## Just thought of something else. Putting a coil inside of an aluminum
cylinder will decrease the inductance of any coil.
I'm not sure it actually changes the inductance.. The parasitic C in
parallel can change the "end to end" reactance so that it looks like L
is lower.
Coil Q is reduced in the cylinder, especially when the distance from the
coil to the cylinder is uniform in diameter and length. I suspect a
moderate to large portion of Mosley trap loss (and overheating on Tx), is
due to Q reduction.
In the mid '70s, I melted the driven element coils of my TA-33 Jr. after I
had just finished building a Heath SB-230 amp as a high school project. The
SB-230 output power was probably 400W. If the antenna instantly melted at
400W, then at 100W, the coils were marginal. At Mosley's suggestion, the
fix required replacement of the entire driven element with one from the
MP-33 model. At least that allowed me to use the amp to its full potential
for the next five years. That was in 1980. I have not owned a tower with
directional antennas since then. After waiting 35 years, that's about to
change in the next two months. My QRZ.com page is updated to show current
progress.
The supreme irony is that I replaced a Gotham 15m 3L Yagi with the TA-33 Jr.
Despite any pattern imbalance from the gamma feed on a short boom, no doubt
the Gotham was a much better performer on 15m since it was a full-size Yagi
with no loading coils. I could not A/B the antennas; the Gotham was
replaced not due to any performance issues. Rather, it self-destructed over
a two year period from wind vibration. I could have dampened the Gotham
elements with rope inserted into the AL tubing, but at 15 years old, I was
too young to reach that conclusion. Not sure if Gotham used an inferior
grade of AL, or perhaps the destruction would have occurred even with better
AL.
Paul, W9AC
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