Electronics > RF, Microwave, Ham Radio
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: ---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
--- End code ---
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.:
--- Quote from: ZigmundRat on May 11, 2021, 12:56:46 pm ---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!
--- End quote ---
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.:
--- Quote from: ZigmundRat on May 13, 2021, 11:50:04 pm ---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.
--- End quote ---
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 :D
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
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