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
Cc: towertalk@contesting.com
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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