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The Three L-Antennas

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ANTENTOP- 01- 2017, # 021

The Three L-Antennas

 

 

Figure 2

Generalized Impedance Graph for Off Center Dipole Antenna

 

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.

 

 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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February 24, 2018 22:20

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