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Hi Doc....
You are on the right track with your thinking WRT the metal cylinder. It is a short section of coaxial transmission line with a Zo of around 160 Ohms.

Using EZNEC, I've modeled this device as an assymetrical sleeve dipole (source at the center) AND as an end fed device. The elevation patterns are essentially the same. The modeled bandwidths are also the same -- ~ 5%.

But my *measured* bandwidth is WAY bigger.

With a sleeve dipole, the bottom of the metal cylinder is "hot" with RF. (It's at the High impedance end of a dipole secton.) And that implies *large* amounts of Common Mode current induced onto the feedline coax shield.

With my design, the impedance where the coax shield attaches to the bottom is at a relatively low impedance location. There is *some* CM, but not a lot.

The biggest "mystery" for me is the unexpectedly wide bandwidth. I'm convinced the Cylindrical element is acting as some kind of a matching section, but I haven't yet figured out how it works!

Anyone with ideas, PLEASE "jump in."

Bill

<blockquote data-quote="kt4ye" data-source="post: 400691" data-attributes="member: 27040">Hi Doc....You are on the right track with your thinking WRT the metal cylinder. It is a short section of coaxial transmission line with a Zo of around 160 Ohms. Using EZNEC, I&#039;ve modeled this device as an assymetrical sleeve dipole (source at the center) AND as an end fed device. The elevation patterns are essentially the same. The modeled bandwidths are also the same -- ~ 5%.But my *measured* bandwidth is WAY bigger.With a sleeve dipole, the bottom of the metal cylinder is &quot;hot&quot; with RF. (It&#039;s at the High impedance end of a dipole secton.) And that implies *large* amounts of Common Mode current induced onto the feedline coax shield. With my design, the impedance where the coax shield attaches to the bottom is at a relatively low impedance location. There is *some* CM, but not a lot.The biggest &quot;mystery&quot; for me is the unexpectedly wide bandwidth. I&#039;m convinced the Cylindrical element is acting as some kind of a matching section, but I haven&#039;t yet figured out how it works!Anyone with ideas, PLEASE &quot;jump in.&quot;Bill</blockquote>

[QUOTE="kt4ye, post: 400691, member: 27040"] Hi Doc.... You are on the right track with your thinking WRT the metal cylinder. It is a short section of coaxial transmission line with a Zo of around 160 Ohms. Using EZNEC, I've modeled this device as an assymetrical sleeve dipole (source at the center) AND as an end fed device. The elevation patterns are essentially the same. The modeled bandwidths are also the same -- ~ 5%. But my *measured* bandwidth is WAY bigger. With a sleeve dipole, the bottom of the metal cylinder is "hot" with RF. (It's at the High impedance end of a dipole secton.) And that implies *large* amounts of Common Mode current induced onto the feedline coax shield. With my design, the impedance where the coax shield attaches to the bottom is at a relatively low impedance location. There is *some* CM, but not a lot. The biggest "mystery" for me is the unexpectedly wide bandwidth. I'm convinced the Cylindrical element is acting as some kind of a matching section, but I haven't yet figured out how it works! Anyone with ideas, PLEASE "jump in." Bill [/QUOTE]

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