Pete,
You'll pretty much be required to model it to get any semblance of accuracy.
The T will produce a more symmetrical azimuth pattern than an inverted L and
will also do a better job of canceling radiation from the horizonal wire
section.
You can experiment with different T-Top length values in 4Nec2 or EZNec. When
space is available, my preference is to over-resonate the T down to about 1400
kHz. Doing so allows the maximum current point to shift up off the ground to
the middle of the T's vertical radiator, makes the vertical less dependent on
the extent of the radial field, and raises the R portion of the feed-point Z to
50 ohms such that a simple capacitor can be used to cancel the inductive
reactance.
The 160m T that N4CC and I are currently using has a 100 ft. vertical section
composed of "LadderSnaps." The LadderSnap wires are shorted together at the
top and at the feed. The symmetrical T section is supported by 140 ft and 100
ft towers separated about 180 ft apart. The overall horizontal T top length is
80 ft, but I want to extend it to 100 ft to produce a nearly linear current
distribution on the vertical section. This also increases radiation resistance.
Of course, you can always find a T length value that results in resonance at
some point on the 160m band. Many folks do that to ensure they have a resonant
antenna, either not having space for a long T top or they simply don't
understand the over-resonating benefits.
Paul, W9AC
-----Original Message-----
From: Topband <topband-bounces+w9ac=arrl.net@contesting.com> On Behalf Of N4ZR
Sent: Tuesday, February 18, 2020 11:45 AM
To: topband reflector <Topband@contesting.com>
Subject: Topband: T-loaded vertical
I keep seeing references to people using symmetrical wire top loading instead
of the single wire of an inverted L. My question is how to calculate the
appropriate length for the two top-loading wires.
--
73, Pete N4ZR
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