Good info Jim! I have wondered about this current moving technique. It
has been a while since I had a 160m vertical, I used to cut the wire a
little short and use a hairpin coil to match 50 ohms.
John KK9A
Jim Brown K9YC Wrote:
I agree with all of this advice, and it's how I matched my Tee. I did
not use the ladder snaps, but I did make the vertical section two wires
spaced with PVC conduit to increase the SWR bandwidth, and I did model
it in NEC.
With respect to current distribution with the lowered resonance -- N6BV
modeled it in his NEC-4 licensed software, and found no increase in the
far field compared to a resonant antenna.
73, Jim K9YC
On 2/18/2020 9:29 AM, Paul Christensen wrote:
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
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