(double balanced) bobtail curtain?

[Uncle Ronnie 336]

my 11m base station is all 450 ohm ladderline once it gets to the balun outside my shack window, inside said window on the other end of a 2' lmr400 passthru jumper is a tentec 229 tuner....looking to do a vertically polarized curtain array, and i think putting 2 bobtail curtains in a balanced configuration is the way to go. there are a few writeups and videos on a single coax-fed BC, but I never seen anyone take 2 of them and fed in the middle with balanced feeder. I might have to be the 1st, unless someone here tried it, then I might have to be the 2nd.

the questions to be answered in this fact finding mission:
-will the 4:1 or 9:1 balun get me closer?

-will it magically load up with the tuner in bypass mode? (my folded dipole main vertical has a 1.2 across the band and then some in bypass using the 9:1 balun.)

 

[Henry HPSD]

That sounds like a fun experiment !

I made a 2el bobtail curtain years ago... say 1/4 wave apart, with a phasing line...
That must have been 20 years ago..and i only remember my somewhat dissapointment in performance.
In all honesty...that could have been cause i didnt examined it into detail...perhaps ground losses was one of the reason.

But i think you are intending to make it in a different configuration ? ...2 BC's in perhaps an X shape (from view above).
If so... and the two driven elements stand close... they will act as an transmission line and impedance can be hard to predict (and possibly complex).

Im not sure i visualize your idea correct though.. maybe you could draw a sketch ?

 

[Uncle Ronnie 336]

you got it

 

[BJ radionut]

Ham Radio Antennas for Amateur Radio

over 460+ options, including Dipole, End-fed, Loop, Vertical, Yagi, G5RV, ZS6BKW, Quad and many more types.

ham-ant.com

 

[Uncle Ronnie 336]

Ham Radio Antennas for Amateur Radio

over 460+ options, including Dipole, End-fed, Loop, Vertical, Yagi, G5RV, ZS6BKW, Quad and many more types.

ham-ant.com

lots of stuff in there, yet no LL fed bobtail curtain

 

[Henry HPSD]

Hello,

You put a hugh smile on my face !
As I dont think i have seen that before.
Good idea thinking out of the box !

It reminds me somewhat of the sterba curtain... (though that is differently of course) .

Anyway...
I made a quick run using EZNEC:
The impedance will still be high... thousands of Ohms.. and will vary a lot with frequency. (that is also true for the "normal" end-fed bobtail curtain, they are rather narrow bandwidth in aspect to SWR). and will also vary with height above ground ofcourse.

Looks like you will gain 0,5 maybe 0,8 dB.


But...certainly worth a try !

Im not sure if you are only after vertical polarisation ? if not...
perhaps looking into the Lazy-H or the mentioned sterba curtain is something to consider, (they tend to be horizontal due to the mechanical construction...)

Thanks you for sharing your interesting idea !

Kind regards, Henry

 

[brandon7861]

What if, instead of insulators at the ends, there are shorts. I think that would lower the feed point impedance.

Start from the insulator at the end. It is an infinite impedance. A quarter wavelength away is a low impedance. The half wave wire maintains that low impedance and presents it to the ends of the driven (which should be higher impedance points relative to the feedpoint). But since there is a low impedance there, where the feed point is would require a high impedance since it is a quarter wavelength away. So, why not replace those insulators with a short and then see where you are with feed point impedance? I could be wrong, IDK,

 

[Uncle Ronnie 336]

Hello,

You put a hugh smile on my face !
As I dont think i have seen that before.
Good idea thinking out of the box !

It reminds me somewhat of the sterba curtain... (though that is differently of course) .

Anyway...
I made a quick run using EZNEC:
The impedance will still be high... thousands of Ohms.. and will vary a lot with frequency. (that is also true for the "normal" end-fed bobtail curtain, they are rather narrow bandwidth in aspect to SWR). and will also vary with height above ground ofcourse.

Looks like you will gain 0,5 maybe 0,8 dB.


But...certainly worth a try !

Im not sure if you are only after vertical polarisation ? if not...
perhaps looking into the Lazy-H or the mentioned sterba curtain is something to consider, (they tend to be horizontal due to the mechanical construction...)

Thanks you for sharing your interesting idea !

Kind regards, Henry

so this will be about 80' off the ground, so a couple+ wavelengths up.
i read a normal BTC does about 3dbd broadside, but i guess that all is in the spacing of it. tried downloading eznec but i can't wrap my brain around making the actual program work.

does sound like the 9:1 balun would prob work better at the window.

sometimes when i talk about antennas on my feed system, people's brains will revert back to coax/low ohm, when i see a high impedance feedpoint, it's just a question of tuner settings if the balun doesn't correct it

 

[Uncle Ronnie 336]

AI is telling me the impedance around the end of the middle element is 500-5000ohm. fingers crossed its closer to 500-1000. i'll be tickled pink if the balun corrects it with the tuner in bypass