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Re: [TowerTalk] TL antenna ?

To: "Terry Conboy" <>
Subject: Re: [TowerTalk] TL antenna ?
From: "W5CPT" <>
Date: Thu, 8 Oct 2009 19:16:53 -0500
List-post: <">>
Thanks to Terry, N6RY, for the answer and the plots.  Now that I have them I am 
going to build a version.  I have figured a way to build it with just one 
support by using PVC pipe to form the T section.  It will take two or three 
suspension points to keep the upper part of the T level but that is very 
do-able.  I will also be able to raise & lower the whole assembly using a 
pulley to see the actual change in the match. 

Clint - W5CPT

  ----- Original Message ----- 
  From: Terry Conboy 
  To: W5CPT 
  Sent: Thursday, October 08, 2009 6:19 PM
  Subject: Re: [TowerTalk] TL antenna ?

  On 2009-10-07 3:09 PM, W5CPT wrote:
  > In the Antenna Compendium Volume 3 is an antenna the writer calls a TL DX 
antenna. It is a T topped vertical over a 2 wire counterpoise.  I was wondering 
if anyone has ever modeled this antenna.  In the text he (AL7KK) says the lower 
(less than 15 degrees) radiation is useful for DX but he did not provide plots 
for antenna.  I need a DX antenna for 30 Meters and am considering this one 
since I can hang it in a pine tree.
  > For 30M the top of the T is 17.2" over a 12.9' vertical section with two 
21.6' counterpoise wires at a 90 degree angle to each other.  The top of the T 
is at 35.2'.   The antenna looks like a T over a ^ (T over a Lambda) 
  > Clint - W5CPT
  This is a nice antenna with a direct match to 50 ohms.  Using AL7KK's 
  dimensions and bare #12 copper wire, an EZNEC model shows 0.44 dBi gain 
  at 19.5 degrees above the horizon over "average" ground.  The feed Z is 
  44.9 +j 15.5 ohms at 10.125 MHz.

  I lengthened the lower two wires to 23.1 feet each and reduced the top 
  wire to 14.95 feet end-to-end to get the impedance a little closer to 50 
  ohms and resonant (47 ohms).  The gain is 0.48 dBi at 19.5 degrees.  The 
  2:1 SWR bandwidth is about 660 kHz.

  Raising the antenna will lower the takeoff angle, but will reduce the 
  feedpoint Z somewhat.  For example, raising the tweaked model by 5 feet 
  (40.2 feet at the top), lowers the takeoff angle to 17.5 degrees with a 
  gain of 0.65 dBi.  The feed Z drops to 41.5 ohms.

  You can raise the feed Z by increasing the droop angle of the lower 
  wires.  For example, with the top at 40.2 feet, increasing the droop 
  from 45 degrees to 58 degrees below the horizon and shortening them to 
  22.6 feet give a 50 ohm match and 0.57 dBi gain at 18 degrees.  2:1 SWR 
  bandwidth is 680 kHz.

  Of course, local conducting objects and ground characteristics will 
  change the gain and impedance somewhat, but the antenna is pretty 
  forgiving.  Be aware that if you don't include a feedline choke at the 
  feedpoint, there will be currents on the shield of the coax that can 
  modify both the pattern and feed impedance.

  This antenna is a close cousin of the balanced T, such as the Force 12 
  Sigma series.  There are usually dimensions for this type of antenna 
  that give 50 ohms at resonance.  Despite some claims, the elevation 
  patterns are largely determined by the height above ground and there 
  isn't any clear advantage over a standard ground plane of placing the 
  maxima of the current distribution at the center of the vertical 
  conductor.  The main advantage is the reduced height of the antenna 
  structure itself and no need for an extensive ground system.

  73, Terry N6RY

  PS - I'll send Clint .jpg's of the elevation patterns, but they are 
  pretty much like any ground plane vertical with the height above ground 
  adjusted so no high angle lobes appear.  If you want copies of the plots 
  and/or models, drop me a note off list.

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