[Antennas] Re: full wave INV-L for 160m

[Terry Conboy]

At 10:33 PM 2008-03-04, Igor Sokolov wrote:
>I would make it a bit longer then 3 quarters and then bring to 
>resonance with the serial capacitor.  That will make the impedance 
>higher and improve efficiency of the antenna somewhat. MMANA is a 
>good tool for modeling and understanding what is going on.
>
>73, Igor UA9CDC
>
>    Roy, K6XK, is right. The only function of the horizontal wire is 
> to bring the antenna to resonance. The antenna is basically a short 
> vertical radiator loaded by the horizontal wire, and the minimum 
> total wire length needed to bring it to resonance is a quarter 
> wavelength. The next length that works is three-quarter wavelength, 
> not full wavelength.
>
>    Keep it simple!
>    73, Brian KF2YN

The 3/4 wl version impedance is pretty close to 50 ohms at resonance, 
so you shouldn't need to lengthen it to get the impedance 
up.  However, the presence of an extra horizontal half-wave of wire 
(beyond the first 1/4 wl) will radiate a very significant 
horizontally polarized signal, mostly straight up, since the current 
maxima in this section is essentially equal to the current in the 
vertical section at the feedpoint.  In addition, the low angle 
pattern of the 3/4 wl version isn't very omnidirectional, with a 
broad null off the open end of the top wire which is down about 6-9 
dB from the broadside signal.

Keep it simple by staying close to 1/4 wl, unless you want to favor 
NVIS operation.  Of course, if you just want NVIS, use a dipole and 
save the pain of installing the ground system.

73, Terry N6RY