[TowerTalk] Topband: T-loaded vertical

[Jim Brown]

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