian Field"

I can't get over the bit where a shell containing3 tubes is fired out of a
gun that can fire the shell more than 20,000' straight up!

Jim Yanik wrote:

"Ian Field" <gangprobing.alien_at_ntlworld.com> wrote in
news:eGfSq.67$Ud1.41_at_newsfe01.ams2:


"Jim Yanik" <jyanik_at_abuse.gov> wrote in message
news:Xns9FDFF145870E4jyaniklocalnetcom_at_216.168.3.44...
"Ian Field" <gangprobing.alien_at_ntlworld.com> wrote in
news:9LZRq.478$fZ7.93_at_newsfe09.ams2:


I can't get over the bit where a shell containing3 tubes is fired
out of a gun that can fire the shell more than 20,000' straight up!




don't forget that the shell had a battery or batteries in there to
power the tubes!

From what I've read so far, it seems they had dry-charged lead acid
batteries and a glass ampule of acid that shattered when the shell was
fired and soaked the plates.

There was also the generator type with a small turbine ring on the
nose cone.




lead-acid batteries doesn't seem like enough voltage to run vacuum tubes.
2v per cell,and even the tiny VTs used probably needed 30V or more for
the
plate V.
Hmm,ISTR vaguely that TEK used 12V on Nuvistor plates used in 400 series
O'scopes,not 100% sure though.

maybe they used the L-A cells for filaments,and the turbine for the plate
V.

The wet cell batteries in AA shells were 67.5 volts (according to
Baldwin).
They were carbon zinc with a chromic acid electrolyte in an ampule that
was
broken when the shell fired.

In article <jfcsn2$f48$1_at_dont-email.me>, John S <Sophi.2_at_invalid.org> wrote:
On 1/18/2012 4:46 PM, John Larkin wrote:
On Wed, 18 Jan 2012 16:18:05 -0600, John S<Sophi.2_at_invalid.org
wrote:

On 1/18/2012 3:30 PM, John Larkin wrote:
On Wed, 18 Jan 2012 14:31:07 -0600, John S<Sophi.2_at_invalid.org
wrote:

On 1/18/2012 1:59 PM, John Larkin wrote:
On Wed, 18 Jan 2012 10:14:37 -0800, "Paul Hovnanian P.E."
paul_at_hovnanian.com> wrote:

Here's a question for all you old-timers.

I've been reading a few articles about the WWII vintage proximity
(VT) fuse.
Some describe it as using the change in radiation resistance caused by the
proximity of a conductive object. Others say it worked on based on the
Doppler effect.

My vote is for radiation resistance, but I've only (briefly) examined a
poorly documented schematic of one version.

So, which is it?

The signal detected is definitely the doppler frequency.

Some schematics, about midway down:

http://johnlarkin.yolasite.com/pics.php

John

Not doppler -- proximity. So sez your schematic.

"Proximity" means "near." The signal that fires the thyratron is
mathematically the doppler frequency. In as much as the shells
detonated several wavelengths (up to 100 feet stated) away from an
airplane or the ground, it's far-field RF, not some capacitive thing.

http://en.wikipedia.org/wiki/Proximity_fuze#Radio_frequency_sensing

John

It is pointless to argue with you, John, because you are as tenacious as
a booger when it comes to "sticking" to your point even if wrong.


Well, if "proximity" is a measurable electrical effect, what is it?

John


"Technically, the Allied fuze used constructive and destructive
interference to detect its target.[8] The design had four tubes.[9] One
tube was an oscillator connected to an antenna that would both transmit
and receive. When there was no target nearby, the received signal would
be small and have little effect on the circuit. When a target was
nearby, it would reflect a portion of the oscillator's signal back to
the fuze. This reflected signal would affect the oscillator depending on
the round trip distance from the fuze to the target. If the reflected
signal were in phase, the oscillator amplitude would increase and the
oscillator's plate current would also increase. If the reflected signal
were out of phase, then the plate current would decrease. When the
distance between the fuze and the target changed rapidly, the phase
relationship also changed. A low frequency signal developed at the
oscillator's plate. Two additional amplifiers detected this low
frequency and triggered the 4th tube (a gas-filled thyratron) to set off
the detonator."

The phrase "used constructive and destructive interference" made me
think of proximity. However, they go on to mention the low frequency
signal generated, and that makes me now think in terms of Doppler.

Was the RF tube run as a super-regen detector?

On Wed, 18 Jan 2012 17:07:42 -0800, "Paul Hovnanian P.E."
paul_at_hovnanian.com> wrote:

Tim Wescott wrote:

On Wed, 18 Jan 2012 10:14:37 -0800, Paul Hovnanian P.E. wrote:

Here's a question for all you old-timers.

I've been reading a few articles about the WWII vintage proximity (VT)
fuse. Some describe it as using the change in radiation resistance
caused by the proximity of a conductive object. Others say it worked on
based on the Doppler effect.

My vote is for radiation resistance, but I've only (briefly) examined a
poorly documented schematic of one version.

So, which is it?

My understanding is that it is the former, at least for the commonly
used
fuses. They basically built a directional metal detector that depended
on the rotation of the missile to provide and AC component to the
detection, then they looked for that wobble in the oscillator bias.

Certainly the detailed descriptions that I've read (there was one in the
IEEE Spectrum several years ago) were consistent with the "metal
detector" idea, and had schematics that matched.

There probably _were_ fuses that were designed to use Doppler, but I
think the ones that actually worked in WWII didn't. Instead, I suspect
that the "Doppler" effect was being cited by journalists who either
didn't know what the heck the real "Doppler effect" is (and think that
_any_ change in the signal due to motion must be "Doppler"), or were
confused by later, more sophisticated fuses, or both.

Not really. The antenna dipole was symmetrical with the axis of the shell
(between the nose cap and shell body). So it wasn't directional in a sense
that it could sweep an off axis target to modulate the return.

Versions of VT fuses were deployed in non rotating projectiles (mortars,
missiles and bombs).

At one point, some people thought its signal was being modulated by
airplane propellers. But that was put to rest when VT fuses proved to work
very well for detonating shells at preset altitudes above the ground.

Its an interesting subject. Until around 1940, the two primary areas of
investigation were photoelectric fuses and remotely (radio) triggered
detonators. Around 1940, the emphasis was shifted to radio proximity
fuses.

All of the from 'The Deadly Fuze' by Ralph Baldwin

I just finished reading 'Hedy's Folly' by Rhodes. There's a bit about Hedy
Lamarr and George Antheil submitting a proposal for a proximity fused
antiaircraft shell to the National Inventor's Council (the wartime group
tasked with promoting R&D). That resulted in a temporary falling out
between Hedy and George when he came back from a meeting in 1940
empty-handed. Hedy suspected him of trying to cut her out of the proceeds
of that potential invention.

But given the secrecy of the VT fuse development at the time, that's
exactly the reaction an outsider (a piano player/composer) would have
gotten at the time. 'Run along now, buddy. That's a stupid idea.'

By the way, VT stands for variable time, a fuse type with a settable at
use time delay from being fires to time to explode. A very different fuse
from a proximity fuse.

Simple arithmetic and muzzle velocity. Grade school stuff.