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
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