Double T bar antenna - Page 1
Forum section: electronics / rf-microwave

author: A.Z.
author_rank: Super Contributor
timestamp: May 10, 2021, 10:53:59 am
content: Had some time in my hands and decided to explore the "double T bar antenna" by creating a 4NEC2 model, the model is the followingCode:CM .CM 20m Double T bar verticalCM .CE' setup paramsSY freq=14.100 ' resonanceSY wdip=0.005 ' dipole radiusSY what=0.005 ' hat radiusSY base=0.80 ' height from groundSY arms=2.15 ' arms lengthsSY hats=2.1 ' hats lengthsSY uppr=(arms*2)+base ' top cap heightSY cent=arms+base ' feedpoint heightSY lowr=base ' bottom cap heightSY segv=51 ' vertical segmentsSY segh=51 ' horizontal segments' vertical dipole armsGW 1 segv 0 0 cent 0 0 uppr wdipGW 2 segv 0 0 cent 0 0 lowr wdip' top hatGW 3 segh 0 0 uppr hats 0 uppr whatGW 4 segh 0 0 uppr -hats 0 uppr what' bottom hatGW 5 segh 0 0 lowr hats 0 lowr whatGW 6 segh 0 0 lowr -hats 0 lowr what' standard groundGE-1GN2000130.005' loading (aluminium)LD500037700000' feedpointEKEX 0 1 1 0 1 0 0' frequencyFR0100freq0ENand 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 imageas 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 difficultNothing special, but I hope it may be useful

author: ZigmundRat
author_rank: Regular Contributor
timestamp: May 11, 2021, 12:56:46 pm
content: 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!

author: A.Z.
author_rank: Super Contributor
timestamp: May 11, 2021, 01:38:07 pm
content: Quote from: ZigmundRat on May 11, 2021, 12:56:46 pmAs 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 goFor further infos and ideas, you may also check thesehttps://www.qsl.net/hb9mtn/hb9mtn_vertdip.htmlhttp://radio-amador.net/pipermail/cluster/attachments/20100517/6b2f8172/antennas-M3KXZPortableDoubleT.pdfhttp://users.skynet.be/on4bai/VDIPHAT/4BAIVHAT_bis.htmhttps://sites.google.com/site/lofturj/tf3lj_verticalHTH

author: ZigmundRat
author_rank: Regular Contributor
timestamp: May 13, 2021, 11:50:04 pm
content: 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!

author: A.Z.
author_rank: Super Contributor
timestamp: May 14, 2021, 08:00:55 am
content: Quote from: ZigmundRat on May 13, 2021, 11:50:04 pmWell, 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.) thisthat would allow to further reduce the antenna size; also notice that the "double T bar" can be mounted horizontally as shown herein 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" areasAs 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 seenhere(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

author: A.Z.
author_rank: Super Contributor
timestamp: May 17, 2021, 12:59:29 pm
content: Quote from: A.Z. on May 14, 2021, 08:00:55 amHere's an example; the 4NEC2 model is as followsCode:CM .CM 20m Double T bar verticalCM .CE' setup paramsSY freq=14.100 ' resonanceSY wdip=0.005 ' dipole radiusSY what=0.005 ' hat radiusSY base=0.80 ' height from groundSY arms=1.49 ' arms lengthsSY hats=1.31 ' hats lengthsSY uppr=(arms*2)+base ' top cap heightSY cent=arms+base ' feedpoint heightSY lowr=base ' bottom cap heightSY segv=51 ' vertical segmentsSY segh=51 ' horizontal segments' vertical dipole armsGW 1 segv 0 0 cent 0 0 uppr wdipGW 2 segv 0 0 cent 0 0 lowr wdip' top hatGW 20 segh 0 0 uppr (hats*sin(45)) (hats*cos(45)) uppr wdipGW 21 segh 0 0 uppr (hats*sin(90)) (hats*cos(90)) uppr wdipGW 22 segh 0 0 uppr (hats*sin(135)) (hats*cos(135)) uppr wdipGW 23 segh 0 0 uppr (hats*sin(180)) (hats*cos(180)) uppr wdipGW 24 segh 0 0 uppr (hats*sin(225)) (hats*cos(225)) uppr wdipGW 25 segh 0 0 uppr (hats*sin(270)) (hats*cos(270)) uppr wdipGW 26 segh 0 0 uppr (hats*sin(315)) (hats*cos(315)) uppr wdipGW 27 segh 0 0 uppr (hats*sin(360)) (hats*cos(360)) uppr wdip' bottom hatGW 50 segh 0 0 lowr (hats*sin(45)) (hats*cos(45)) lowr wdipGW 51 segh 0 0 lowr (hats*sin(90)) (hats*cos(90)) lowr wdipGW 52 segh 0 0 lowr (hats*sin(135)) (hats*cos(135)) lowr wdipGW 53 segh 0 0 lowr (hats*sin(180)) (hats*cos(180)) lowr wdipGW 54 segh 0 0 lowr (hats*sin(225)) (hats*cos(225)) lowr wdipGW 55 segh 0 0 lowr (hats*sin(270)) (hats*cos(270)) lowr wdipGW 56 segh 0 0 lowr (hats*sin(315)) (hats*cos(315)) lowr wdipGW 57 segh 0 0 lowr (hats*sin(360)) (hats*cos(360)) lowr wdip' standard groundGE-1GN2000130.005' loading (aluminium)LD500037700000' feedpointEKEX 0 1 1 0 1 0 0' frequencyFR0100freq0ENas 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 resultsnotice 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

author: ZigmundRat
author_rank: Regular Contributor
timestamp: July 24, 2021, 02:51:58 am
content: 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.

author: Lord of nothing
author_rank: Super Contributor
timestamp: July 29, 2021, 09:43:48 am
content: What Software do you use?Mabye someone can help me build a Helix Antenna for a Freq band.

author: A.Z.
author_rank: Super Contributor
timestamp: July 29, 2021, 09:54:11 am
content: The simulations have been performed using the 4NEC2 programhttps://www.qsl.net/4nec2/if you want to try it, here are some notes which will help; start by downloading the installer herehttps://qsl.net/4nec2/4nec2.zipand while you're at it, also have a look at the guides and the manual found herehttps://www.qsl.net/4nec2/supfiles.htmonce 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 startOnce 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

author: A.Z.
author_rank: Super Contributor
timestamp: July 29, 2021, 10:35:03 am
content: Quote from: ZigmundRat on July 24, 2021, 02:51:58 amYou 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