Friday, February 12, 2010

Inverted V Dipole Performance

This site makes available a program you can download to calculate antenna efficiency based on parameters you input:

Inverted-V antennas are popular because, relative to horizontal dipoles, erection costs are lower, and/or because the size of the site is smaller than the overall length of the full size horizontal dipoles they substitute. Efficiency is less than a horizontal dipole of similar height, but the radiation pattern is more omni-directional which may be considered to be an advantage.

The inverted-V is defined by overall antenna length, straight-line distance between wire ends and height of wire ends above ground. Dimensions calculated by the program are the height above ground of the triangle's apex and the angle it subtends. When the distance between wire ends is set equal to overall wire length a low, but adjustable, horizontal 1/2-wave dipole results.

The resonant frequency of the antenna is continuously re-calculated from its dimensions. All computed data applies at the current resonant frequency. A within-amateur-band change in transmit frequency will not result in a significant change in performance provided the tuner is readjusted to maintain the correct load on the transmitter. Tuner L and C settings are not included in this program. To do so would overcomplicate and reduce its usefulness.

Miscellaneous One-Line Notes

  • When the apex angle is 180 degrees the antenna reduces to a simple low dipole.
  • To obtain 180 degrees set distance between wire-ends equal to dipole length.
  • For small apex angles, bring the ends of the antenna nearer together.
  • Radiation resistance and efficiency fall rapidly at small apex angles.
  • Radiation resistance and efficiency fall as ends of antenna near the ground.
  • Highest efficiency occurs at heights above ground greater than 1/4 wavelength.
  • For small transmitting sites suitable apex angles are from 60 to 120 degrees.
  • Height of the whole triangular model is set by varying height of the wire ends.
  • At low apex height radiating efficiency is poor due to ground induced loss.
  • NVIS propagation may be possible at HF even at very low average dipole heights.
  • At extremely low heights resonant frequency may fall to crudely half normal value.
  • To maximize efficiency raise wire ends as high as possible above ground level.
  • Antenna resonant frequency is defined by feedpoint reactance = zero ohms.
  • Resonant frequency falls fast at low heights due to capacitance of ends to ground.
  • Soil resistivity is in units of ohm-metres = 1/(Soil conductivity).
  • Soil resistivity is seldom accurately known. A guess within +/-50% is okay.
  • Greater than normal uncertainty in estimating performance is unavoidable.
  • A 100 watt tuner will not normally increase overall loss by more than 0.25 dB.
  • The program needs feedline wire gauge to compute line loss and input impedance.
  • Type of line is auto-selected: If Zo is less than 100 then greater than 100 then balanced.
  • Line attenuation per 30 metres is same as dB per 100 feet at same frequency.
Go to this site to download the program:

Run this Program from the Web or Download and Run it from Your Computer
This program is self-contained and ready to use. It does not require installation. Click this link Inv_Vee then click Open to run from the web or Save to save the program to your hard drive. If you save it to your hard drive, double-click the file name from Windows Explorer (Right-click Start then left-click Explore to start Windows Explorer) and it will run.

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