[NLRS] Looking for 2015 RADCOM article by G8AGN

[Doug Reed]

FYI, I got the article from Chris, N0UK. It is a good article about
automated audio signal-to-noise measurement, but not much detail about the
optical photon tube I was looking for. I forwarded the article to Rob K0XL
and Barry VE4MA who expressed interest.

For those who want to know, the "photon tube" is basically a 2ft or longer
PVC pipe with a LED at one end and an optical receiver at the other end.
The pipe is sealed as well as possible to eliminate external light sources
and has a lining of non-reflective flat-black material inside. The LED can
be driven with any desired test signal and the operating current is set
very low for minimum light output. The optical receiver can be swapped out
to test different designs and to determine relative sensitivity and
signal-to-noise with a "known & repeatable" light source. The intent is to
create something that looks like a point source and eliminate off-axis
reflections with the flat-black lining. One variation was to add a pinhole
shield a couple inches from the LED and another shield some distance away
from the optical detector with a hole diameter near the detector's size.
Another variation created a folded or bent tube to reduce the size, but you
also need to keep the active LED circuit away from the optical detector to
prevent measurement error. A white piece of paper could be a corner
reflector or an attenuator.

73, Doug Reed, N0NAS.
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