Topband: Inverted L Directivity

[Tom Rauch]

> Admitting the inverted L lenght is such to determine enough current
> flow in the horizontal section, how does this field can have the same
> polarization and consistent energy to add "efficiently" with that
> produced in the vertical portion? More important, what's the practical
> impact of this at low elevation angles?

The directivity change is very small in most cases, I am sure it is 
not worth worrying about. The antenna would have to be very poor in 
performance to have any noticeable directivity from the horizontal 
wire, and if it was that bad directivity would be the least of your 
worries! 

With a very poor ground system, a short vertical section, and long 
horizontal section you can get some directivity. In exaggerated cases 
the directivity change can be significant for the reasons Geoff 
already outlined, as much as three dB or so in the extremes of poor 
construction.

If the ground system is reasonable below the antenna (especially 
under the flat top area), the difference in directivity is nearly 
zero dB even with a very short antenna. This is because radiation 
resistance of the flat top area is greatly reduced by opposing 
currents in the low-loss earth (the ground system current opposes any 
radiation effects of the horizontal wire) and radiation from the flat 
top shrinks compared to radiation from the vertical wire (which 
greatly improves with a better ground).

This is all VERY similar to the effects we see in Beverages, where 
the low horizontal wire quits radiating a vertically polarized signal 
when the ground below it is very good...and where the long horizontal 
wire radiates a very good vertically polarized wave when the ground 
is very lossy. It is always interesting how one antenna ties directly 
into another, if you look at it closely enough and get a feel for the 
rules all antennas follow.



    73, Tom W8JI
W8JI@contesting.com