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.
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
The principles of OCF apply
very well for L-antennas.
The Wide or Vertical characteristic
can emphasized by making OCF ratios smaller
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
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
Based on modeling software,
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.