The 5-band Cubical Quads require either relays to switch bands or a window line and an ATU to handle high SWR (DK7ZB design). Although such antennas are built and used, I always considered them as a bit imperfect. My initial attempts to design a 5-band Cubical Quad not requiring relays and having low SWR failed.
The main reason for that is the fact that a square loop has a harmonic resonance at about 2F. The 14 MHz band loop resonates also at about 27.45 MHz, what interferes with 12 m and 10 m band loops. At the second harmonic resonance, the loop radiates up and down, left and right, but not forward and backward as it does at the fundamental harmonic. I had tried various antenna shapes, but nothing helped. I was convinced it was impossible.
But never say never!
Let's compare a classic square loop with a loop that is fed in the centers of the top and bottom sides.
I call the second antenna a split feed point loop or SFP loop. Two transmission lines connect the feed point located now in the center of the antenna with the centers of its top and bottom sides. Let’s compare the impedance magnitude and the phase plots of both antennas.
The classic square loop (5.58 x 5.58 m) has the fundamental resonance at 14.175 MHz and the next low impedance resonance at 27.45 MHz. We may say that this is the second harmonic. See below.
The SFP loop does not have a second harmonic resonance but third harmonic resonance. With the dimensions 5.58 x 5.58 m, and 300 ohms transmission lines are used, 1F equals ca. 11.8 MHz and 3F equals ca. 26.9 MHz.
As a matter of fact, you can regard the SFP loop as a stack of two dipoles with bent ends. If you break the antenna in the centers of the vertical sides and move the upper and lower halves slightly away from one another, the antenna performance will remain unchanged. It is shown below.
The SFP loop and the stack of two dipoles with bent ends are electrically identical. But that’s not all.
If you change the transmission lines impedance (e.g. 300, 450, or 600 ohms), you can change antenna resonance impedance. Also, the length of the transmission lines matters.
Another interesting fact is you can try to use either the low impedance resonance at about 11.8 MHz or the high impedance resonance at 16.1 MHz (see the |Z|/Phase plot of the SFP). Three reasons back the choice of the high Z resonance:
- impedance magnitude and phase change more gently around it what should make achieving wide bandwidth easier
- high Z value is relatively low (about 760 ohms) and there are prospects of bringing it even more down by choosing the transmission lines characteristic impedance and adjusting their length
- antenna gain is greater at the high Z resonance than at the low Z resonance (3.75 dBi vs. 2.82 dBi)
And there is one obvious drawback of the high Z resonance choice. For the same operational frequency, the loop needs to be larger. Nevertheless, I opted for the high Z resonance and succeeded in designing the 5-band SFP Cubical Quad.
The antenna looks like that.
In the model, all the wires are made of copper, 2 mm in diameter. Transmission line characteristic impedance is 450 ohms. Both lines are 2.5 m long in the NEC model, but when building this antenna, one must multiply this value by the VF (Velocity Factor) of the transmission line used. VF usually equals 0.9 - 0.91 for a 450 ohm window line. So, in the real world implementation, each TL should be 2.25 +/-0.05 m long. The feed point will be a little closer to the driven element than shown in the picture above. The main feed point is on the boom, the split feed points are 1.87 m above and below the boom.
The antenna impedance is approx. 450 ohms. So, you will need a 9:1 balun to feed it. A three core Guanella balun seems to be the best choice.
The distance between the driven element and the reflector is 3.2 m.
The loops dimensions are provided in the table below.
Find below the SWR, gain and F/B ratio plots of the 5-band SFP Cubical Quad.
I compared the SFP Quad with the Flat Cubical Quad and the Spider Cubical Quad. See the graphs below. Relative gain is calculated with 30 meters of RG-58 coax cable (A+TL = Antenna + Transmission Line). In this way, the additional losses in coax caused by SWR greater than unity are included in the comparison. The gain is related to the gain of a half-wave dipole fed with the same coax cable. That's why the units are dBd.
SFP Quad gain is on par with the classic quads. F/B ratio seems to be somewhat worse on 12 and 10 m, but I designed this quad to cover the whole 10 m band with low SWR what can not be said about the other two.
All in all, the SFP Cubical Quad is quite close in performance with the other quads and does not require any relays for band switching. The goal of this design has been met.
Click here to download this antenna models.
Note: a model for MMANA-GAL has not been created because this simulator does not support transmission lines.
I also design the diamond version of the SFP Quad. This is how it looks like.
The driven elements and reflectors are 2.5 meters apart. The split feed points are 2.225 m above and below the boom. The ideal 450-ohm transmission lines are 2.5 m log in the model, but when corrected by VF, they will be about 2.25 m long. It means that they will be almost exactly in the same plane as the driven element. The dimensions of the loops are presented in the table below.
Find below the SWR, gain and F/B ratio plots of the diamond version of the 5-band SFP Quad.
And the comparison with the other quads:
The SFP Diamond Quad has quite decent gain. Its F/B is not extremely high but quite stable with frequency. That means that its radiation patterns will not change dramatically throughout the band. See below the patterns for the 20 m band, for the lowest, center and highest frequencies. 11 m above the ground.
I can not build and test this design because I do not have enough space in my backyard to raise a full size Cubical Quad. But perhaps somebody else will do that one day...
Click here to download this antenna models.
Note: a model for MMANA-GAL has not been created because this simulator does not support transmission lines.