>"I wonder when the theory was developed, as opposed to empirical
observations. I'm sure that empirical observations (1910, 1920) showed
the bandwidth increase (as all of you running a flattop T antenna from
your Alexanderson alternator on Straight Key Night did)."
The bandwidth benefit from caged construction was secondary to the primary
reason. When spark was king, wattmeters and SWR indictors were unheard of.
Nor were they needed because transmission lines weren't generally used nor
were they understood. The feed for the antenna was attached to a tuned
network immediately following the spark contact device.
But the ham of the day did have a powerful tool that lasted until these
effects were better understood. That tool was the RF ammeter. Many photos
of the era show large hot-wire ammeters placed at eye level at the operating
position to watch current. More current meant better radiation.
The cage came about because of empirical observations. Someone along the
way added more wires to the tops of T and Inv. L antennas -- and saw that as
more broadly-spaced conductors were added, the ammeter would show a rise in
current, mostly due to an increase in C between the flat-top and ground.
Same with the vertical portion. More wires produced incrementally more
current after tuning was complete. More conductors were added until some
point of diminishing return on the ammeter was reached.
Paul, W9AC
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