Topband: shunt feed help needed

[Joe Subich, K4IK]

> From: k6se@juno.com
> Sent: Monday, October 08, 2001 11:41 AM
> To: i4jmy@iol.it; topband@contesting.com
> Subject: Re: Topband: shunt feed help needed
> 
> To encounter capacitive reactance when shunt feeding a tower is only
> possible when that structure is greater than 1/2-wave electrically.  If
> it is, there will be two 50-ohm tap points, one capcitively reactive and
> the other inductively reactive.
> 

Inductive reactance when shunt feeding a tower is a major "red flag!"  
As Earl correctly states, it means the tower is greater than 180 degrees  
electrically.  Tall towers have a significant high angle component as 
a second lobe begins to form when the tower height extends beyond 1/2 
wave.  Once the tower is greater than about 230 degrees electrically 
the second, higher lobe may actually be stronger than the first lobe.  

If one has a tall tower (ca. 180 feet with a large full size 40 or 80 
meter yagi), one must "sectionalize" or detune the top to avoid the 
high angle lobe.  The "half wave problem" often comes into play on 80 
meters (150' tower with a large 20 meter antenna) and is one reason 
shunt fed towers (even 70' with a tribander) are generally dogs on 40!  

73, 

   ... Joe, K4IK