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Re: [TowerTalk] TL antenna ?

To: W5CPT <>
Subject: Re: [TowerTalk] TL antenna ?
From: Terry Conboy <>
Date: Thu, 08 Oct 2009 16:19:37 -0700
List-post: <">>
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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