[TowerTalk] 3/8 Wave Vertical Questions

[Terry Conboy]

Larry,

The high current point will be very close to 1/4 wl from the open end of 
the inverted L, for any configuration whose length exceeds 1/4 wl.

However, although increasing the length of an inverted-L to 3/8 wl will 
elevate the current maxima, raise the feedpoint Z, and increase the 
overall efficiency, the improvement in signal is usually horizontally 
polarized and at high takeoff angles (if the added length beyond 1/4 wl 
is horizontal).  This is fine for close in contacts, but normally 
doesn't help for low angle DX.

The antennas shown in ON4UN, 1st ed, page II-21, figure 2.25 are for 
definition of radiation resistance (which is always referred to the 
current maxima).   (In ON4UN, 3rd ed, these are on page 9-8, figure 
9-10.)  For all four antennas shown, the feedpoint is between ground and 
the bottom of the wire.  (It's not clear to me what the capacitor shown 
is for.)  My EZNEC model of a straight, 3/8 wl wire, bare #14 copper 
(98.357 ft at 3.75 MHz), has a feed Z of 237 + j 651 ohms (inductive).  
The current max is on segment 36 of 100, or about 0.24 wl from the top.  
Of course, an inverted L will have a different feed Z, depending on the 
ratio of vertical to horizontal wire, etc.

73, Terry N6RY

On 2009-11-17 4:25 AM, K3VX wrote:
> Antenna gurus:
>
> I'm feeding a 3/8 wave (for 80M) inverted L against ground using an AH4 tuner.
> The idea was supposed to raise the point of maximum current above the base of the L.
> But on page II-21 of Low Band DXing the 3/8 wave vertical is fed 1/8 wave above the low end of the antenna.
> It also seems there's no ground connection - coupling indicated by a capacitor symbol. 
> ON4UN shows the impedance as 300 ohms - is this balanced or unbalanced in his configuration?
>
> So am I fooling myself thinking that the high current point on my inverted L has moved up the antenna?
>
> TIA and 73
>
> Larry K3VX
>
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