Topband: Extended Inverted "L" Antennas

[Terry Conboy]

At 12:34 PM 2007-09-07, Eddy VE3CUI - VE3XZ wrote:
>Would I benefit from (possibly) optimizing the low angle radiation 
>of my inverted "L" antenna, if it was to be cut exactly 
>1/4-wavelength --- as opposed to it being 170' long (overall), and 
>tuned by way of a variable capacitor at the base...?
>
>Note: the vertical portions of each "L" here is about 70' tall...the 
>horizontal 100' balance is true horizontal, away from the vertical portion.

It would probably be difficult to detect much difference in low angle 
signal between your current configuration and a shorter version.  The 
signal for close-in stations via high angle paths could drop about 6 
dB with a shorter top wire due to reduced horizontally polarized energy.

My EZNEC model of your inverted-L show gains at a 15 degree elevation 
of +0.4 dBi forward and -1.0 dBi reverse (in the direction the open 
end of the top wire points), with -1.6 dBi at the zenith.  It is 
tuned with 321 pF series cap (Q=1500) for resonance with a resulting 
44 ohm feed impedance.  SWR 2:1 bandwidth is about 66 kHz (50 ohm 
reference) or 63 kHz (44 ohm reference).

With a simple 1/4 wl resonant inverted-L, with a 64.3 foot top wire, 
the gains are +0.1 dBi forward and -0.7 reverse, with -7.7 dBi at the 
zenith.  Feed impedance is about 28 ohms.  SWR 2:1 bandwidth 77 kHz 
(referenced to 28 ohms).

For an inverted-L with 70 feet vertical and 60.8 feet horizontal, 
tuned with a 4.73 uH (Q=200) shunt at the feedpoint to match 50 ohms, 
the gains are +0.0 dBi forward and -0.7 dBi reverse, with -8.5 dBi at 
the zenith.  SWR 2:1 bandwidth 77 kHz.

"Average" ground (MININEC), a 5 ohm ground loss resistance, and #14 
bare copper wire are assumed.  (The top wires are only 0.13 wl high, 
so using MININEC ground approximations will introduce some errors, 
which I hope are similarly optimistic for all three versions.)

73, Terry N6RY