Weeel - a light bulb current indicator is a pretty fair indicator of the
presence of RF.
And it's been used as one for many years. But tests back in the 1930's
comparing brightnesses to determine the optimum ratio between brightness and
lamp
life found the human eye cannot reliably detect brightness changes of less than
15% or so.
Personally, my peepers aren't calibrated, and I do not do well resetting a
100 watt bulb to an exact brightness - using a Variac with the light bounced
off
a white card and measured with my Gossen spotmeter. Within an F stop, plus or
minus 100 percent, easy. A half F stop takes some care, much closer than
that is well nigh impossible. It's a good thing most films have a great deal of
exposure latitude. And I seriously doubt that a one bulb indicator would do
for an indicator of feedline balance. Or unbalance as the case may be.
Of course, the Gossen doesn't work very well when you want to to compare the
intensity of two different lamps, since the photocell blocks at a fairly low
level. I have a new box of 757's on hand and I wanted to compare the
brightness of several lamps. So I turned to my Cooke extinction photometer,
which uses
two fairly dark wedges of optical glass. Just roll the wedges over each
other, and read the intensity of the light from the scale at the point you can
no
longer see the light. Hence, extinction photometer. Probably not calibrated to
NBS standards - but I get extinction at the same place every time.
I get quite a bit of difference in lamp intensity between two lamps out of
the same box connected to the same AC supply voltage at the same time. One
lamp
of a pair always disappeared before the other, and the one '"extinguished"
first swapped when I changed bulbs in holders. So I would not say that a single
lamp swapped between the two wires of a transmission line would be a good way
to prove or disprove feedline balance.
If you insist on disproving the old adage that feedline voltages and currents
are balanced to within a small fraction of a wavelength of the load, and if
you insist on using lamp indicators, it would probably be best to start with a
matched pair of bulbs in a paired arrangement and use a photometer to make the
measurements.
73 Pete Allen AC5E
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