i4jmy at uugate.wa7slg.ampr.org i4jmy at uugate.wa7slg.ampr.org
Sun Sep 22 15:08:36 EDT 1996

    There is a lot of confusion in the matter of vertical antennas, ground
  planes, elevated radials and counterpoises.

  This will remain until the concept of NEAR and FAR FIELD won't be taken 
  in consideration and understood as it would deserve.

  In a classical Ground plane antenna, disregarding the other parameters, 
  important but not related to the actual topic, unless the radial lenght 
  is ENORMOUS, what is done varying their number, size and position, just
  under the antenna, is not worth to affect substantially the type of Far
  Field produced.

  This is the reason why the best set of radials will bring to have the best
  possible antenna efficiency, but without changing SOIL and AREA type, the
  inherent performance won't change ever.

  The most efficient Ground Plane will be a poor DX radiator when on a desert
  area, a "poor" Ground Plane will be an outstanding antenna over sea water.

  A real Ground plane antenna then is that placed at ground level with ground
  electrical connection to the soil, where the ground works as the "missing" 
  half of itself and for the current return path to the generator.
  Return current that's increased with the aid of a number of radials.

  The antenna radials will work for ONLY for increasing efficiency and NOT
  for other purposes.

  There is infact no need or electrical reason to make them quarter wave long.

  If the ground were perfecly conducting the radiation resistance,in case
  of a quarter wave radiator, would be exactly the half of a dipole and the
  maximum radiation in the vertical plane would occur at an hypothetical 0 
  (zero) angle.

  This situation is nearly reached (Brewster angle is always present) over 
  sea water.

  A Ground Plane elevated from ground is then a different antenna than the
  ground plane in the strict sense of the word. 

  The radials now are an electrical COUNTERPOISE, the term ground plane is
  then largely improper.
  A quarter wave elements antenna of this type, with its center just more
  than 1/4 lambda from ground, performs like an half wave vertical dipole
  with the center at the same height.

  One million of radials in such an antenna will'not make it more effective.

  They can be only 2 (but opposite to cancel their radiation) or they can be
  collapsed straight down from the feed point toward the ground, in the way 
  that the antenna is transformed to a vertical dipole.

  Of course if fed point height is much less than 1/4 wave, there is no space 
  for a full size vertical quarter wave element and the need of a counterpoise
  is the obvious consequence.
  Counterpoise has to be bigger (in number of horizontal radials) as much as 
  the ground level is approached, reaching the classical requirements at zero

  It is not worth for HF communications to elevate a vertical antenna a lot
  over the ground. Unless the antenna will be several wavelenght from the
  ground, there will be two lobes, the first at low angles but small(er) and
  the second, reinforced one, but too high for effective DXing.

  There are existing a number of PC programs for Antenna Analysis, if well
  used they can show what really happens even to people who is not involved
  in the antenna & communication field and the complex math of the antenna

  What's commonly assumed is often not true, or better, what's read on books
  or spoken about is often misunderstood or applied where it shouldn't be.


  E-mail: i4jmy uugate.ampr.org  

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