Topband: T-loaded vertical

[Richard (Rick) Karlquist]

On 2/18/2020 8:45 AM, N4ZR wrote:
> 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.
> 

At least 3 different answers:

1.  If you want the antenna to be resonant on 160 as is (without a
loading coil, etc), then as a rule of thumb, the length of the
vertical section plus the length of ONE of the legs of the top
loading should be about 1/4 wavelength.

2.  If you want to move the current maximum above ground,
you can make the top wire even longer than in #1 above.  You
will need a series capacitor to tune it to 160.  The limit
to this is when the vertical section plus one leg
equals a 1/2 wavelength.  This configuration is difficult
to match, but gets out well if you go to the trouble.

3.  If you are limited in the size of the top loading, you
can install just enough to get near uniform current in the
vertical section.  Then use a loading coil to tune it to
160 meters.  W8JI advocated this.  Perhaps his posts are
in the top band archive.

You really need to model your system and consider all the
tradeoffs including your grounding/radial system.

Rick N6RK