[TowerTalk] Topband: T-loaded vertical

[john at kk9a.com]

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