Topband: Cone of silence

[john battin]

  ----- Original Message ----- 
  From: Tom Rauch<mailto:w8ji at contesting.com> 
  To: john battin<mailto:jbattin at msn.com> ; Topband at contesting.com<mailto:Topband at contesting.com> 
  Sent: Thursday, May 12, 2005 9:54 PM
  Subject: Re: Topband: Cone of silence


    I believe a good start is to leave the feed system with
  the matching transformer in place and just cut the beverage
  off, leaving only the vertical piece.>>

  When the antenna wire is just hanging there it is so
  reactive it could have a very wide range of matching loss
  due to the complex interaction of the reactance and
  transfomer reactance. When the T is added with the resistor,
  we now have added 450 ohms of intentional loss resistance at
  the same time we eliminate most reactance. I wouldn't even
  try to guess what happens to mismatch loss without looking
  at the impedance back into the transformer (the transfomer
  might not be bilaterial, especially when misterminated
  severely) and receiver and knowing the antenna impedance. We
  can be pretty sure the radiation resistance of the vertical
  wire increased by a factor of three or four due to current
  distribution changing from triangular to nearly uniform, but
  then we added losses with the resistor.



  THESE issues are exactly why I took two months in the summer of 2003 to make careful measurements before re-doing my beverages. The impedance into the transformer was exactly 400 ohms ... a W8JI transformer with a pad between it and the receiver to stabilize the input impedance of the receiver. The point here is that if the signal goes down by 20 db. when the beverage wire is disconnected, then that level represents the minimum level of signal that will be picked up by the vertical feed. So if someone is trying to null a signal or noise by more than 20 db. this is a major factor.

   As we add wires to the top of it, we increase the current in the vertical wire and increase its' receiving capability. I tried a 1/4 wave single wire running toward and away from the source and the signal went up 15 db. (looks like short unterminated beverage). When I ran the wire at right angles to the source, the signal went  down about 11db. to where the level was about 4 db higher than the vertical alone. The signal was not stable, and varied a lot as I walked around. When I added the other 1/4 wire to make a T, the signal stabilized at about 3db. over the vertical feed or about 17 db down from the beverage starting point. At this point I was convinced the signal was not being picked up by the T. These measurements used the 400 ohm resistor between the vertical and the T to simulate the impedance of the beverage when it is properly terminated. Without this resistor the low impedance of the T would cause more top loading of the feed wire.

  But even if you disregard these T measurements, the fact that the vertical feed is only 20 db down even without top loading makes it a major contributor the the pattern of a beverage and getting back to the original question of COS, a major factor in that as well. 

  OH, by the way, after all these measurements I converted some of my beverages to elevated T terminations. The F/B ratios were much better, but they did not receive one tiny bit better. In my location, the noise is usually omni-directional, and hence the width of the front lobe dominates  performance.

  John K9DX