Topband: [TowerTalk] 160 vertical question (Top Hat)

[Charles Moizeau]

Yes, the maximum quantity of total current in the vertical section of an inverted T, or inverted L antenna is achieved when the antenna's current loop is placed at the center of the vertical leg.  As in all antennas this current loop is located a quarter wavelength back from an open end.


However, raising the point of maximum current to a location above ground level, where it occurs in an antenna whose overall length up and out is 0.25 wavelength, means that the near-field ground reflection zone moves further away from the base of the vertical leg.  And this has implications for a field of in- (but preferably on-) ground radials.


I like to think that the function such radials is to collect the current radiated by the vertical that splashes on the ground and return it to the feedpoint for recycling.  But with the point of maximum current in the vertical raised above ground level, the splashed radiation will strike the ground further from the feedpoint.  This suggests that longer radials will be needed.  Since there will be less radiation splashing in close to the feedpoint, it would be economical to not add more and longer radials radiating out from the feedpoint, but rather connect crow-foot extensions of additional wire at intermediate points on the existing radials.


73,


Charles, W2SH


________________________________
From: Topband <topband-bounces at contesting.com> on behalf of Paul Christensen <w9ac at arrl.net>
Sent: Sunday, February 4, 2018 7:35 AM
To: topband at contesting.com
Subject: Re: Topband: [TowerTalk] 160 vertical question (Top Hat)

>"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 feedpoint 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, 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.

Paul, W9AC


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