Tuning: If the impedance is high at 0.67, the ratio is
high. If the impedance is low, the ratio is low.
Adjust accordingly.
Simple!
Advice: Use a choke
balun at the feedpoint.
The farther off-center, the greater the current imbalance between
arms, the more "common mode" current on a coaxial feed
line. Common mode current problems cause change in tuning if you
touch the cable, sometimes tingling or "bites" to face
and lips when transmitting. RF radiating from the coax can couple
with nearby electronics or AC power lines to affect TV, radios,
clocks as well as control devices like alarms, thermostats and
monitors.
Equal, Wide and Tall L-Antennas
The Equal L-Antenna
With "equal" arms,
the horizontal arm is slightly longer at resonance due to the
effect of ground. The arm-to-arm ratio therefore is not exactly
0.5. Impedance is around 40 ohms. SWR can be as low as 1.2-1.3:1.
RF polarization is equally horizontal and vertical.
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The principles of OCF apply
very well for L-antennas.
The Wide or Vertical characteristic
can emphasized by making OCF ratios smaller
or larger.
The Wide L-Antenna
As the off-center feedpoint ratio goes less than 0.5, the L-antenna
becomes wider and the impedance rises towards 50 Ohms. The antenna
hears and talks more like a horizontal dipole as the SWR approaches
1:1.
The Tall L-Antenna
As the off-center feedpoint ratio goes greater than 0.5, the L-Antenna
becomes taller and the impedance rises towards 50 Oms. It hears
and talks more like a vertical antenna as the SWR approaches
1:1.
Based on modeling software,
Figure
3 below summarizes the results as the feedpoint
of an L-Antenna is changed by 0.05 (5%) increments
around the center 0.50 ratio.
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