# Double Cross antenna: what's wrong with my model?

Important note: After consulting another source, I learned that the article calls for the first pair of opposing dipoles to be fed in phase, with the second pair fed in quadrature to the first pair (but in phase with each other). Sadly, the results are the same, so I still don't know what's wrong with the model.

An earlier question asked about the tuning of the four dipoles that comprise the "Double Cross" antenna, intended for hemispherical reception of satellite signals:

While attempting to respond to the question, I found that I was unable to reproduce the free-space radiation pattern claimed in the article:

Using the dimensions specified in the article for 435-MHz:

I constructed a free-space NEC-2 model with increases in source phase of $$90^o$$ proceeding around the circle of elements. I obtained these results with the elements tilted $$45^o$$ from the vertical:

The "lightbulb" shaped pattern is clearly not what the author intended, although the impedance at the feedpoint of each dipole seems to approach the author's goal, re:

The input impedance of each of the λ/2 dipoles when configured as shown will be very close to 50 Ω. Each pair is wired in series to have an impedance of 100 Ω. After connecting the two pairs in parallel, as shown in the harness diagram of Figure 9, we end up with the desired 50 Ω for the run to the radio.

Tilting the elements more toward the vertical reduces the "lightbulb" shape of the pattern, but approximately halves each dipole's feedpoint impedance:

Can anyone clue me in to what I have done wrong in the model?

• interesting! I got sooo many questions. Reading that article: I'm not quite sure how combining two Figure 5s would lead to a null in negative z, but only a moderate dip in positive z direction. Maybe their simulation included a reflective ground plane in a specific distance? – Marcus Müller Jun 20 at 11:38
• Also, text and pictures contradict: on the first page, they say the two elements of a double cross are "fed in phase"; but the wiring harness fig. 9 of page 3 says the opposite. Do you get Fig. 5 when you enable only two opposing ones of your 4 dipoles? – Marcus Müller Jun 20 at 11:42
• oh, and I don't know EZNEC well enough, but can you add a (very) long grounded vertical rod from $x=0=y, z\to-\infty$ to the center of your antenna? That would force a null in $-z$, but the question is what happens to the rest of the antenna. – Marcus Müller Jun 20 at 11:51
• @MarcusMüller Yes, I checked that out, too. I've tried everything I can imagine, including "reversing" the tilt between groups, but nothing gets the article's results. – Brian K1LI Jun 21 at 9:53
• I can practically taste your disappointment with the situation! But honestly, although the author certainly seems very experienced, I trust your simulation no less than his description of the system! (honestly, such articles shoul,d come with downloadable design files that he has, anyway, if he can do a simulations!) Maybe cross-validate with amsat.org/articles/w6shp/lindy.html ? That suggests larger distance and a lower instead of a steeper angle. Again, I'm not at all versed in EZNEC, but can one script that to iterate towards a good design? – Marcus Müller Jun 21 at 11:05

From the Answer of Marcus Müller: I simulated the Lindenblad 2m antenna (which should scale, right?) that came with the xnec2c examples. ...

The unedited *.nec file included in Marcus' answer was downloaded into 4nec2 and analyzed using the NEC4.2 Engine — which produced the graphic shown below. Two items of interest appeared...

(1) The az/el radiation patterns are ~perfectly circular, but h-pol gain in the horizontal plane is about 5 dB less than v-pol gain. This means that the axial ratio of its net radiation pattern is not unity.

(2) The calculated feedpoint terminal impedance (outlined in red) does not provide a good match to a 50 Ω source connected to it.

Some refinement of the geometry of this design might be worth pursuing before such a wire model is actually constructed.

This is as close to an answer as I'll be able to give:

I simulated the Lindenblad 2m antenna (which should scale, right?) that came with the xnec2c examples.

view from side

view from top

In conclusion: yeah, in my simulation, things are symmetrical, as promised for the 30° slant.

So, Martes' design seems to stand out pretty singularly, and simulation can't seem to reproduce it: Hm. Maybe it's plain wrong, maybe the description doesn't really match what he did, maybe we're too stupid to do it right. Hm.

Double Cross antenna: what's wrong with my model?

nothing, maybe.