Topband: Close to earth Beverage.

[john battin]

  ----- Original Message ----- 
  From: Tom Rauch<mailto:w8ji at contesting.com> 
  To: john battin<mailto:jbattin at msn.com> ; k1fz<mailto:K1FZ at prexar.com> ; topband at contesting.com<mailto:topband at contesting.com> 
  Sent: Friday, November 19, 2004 6:52 AM
  Subject: Re: Topband: Close to earth Beverage.



  I believe the advantage you are seeing with a low beverage
  is the reduction of pick-up from the feeds. When you think
  about it, the 8 foot vertical at each end of the higher
  beverage is 1/4 wave on 10 meters and this signal dominates
  the signal picked up by the beverage. At lower frequencies
  the effect is lesser, but still effects f/b and f/s
  signals.>>


  I don't see how that could be. The radiation resistance of a
  short vertical with uniform current can be closely
  approximated by multiplying 1580 times height in fractions
  of a wavelength squared.

  A 1/100th wavelength tall vertical with uniform current has
  a radiation resistance of  .15 ohms.

  .15 ohms radiation resistance is a tiny part of 400-600 ohms
  antenna impedance.

  The mechanism going on (I've never seen an improvement here
  laying a Beverage on the ground) is probably the velocity
  factor of the wire has decreased increasing effective
  length, or he has managed to position a null on a dominant
  local noise source.

  What everyone seems to ignore is the entire Beverage from
  end-to-end receives vertically polarized signals. It is very
  unlikely that two mismatched 1/100th WL long verticals would
  ever inject meaningful noise or unwanted signal to a wire 1
  or 2 wl long that the 1-2 wl long wire wouldn't already be
  receiving.

  If the ends had enough sensitivity to change noise level,
  the pattern would be terrible. It all comes back to antenna
  pattern.

  73 Tom

  My measurements (done with care) show quite the opposite... that the reason hardly any of our beverages have the text book pattern is that the pattern is determined by the vertical component of the feeds. To take it further, it became more obvious that if the length of the beverage or the  termination resistance seems critical, it is most likely related to pick-up by the feed wire. For instance, take the limit case of an unterminated beverage that is an odd multiple of 1/4 wave long. The beverage presents a low impedance to the top of the feed wire and increases the current in it .... This could be looked at as a low inverted L..... As we terminate the beverage, the top loading on the feed wire increases to several hundred ohms and is quite independent of frequency. Even at that, the top loading still enhances the signal picked up by the feed. 
  Along with the expected advantages, phasing of  beverages may cancel the feed pick-up, although the effect is likely to be in the form of deep nulls rather than a broader pattern.
  The good news is that great front to back and front to side ratios are not very important unless we are trying to null noise from one  particular direction. Even with only 15 db. rejection, the noise picked up by the front lobe can be expected to dominate the S/N ratio.

  John 
  K9DX
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