Expensive Jammers

[jamesinnewcastl]

Hi

In another thread about WW2 wireless operators the subject of 'jamming' was raised. This reminded me of an item I had bought as curiosity - photo attached.

It uses a natty photomultipler lit by an incandescant bulb to generate what I assume is white noise of a quality that when used to broadcast would appear to be coming from a natural source and so lead the listener to assume that their equipment was faulty.

There are e valve stages to it - I wonder if these were shaping and forming the noise more?

I assume that as the valves are all 'JAN' then the item is American?

Anyway does anyone know anything more about this unit? It clearly drops into some larger piece of equipment given its flange mounting.


Cheers
James

[jamesinnewcastl]

I also just noticed that there is no power connector, only the coax output, so the power must have come from the array of threaded rods along its length. Not a connection technique I have seen before - why so many - were they wired individually or was it one half of a makeshift two-part connector?

Cheers
James

[HamishBoxer]

Interesting and certainly looks American.

[Nymrod121]

An interesting device. Any chance you could post some more photos of the underside wiring (assuming it's visible)?

Best wishes
Guy

[jamesinnewcastl]

Hi

I'll post more pictures when I get home.

It is very clean inside no wax capacitors or dog bones, all the caps are rectangular and light brown 'plastic' from memory. Resistors are the white ceramic sort with the brown 'filling' at the ends.

The photomultipler (still available at about £8, pity, I'd hoped it was a rarity) is in a can to keep the light out - the bulb is in a little can attached to it. it's just occurred to me that the bulb would be massively too bright for a photomultiplier at such close range so I am now assuming that there will be just a tiny hole between the two cans. I'll see tonight.


Cheers
James

[Nymrod121]

I look forward to the next photos - thanks James
All the best
Guy

[turretslug]

The capacitors sound like "Micamolds"- looking like Bakelite-cased mica capacitors (which may also be present in the lower values)- a type very common in US equipment of the era that actually uses paper and not mica as dielectric. They're guaranteed (pretty much) to be very leaky by now, should you ever seek to get it into action!

The threaded rods look rather like tuning adjusters, the 6AC7 is a low-VHF pentode broadly comparable to the EF50- I wonder if this was a part of test gear, say a noise source for checking radar IF strips?

[ms660]

Part of AN/APT-1:

https://archive.org/stream/GraphicSu...e/n29/mode/2up

?

Lawrence.

[Refugee]

Could it be a very early true RMS volt meter chassis?

[jamesinnewcastl]

Hi

I don't quite remember why I bought the thing but now I'm getting interested in powering it up. I'm suspecting that the last valve is just used as an amplifier, the intermediate ones must be shaping the generated spectrum I think.

Using the engines or the rotary generator might sound like white noise but I imagine that a spectrum analyser (or a musician) might readily reveal peaks of noise at frequencies corresponding with the rpm and harmonics of the rotating parts.

So it would be interesting to see the spectrum from the Photomultiplier - are there peaks that would give the game away? What do each of the sucessive stages do to the spectrum? What noise comes from just the valves themselves?

How does changing the lamp brightness affect the system?

The other different aspect to it is that one would be actively seeking huge amounts of noise and studying it rather that doing your best to reduce and eliminate it!

Cheers
James

[Radio Wrangler]

Hmm, suppressed carrier.

So there's an RF oscillator and the amplified noise from the photomultiplier and the RF meet in a balanced modulator then to a power amp. There will be a string of amplifiers after the photomultiplier to bring the noise up to a useful level.

As said, true random noise will leave the victims not too suspicious of deliberate interference. In addition to shielding a bomber, it might also waste effort of higher echelon enemies trying to fix nonexistent faults.


...... was code named "Tinsel"

while actual tinsel was code-named "Window"

That ought to confuse them.

David

[jamesinnewcastl]

Originally Posted by Radio Wrangler

while actual tinsel was code-named "Window"

David

Which the Americans called 'Chaff'


Hang on I found a circuit - not of my one but similar I suspect.



James

[turretslug]

Originally Posted by ms660

Part of AN/APT-1:

https://archive.org/stream/GraphicSu...e/n29/mode/2up

?

Lawrence.

Thanks for that very interesting and informative link Lawrence- it's slightly sobering to think that safety in the teeth of danger was dependent on a humble little #47 (or whatever) bulb continuing to shine....

