Double T bar antenna

[A.Z.]

Had some time in my hands and decided to explore the "double T bar antenna" by creating a 4NEC2 model, the model is the following

Code: [Select]

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=2.15                ' arms lengths
SY hats=2.1                 ' 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  3 segh 0 0 uppr  hats 0 uppr what
GW  4 segh 0 0 uppr -hats 0 uppr what

' bottom hat
GW  5 segh 0 0 lowr  hats 0 lowr what
GW  6 segh 0 0 lowr -hats 0 lowr what


' 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

and describes an antenna designed for resonance on the 20m band, the antenna is made using 10mm aluminium pipes and is placed at 80cm from ground, after running some simulations of the model I gathered the informations shown in the attached image



as you can see, the antenna is a vertical dipole, fed at middle and shortened using two capacity hats, the advantage of such a design is that even if the antenna sits low on ground, the radiation angle is pretty low and due to the Q decrease due to the capacity hats, the bandwitdh is wide, the disadvantage is that the antenna offers no gain, but in my opinion, the lack of gain is compensated by the low radiation angle and by the ease of installation (it can sit almost on ground and doesn't need radials)

It may be possible to use the antenna on other bands, and probably with a tapped coil, even on 40/30 meters, ran some simulations on 40/30/15/10 and the pattern is still low, but didn't go on with the matching, although it shouldn't be so difficult

Nothing special, but I hope it may be useful

[ZigmundRat]

Interesting. Maybe I’ll play around with this a bit.  I know the parameters are posted there but can you attach the .nec file too (for completeness)? Thanks!

[A.Z.]

Interesting. Maybe I’ll play around with this a bit.  I know the parameters are posted there but can you attach the .nec file too (for completeness)? Thanks!

As you like; the "4NEC2" file is in my post, just select it, copy the text and paste it into an editor, save it as (say) tbar.nec and there you go

For further infos and ideas, you may also check these

https://www.qsl.net/hb9mtn/hb9mtn_vertdip.html

http://radio-amador.net/pipermail/cluster/attachments/20100517/6b2f8172/antennas-M3KXZPortableDoubleT.pdf

http://users.skynet.be/on4bai/VDIPHAT/4BAIVHAT_bis.htm

https://sites.google.com/site/lofturj/tf3lj_vertical

HTH

[ZigmundRat]

Yes, that helps. It’s been some time since using 4NEC and I was asking to be sure everything needed to play around with the design was present. Thanks for the reply. Cheers!

[A.Z.]

Yes, that helps. It’s been some time since using 4NEC and I was asking to be sure everything needed to play around with the design was present.

Well, if 4NEC isn't your cup of tea, the model could be rebuilt in EZNEC, MMANA or whatever you like, it's pretty simple and while I splitted the model in 6 wires, the model could be built using just 3 wires if desired.

As for modeling it, I tried to keep the model simple, but the antenna could be reduced in size by adding more arms (like radials) to the loading hats or turning them into "discs" like (e.g.) this



that would allow to further reduce the antenna size; also notice that the "double T bar" can be mounted horizontally as shown here



in such a case to obtain a decently low radiation angle it should be raised from ground, the advantage of the horizontal configuration is that the dipole will now be "directional" and, due to the smaller size, it could be mounted on a small antenna rotator. I like the vertical setup more, since it may be built as a portable antenna (e.g. using telescoping elements and a foldable tripod) and since it doesn't need to be raised over ground, plus if built using wires, it may even become an almost stealth antenna which may fit in HOA "infested" areas 

As for multiband operations, it may be possible to add a relay box at the feedpoint which controls some L/C to allow operating the antenna on different bands w/o having to change the elements size, an example of such an antenna can be seen here (but that "TransWorld" - now "DX engineering" one costs an arm and a leg !!)

At any rate, if you'll go on and model or even build it, please post some infos, I'm really curious

[A.Z.]

the antenna could be reduced in size by adding more arms (like radials) to the loading hats

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

Code: [Select]

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

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 Life^TM” has interfered lately and I just have had no time to get back to this. 

[Lord of nothing]

  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.]

I just wanted to say thanks for the detailed follow up. “Real Life^TM” has interfered lately and I just have had no time to get back to this. 

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