R: [TowerTalk] Tall verticals

K7GCO@aol.com K7GCO@aol.com
Wed, 10 May 2000 18:14:22 EDT


In a message dated 10.05.00 13:34:22 Pacific Daylight Time, 
n4zr@contesting.com writes:<< 
 At 09:59 PM 5/10/00 +0200, Maurizio Panicara wrote:
 >
 >It's practically impossible to prevent the beam from (top) loading the
 >tower.  Even isolating the yagi antenna from the tower, its coaxial cable 
will run
 >along the tower and will consequently follow the current distribution of the
 >antenna.  The yagi, isolated or not from the tower, will consistently load 
the
 >vertical structure as much as the horizontal antenna is approaching (or
 >close) to a high voltage point of the vertical radiator.
 
 Interesting.  My tower is 30m of Rohn 25 with a Force 12 C-3 and an EF-240S
 (total about 1 square meter of wind area) at the top.  I'm planning to add
 another C-3 (.5 sq. meter) side-mounted at the 20-meter height point on the
 tower to stack on 20-10.  
 
 I would also like to shunt feed the tower on 160.  Is there any way to
 ascertain analytically whether the electrical length of the tower or the
 loading at 20 meters above ground will cause serious complications so far
 as the impedance to be matched, and the height of the shunt-feed point?
 I've looked at ON4UN's book for guidance on estimating the loading effect
 of antennas on a tower, and have followed this discussion, but don't know
 whether the bottom line is that the shunt feed is just a bad idea, or
 whether it can be done practically.  I lack the engineering skills to do
 the analysis.
 
 73, Pete Smith N4ZR
 n4zr@contesting.com 
  >>
Pete:  You don't need any. Try this.  Connect the shield of a coax to the 
tower starting at 5' off the ground and run a 1/4 wave radial connected to 
the center of the coax horizontal for it's full length.  Run a SWR curve with 
the MFJ Analyzer.  Keep doing that raising the shield connect point 5' higher 
at a time and try and keep the radial horizontal if possible but not fully 
necessary.  There will be a point where low SWR will be obtained or at least 
I've been able to find the magic spot on a couple of my towers.  It sometimes 
helps to fine tune the tower connect point and radial length.  Sometimes it's 
fairly high on the tower like for 40M and then it's called a Slopper.  The 
tower radiation may be more prominent than the sloping wire so it may be 
mis-named.  It's a different form of a gamma match without a variable. I'll 
have to try that connected to a selsyn and I should be able to tune the 
antenna over the whole band--how about that?  This is a simple solution for 
needed bandwidth on 160,80&40M.  (Resonant it low in the band with the 
Variable Xc shorted out.  Bent a plate over to touch at full mesh.)  The 
magic spot will be where the tower and the top loading will be equivalent to 
a 1/4 or 3/4 WL.  The tower presents mostly a resistive load to the coax 
shield at the resonant tap points.  The shorter length of tower below has 
less affect---usually.  Sometimes low SWR is obtained even without 
disconnecting the beam coax and rotator cables at the base of the antenna.  
Actually the coax feeding the tower is of the same RF potential.  It's a form 
of an L antenna with some directivity fed in a "sneaky way.".  To reduce the 
directivity add a radial from the center lead going the other way.  Add 3 or 
4 radials at 120 degrees to even out the pattern.  I was just about to run 
some patterns on this in Eznec. 

If you get this to work say on 160 or 80M, have a coax switch hooked up to 
connect the beam on top coax to the receiver input.  Some transceivers have a 
separate input jack.  It will be a good listening antenna as it's mostly 
horizontally polarized and gives a dual use for the beam and tower.  
Sometimes a preamp helps and an L network or Matchbox to match it to 50 ohms 
can help for receive.
With a switching circuit one could quickly compare receiving antennas.  

K7GCO 

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