Apparently, the folks developing "Window" were rather alarmed to hear of the "Tinsel" code-name, as it seemed a worryingly obvious pointer, if only to another countermeasure!,

Colin

[ms660]

No problem, the link was a second attempt, the first one I posted up for a different website didn't work.

Lawrence.

[jamesinnewcastl]

Hi

New photos attached - as Turretslug suspected there are 9 tuned coils, what looks like decoupling coil for each valve and the power connections come through the small shield boxes with more filtering.

A lot of decoupling for something making white noise but again I suppose that you wouldn't want any extraneous RF or the ripple of the motor generator supply getting onto your pure noise - did I just type that?

The hole in the can is huge - my theory out the window - is it the Photomultiplier that is noisy or the light?

I think that the light is connected to the empty screw on the larger box and fed from elsewhere by another wire on the same screw.


Cheers
James

[jamesinnewcastl]

Some more

[turretslug]

Originally Posted by jamesinnewcastl

Hi

I don't quite remember why I bought the thing but now I'm getting interested in powering it up. I'm suspecting that the last valve is just used as an amplifier, the intermediate ones must be shaping the generated spectrum I think.

Using the engines or the rotary generator might sound like white noise but I imagine that a spectrum analyser (or a musician) might readily reveal peaks of noise at frequencies corresponding with the rpm and harmonics of the rotating parts.

So it would be interesting to see the spectrum from the Photomultiplier - are there peaks that would give the game away? What do each of the sucessive stages do to the spectrum? What noise comes from just the valves themselves?

How does changing the lamp brightness affect the system?

The other different aspect to it is that one would be actively seeking huge amounts of noise and studying it rather that doing your best to reduce and eliminate it!

Cheers
James

Not knowing much at all about photomultipliers (lazy me in the era of the instant search....) but I wonder if the bulb serves to bias it into a region of optimum/maximum noise?

I also wondered if the 6V6 was acting as a PA, it just goes to show how high a frequency something marketed as an audio valve will stretch within the limitations of pinch construction, base and connecting wires. The 6V6 must surely be one of the most and most widely produced valves ever, its big brother 6L6/807 also in the running.

Apparently, German radar operators learned to distinguish between actual aircraft and "Window" returns by listening to the detected IF- Window shimmering down through the air producing faint white (-ish) noise, real aircraft giving an audio-range buzz of propellor reflections. I wouldn't be surprised if their later development introduced at least a degeee of electronic processing and discrimination. Russia's four-turboprop Cold War monster, the Tu-95 "Bear", is said to be the least stealthy aircraft ever with its 6m-diameter and counter-rotating propellor arcs giving a very distinctive return signature.

[Nymrod121]

This might be of interest:

http://www.r-type.org/exhib/aab0093.htm

Cheers
Guy

[Radio1950]

[QUOTE]Apparently, German radar operators learned to distinguish between actual aircraft and "Window" returns by listening to the detected IF- Window shimmering down through the air producing faint white (-ish) noise, real aircraft giving an audio-range buzz of propellor reflections. I wouldn't be surprised if their later development introduced at least a degeee of electronic processing and discrimination. Russia's four-turboprop Cold War monster, the Tu-95 "Bear", is said to be the least stealthy ^ ever with its 6m-diameter and counter-rotating propellor arcs giving a very distinctive return signature[/ QUOTE]

Modern search radars can eliminate "Window" by determining the target phase change per pulse repetition, and hence determine the target velocity.
WW2 window generally floated around and descended to earth at no more than about 15 knots, as it was quite light and had resistance to falling due to its surface area, except in storms etc.

Radar Moving Target Indicators reduce or remove targets with a radial velocity of about less than 40 knots by this phase change measurement technique. It is a variable parameter for setup.

In a previous life, I worked on a French ground ATC primary radar designed in the late 1950s.
Its MTI front panel had a switch marked "Window", which increased the target detection minimum speed to 45 knots.

Modern radars still use MTIs in one form or another, and the same basic techniques are used, along with some interference and jamming reduction.

I dont know when basic MTIs became available, but I doubt it before the end of WW2, as I believe suitable delay devices were not available.

But I digress .....

[Nymrod121]

Originally Posted by Nymrod121

This might be of interest:

http://www.r-type.org/exhib/aab0093.htm

Cheers
Guy

Well, well ... what a coincidence.

Gwarpy [Han] & The Philpott [Dave], take a bow

G