Electronics > RF, Microwave, Ham Radio

Double T bar antenna

**A.Z.**:

--- Quote from: A.Z. on May 14, 2021, 08:00:55 am ---the antenna could be reduced in size by adding more arms (like radials) to the loading hats

--- End quote ---

Here's an example; the 4NEC2 model is as follows

--- Code: ---CM .

CM 20m Double T bar vertical

CM .

CE

' setup params

SY freq=14.100 ' resonance

SY wdip=0.005 ' dipole radius

SY what=0.005 ' hat radius

SY base=0.80 ' height from ground

SY arms=1.49 ' arms lengths

SY hats=1.31 ' hats lengths

SY uppr=(arms*2)+base ' top cap height

SY cent=arms+base ' feedpoint height

SY lowr=base ' bottom cap height

SY segv=51 ' vertical segments

SY segh=51 ' horizontal segments

' vertical dipole arms

GW 1 segv 0 0 cent 0 0 uppr wdip

GW 2 segv 0 0 cent 0 0 lowr wdip

' top hat

GW 20 segh 0 0 uppr (hats*sin(45)) (hats*cos(45)) uppr wdip

GW 21 segh 0 0 uppr (hats*sin(90)) (hats*cos(90)) uppr wdip

GW 22 segh 0 0 uppr (hats*sin(135)) (hats*cos(135)) uppr wdip

GW 23 segh 0 0 uppr (hats*sin(180)) (hats*cos(180)) uppr wdip

GW 24 segh 0 0 uppr (hats*sin(225)) (hats*cos(225)) uppr wdip

GW 25 segh 0 0 uppr (hats*sin(270)) (hats*cos(270)) uppr wdip

GW 26 segh 0 0 uppr (hats*sin(315)) (hats*cos(315)) uppr wdip

GW 27 segh 0 0 uppr (hats*sin(360)) (hats*cos(360)) uppr wdip

' bottom hat

GW 50 segh 0 0 lowr (hats*sin(45)) (hats*cos(45)) lowr wdip

GW 51 segh 0 0 lowr (hats*sin(90)) (hats*cos(90)) lowr wdip

GW 52 segh 0 0 lowr (hats*sin(135)) (hats*cos(135)) lowr wdip

GW 53 segh 0 0 lowr (hats*sin(180)) (hats*cos(180)) lowr wdip

GW 54 segh 0 0 lowr (hats*sin(225)) (hats*cos(225)) lowr wdip

GW 55 segh 0 0 lowr (hats*sin(270)) (hats*cos(270)) lowr wdip

GW 56 segh 0 0 lowr (hats*sin(315)) (hats*cos(315)) lowr wdip

GW 57 segh 0 0 lowr (hats*sin(360)) (hats*cos(360)) lowr wdip

' standard ground

GE -1

GN 2 0 0 0 13 0.005

' loading (aluminium)

LD 5 0 0 0 37700000

' feedpoint

EK

EX 0 1 1 0 1 0 0

' frequency

FR 0 1 0 0 freq 0

EN

--- End code ---

as you can see, I added some radials to both hats and reduced the sizes for both the hats and the vertical section, the resulting antenna has a vertical section of about 3m with the hats covering a "circle" with 2.6m diameter, the pattern is still the same with lower radiation angle, and the antenna works fine at just 80cm from ground, the resulting simulation gave the following results

notice that, with an ATU the antenna will work on the bands from 20m and up, and that following the same approach it's possible to model an 80m version which will then cover the bands from 80 to 10m w/o problems (by the way it will need an ATU) and will fit in a relatively limited space

**ZigmundRat**:

I just wanted to say thanks for the detailed follow up. “Real LifeTM” has interfered lately and I just have had no time to get back to this. :-+

**Lord of nothing**:

:o What Software do you use?

Mabye someone can help me build a Helix Antenna for a Freq band.

**A.Z.**:

The simulations have been performed using the 4NEC2 program

https://www.qsl.net/4nec2/

if you want to try it, here are some notes which will help; start by downloading the installer here

https://qsl.net/4nec2/4nec2.zip

and while you're at it, also have a look at the guides and the manual found here

https://www.qsl.net/4nec2/supfiles.htm

once downloaded the program, run it "as administrator" and select "C:\4NEC2" as the installation folder, that is, do NOT install it under "Program Files" or the like since that would cause issues with permissions (4NEC2 needs to write into its own folder), once installed to start it just click on the "4NEC2" icon (the one showing a tower antenna) on the desktop and ignore any warning message the program may popup upon first start

Once the program will be running click on the "File" menu and select "Open 4nec2 in/out file", browse to "c:\4nec2\models" folder, that's where the antenna models reside, select the "VHFbeams" folder and then select the file "helix.nec" :) (or select the C:\4nec2\models\VHFsimple\Helix.nec which is another helix antenna) by the way that's just an example showing an helix antenna for the 432 MHz, but may give you an idea about how the model is designed just open the ".nec" file with a text editor and then also ensure to check the manual and documentation to understand the file syntax

**A.Z.**:

--- Quote from: ZigmundRat on July 24, 2021, 02:51:58 am ---I just wanted to say thanks for the detailed follow up. “Real LifeTM” has interfered lately and I just have had no time to get back to this. :-+

--- End quote ---

You are welcome, as a note, tried the first model (the simpler one) moving the feedpoint at bottom and found that feeding the antenna through a 2:1 BalUn will give a pretty wide bandwidth, in short, you have an SWR of 1.7 at 13.8 MHz and at 14.8 MHz, and the SWR is 1.2 at 14.3 MHz, not bad :)

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