Intro

This is the second post on phased verticals. If you have not seen the first, here it is >>Half-square antenna....
After half-square antenna, I checked few similar configurations, adding phasing lines and vertical elements. Here is the popular antenna, usually called "double half-square":

As you see, it is basically 2 half-square antennas put together and fed in the center. The length of the phasing lines is still L/2, so left and right radiators emit in phase:

Here is the 4Nec2 model I created and used.
As with the half-square, I am interested in low height placement of the system because that is what I am going to do at my QTH. So, while this model will accept any wavelength and any height, I tested it for 14 MHz and low heights (5.2-6m or 17-20ft). I used good ground because, again, that is what I have at my place here in South Florida.

Results and Discussion

(1) Frequency sweep.
the antenna was optimized to resonate at the frequncy I am interested in (around 14.225 MHz).
The SWR is acceptable and antenna can be fed directly by 50 Ohm coax.
(2) Far field radiation efficiency and pattern.

ok, looking at the data and comparing to the same for original half-square...very similar to original HS antenna, same lossy design (inherent structural loss 12.5% vs 13.9% for half-square); radiation efficiency in far field is also low at this height, even lower then it was for half-square (27.07% vs 30.68%). On the plus side, same high directivity, low takeoff angle, sharper nulls, more gain, all in all a tad better compared to the original half-square.

Verdict

Well...if you have unlimited time and space...otherwise don't bother, go with original simple half-square.