Chuck and others,
No doubt you are right about the small light bulb been used: Here is my memory from EE school:
In ancient times, the 1960’s (if I remember right) a bolometer system was implemented with a common low current fuse. It was arranged in a bridge circuit of sorts: It’s resistance was adjusted with DC current by a “zero setting” arrangement to get zero indication on a meter. Then when RF power was applied, the bridge/feedback circuit automatically rebalanced the thing back to the resistance set earlier - the reduction in (dc?) power drove the indicator to show the RF power that was being applied.
One advantage of this scheme is that the unavoidable non-linearities in the bolometer fuse did not affect the measurement, since the bolometer was returned to its initial resistance/dissipation condition. This basic scheme of “first set null balance, and then return the bridge to null when the unknown is introduced” was used throughout General Radio bridges of all sorts. I would expect the GR instruments to include a bloomer-based power indicator, though I am not familiar with it/them.
One (or more) of the MIT Radiation Lab series of books covers this topic, and quite thoroughly. I have a small group of those books “in storage” here and can look among them before the temperature goes too far below freezing. But see below for an online example of Volume 11.
Likely one of these covers the topic:
8. Principles of microwave circuits - Montgomery, C. G.; Purcell, E. M. and Dicke, R. H. (1948)
11. Technique of microwave measurements - Montgomery, C. G. (1947)
(It appears that my list of the RadLab books came from:
This link no longer works but:
Does and I did get volume 11 from there.
At MIT, I find a page about the series, but not the texts of the volumes:
)
In the second above, page 81 mention is made of “bolometers”, being implemented with thermistors and bolometer, these having negative and positive temperature coefficients. It goes on to describe bridge circuits and the characteristics of the measurement elements.
Roy
On Nov 20, 2018, at 10:20 AM, Chuck Harris <cfharris@...> wrote:
A bolometer is not a thermocouple sensor. What it is is a
small lightbulb filament in a vacuum that is exposed to the
RF power. ...
If I recall correctly, Bolometers were used in a bridge circuit,
where you applied AC power, and the bridge measured the resistance
of the filament. Next, you applied DC power, increasing the
level until the resistance measured by the bridge matched that of
the unknown RF power source. At that point, the DC power was equal
to the unknown RF power source's power.
-Chuck Harris
Roy Morgan
K1LKY since 1958
k1lky68@...
