[time-nuts] Re. DIY atomic "resonator"

[jimlux]

On 4/12/17 10:28 AM, Dave B via time-nuts wrote:
> On 12/04/17 17:00, jimlux wrote:
>
>> Subject: Re: [time-nuts] Re. DIY atomic "resonator"
>> Message-ID: <b3926cda-b4ff-2508-1be6-57c2fecf0ad6 at earthlink.net>
>> Content-Type: text/plain; charset=utf-8; format=flowed
>>
>> On 4/11/17 11:09 AM, Mark Sims wrote:
>>> Apparently fluorescent tubes continuously emit a lot of other microwave signals.  I once built a  homodyne doppler "speed" radar kit (used a coffee can for the antenna).  The way you calibrated it was to point it at a florescent tube and and adjust the reading to a specific value.
>>>
>>> --
>> That's not because the tube is emitting..  It's a target reflector
>> turning on and off at twice line frequency.
>> In most homodyne radars, you filter out the DC (the reflections from
>> stuff that's not moving), so anything that pulses on and off creates
>> nice output.
>
> This patent would seem to confirm that discharge tubes do generate
> microwave noise, and with a DC powered tube too.  (Would different
> gasses produce different microwave spectra?)
>
> http://www.google.co.uk/patents/US2942204

They generate broadband noise - they're used as high power noise sources 
for calibration.

Bracewell (I believe) used fluorescent tubes (driven by a reasonably 
stable source) as a calibration and timing reference at a astronomy array -
http://articles.adsabs.harvard.edu//full/2005JAHH....8...75B/0000077.000.html

R. Bracewell and G. Swarup, "The Stanford microwave spectroheliograph 
antenna, a microsteradian pencil beam interferometer," in IRE 
Transactions on Antennas and Propagation, vol. 9, no. 1, pp. 22-30, 
January 1961.
doi: 10.1109/TAP.1961.1144935

I particularly like the inclusion of "milling machines" in the keyword list
keywords: {Microwave interferometry;Planar arrays;Radio 
telescopes;Reflector antennas, arrays;Solar radiation;Brightness 
temperature;Corona;Frequency;Microwave antennas;Milling 
machines;Monitoring;Moon;Phase modulation;Sun;TV}

And fluorescent tubes have been used as a broad band source in a 
classroom demo
https://www.researchgate.net/publication/276280926_Demonstrating_the_Principles_of_Aperture_Synthesis_with_the_Very_Small_Radio_Telescope



I would suspect that the radiated field has some spectral bumps in it, 
probably related the physical dimensions.  I doubt it would have any 
discrete lines (although maybe it does..)





>
> I recall seeing such things sold surplus back in the early 70's, pity I
> didn't know what they were back then.

There are also gas tubes in waveguides used as Radar T/R switches. when 
the main bang happens, the tube ionizes shorting the waveguide and 
protecting the receiver downstream.


>
> There is also mention of huge mismatches in the guide when the tube was
> not powered, that would support the homodyne speed calibration comment
> by pointing it at a working (AC powered) florescent tube.  As Jim said,
> a 100 (or 120) Hz "modulated reflector."

Yep.  There's been a fair amount of work over the past decades on using 
modulated reflectors for measuring antenna patterns (e.g. on phased 
arrays).  You can have a diode/dipole suspended by resistive leads (with 
an impedance of 377 ohms/square that are invisible) and turn it on and off.

Bolomey (I think) had an array of modulated reflectors, so you could 
measure multiple points in the near field at the same time, and only 
need to scan in one dimension. I can't remember if the reflectors were 
modulated at different rates or with PN codes - either would work to 
separate the responses.


>
> Regards to All.
>
> Dave G0WBX (or G8KBV both still valid.)
>
> ~~~
>
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