[BC] Commercial Station Feeding A Translator

[Cowboy]

On Friday 19 October 2007 12:02 am, RADIO DOCTOR wrote:
 >  On Thu, 18 Oct 2007, Al Wolfe wrote:
 >
 >  > I also contend that it provides a degree of gain and space diversity
 >  > just by having more aluminum in the air in a different space.
 >
 >  Maybe some antenna experts and chime in on this.  Richard Fry, are you
 >  on this list?  My understanding has always been that combined antennas
 >  function as one antenna.  Two stacked and combined yagis would extract
 >  energy from the same frontal area as would a larger one in the middle
 >  with twice the gain.  There would be no space diversity effect.
 >  Indeed, a lower gain antenna might be better than a high gain one if a
 >  broad vertical lobe were desired and gain was not paramount.

  The gain of an antenna array, whether it's a single element or many, is
  proportional to the area occupied by that array.

  Whether they are "combined" by feedline, phasor, or space, is irrelevant.
  How those things are adjusted relative to each other is not.

  We're all somewhat familiar with the concept of adding bays, 
stacking vertically,
  an FM broadcast array to increase gain. The antenna occupies more space
  broadside to the passing wave front, so can extract ( or feed ) a larger
  "surface area" of that wave front.
  Likewise, a long wire ( a true long wire, not the random wires 
commonly referred
  to on these lists ) is an end fire "array" with close to zero area 
exposed to the
  "surface area" of the wave front, but extracts ( or feeds ) that 
passing front over
  time, as the wave traverses the wire linearly. ( disregarding the 
tilt of the passing
  wave for the moment to simplify the discussion )

 >   Two stacked and combined yagis would extract
 >  energy from the same frontal area as would a larger one in the middle
 >  with twice the gain.

  Not exactly, but close. The result is pretty much the same.

  The two stacked will be impacted by twice the "surface ( frontal ) 
area" of the passing
  wave front, but the single higher gain array while impacting less 
initial "surface
  area" will of necessity be longer, and so remain in contact with 
the passing wave
  over a greater distance ( time ) and be able to extract energy over 
greater time.
  That energy may add ( forward gain ) or subtract ( back/side 
rejection ) over time,
  depending on the phasing of the element(s) ( time adjustment ) as 
to exactly how
  the interference pattern of the wave(s) combine at the feed point.

  Space diversity has the additional intent of possibly extracting 
energy from a
  wave, or portion of, that may appear at one, and not at all at the other.

  Remember, an antenna is a four dimensional array.
  It has length, width, hight, and time, ( usually called phase ) and operates
  on a passing, moving wave front, not a static stationary charge.

  Whether elements are considered as parts of a single larger array, or as
  parts of separate "combined" antennae is merely a matter of what's easiest
  to analyze in that particular situation. The wave energy doesn't care.

  Note too, that the rules are the same, whether the array is for 2 
Hz, or 200 GHz.
  Only the physical dimensions of the elements and the real estate occupied are
  different, for our purposes.
  It's true that the 200 GHz array will extract nothing like the 
amount of energy
  from the wave that the 2 Hz array will, since the power density per 
cubic foot
  of space can be the same, but the way the antenna works changes not.

  If we assume ( for the moment ) that the power density of the passing energy
  is static, and evenly distributed per cubic foot, it becomes easy to see that
  the larger array in *any* dimension has a greater chance of extracting more
  energy when properly adjusted for time.
  Resonance is mostly irrelevant, except for ease of matching, and transferring
  the energy extracted to something useful, like a receiver.

-- 
Cowboy