[TowerTalk] 160 vertical question (Top Hat)

Paul Christensen w9ac at arrl.net
Sun Feb 4 11:15:09 EST 2018


>"Another technique that is often used for matching is to make the wires much longer than resonant (that is, resonant around the top of the BC band), so that the feed point Z is 50 +jX ohms (that is, inductive) on 160M, and tune the capacitance out by adding a capacitor of equal value in series.    NEC can easily model this."

When over-resonating with a long T-top on 160m, the maximum current point moves away from the ground and up toward the center of the vertical radiator -- another benefit.   Unless there's a deliberate need to resonate on the operating frequency (e.g., with some phased arrays), then over-resonating a single T-top vertical radiator should be the objective, subject to the availability of long, tall supports on either side of the vertical section.

As the resistive component of base Z increases, matching network losses are minimized, especially if the T is tuned to 50+jX as K9YC cites.  Then, network loss is a function of C, the only matching component needed.  Make it a vacuum type, and matching loss is very low.

When the T-top is resonated into the BC band, be prepared for some very high base voltages if it's located anywhere near a BC station at the top of the BC band.   The better CIA and VNA devices can be operated in these conditions and still provide valid measurements.

Paul, W9AC 




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