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"
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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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