What directive antenna to select ?
Comparison is not reason (I)
After have shortly described the different antenna designs from longwire to yagi and dish, let's see whether a cubical quad or a delta loop performs better than a Yagi or if a log periodic shouldn't be the best solution ? Rightly, what are respectively advantages and drawbacks of these three designs, knowing that the simple dipole and the vertical cannot really fight against these high-gain competitors.
I subtitled this chapter "comparison is not reason" because you will be surprised by the results we get comparing the different designs each other. Most amateurs claim loud and clear that a quad exceeds by far performances of any beam. If this is true, why then have they built or bought a beam ? To explain this choice, we need to compare with accuracy the different antenna designs.
We have to divide the question in two parts : first compare the Yagi with the quad, then compare the log periodic with the Yagi. From these results I can already tell you that it will not be necessary to compare the log periodic with the quad although we will discuss shortly about this comparison.
Yagi vs. Quad
I will not surprise you in saying that the question is widely debated for decades and thus, I would be well unable to give you a final answer. An objective answer is indeed often soiled with a large bias due to environmental considerations. But even if the question is still opens, some technical observations are however interesting to highlight that will help to answer, at least partially, to this question.
As many amateurs know quite well properties and drawbacks of Yagis, we will explore a bit deeper properties of quads and other loops that not all amateurs have the opportunity to use or even to approach, a subject illustrated with some impressive pictures to please you.
In my humble opinion, technically speaking both designs display a similar directivity and give similar gains within 1 dBd.
Only big point in favor of the quad (thanks to N6DBH for the note), each loop is constituted of 2 stacked dipoles : 2 for the vertical radiator and 2 for the vertical reflector, what improve its sensitivity and explain why quad starts hearing earlier than a Yagi and keeps hearing later than a Yagi. They are thus not really on par.
About the gain, several tests conducted by amateurs using both designs have concluded that the quad displays a gain approximately 2 dB over a Yagi for the same array length, with a lower takeoff angle than a Yagi.
But the QST magazine edited by ARRL published some years ago (May 1968) a controversial test. At half-power (-3 dB) the beamwidth of a 4-element UHF quad is 50°/58° in respectively H and E-planes against 47°/56° for a 5-element Yagi, the quad offering a 9.4 dBd gain vs. 9.9 dBd for the Yagi. This was the only test were a quad lost against a beam.
Other significant measurement, the Yagi used a boom 83% longer (i.e. 5.5 m vs. 3 m) than the one of the quad.
At last the Yagi displayed shorter rear lobes in both H and E-planes than the quad, but its side lobes in the H-plane were a bit more extended. They were however are reduced as the ones displayed by the quad in the E-plane.
Searching for a stealth profile
Considering their profile, at first sight quads are not favoured if you are looking for a stealth or a low profile antenna. Indeed, due to their arrangement in the vertical plane, many amateurs consider that quads are bulkier that Yagis. In fact this is a subjective feeling because they are vertically erected, showing a larger profile than beams in that plane. It is obvious that several large X or diamond (+) erected 8 m high one after the other make an impression !But have you ever looked objectively at a beam or a log ? On top of their pylon they look also impressive...
But in fact the turning radius (T.R.) of a quad does not exceed 3m for a 14 MHz loop whatever its shape (delta or cubical), what is on par with several short 5-bander beams : the Cushcraft MA5B has a T.R. of 2.7 m, the Hexa-beam HXP has a T.R. of 2.8m and the Mini-33 from Mosley has a T.R. of 3.3 m only. All other beams have a T.R. close to 5 m.
A beam can also be very cumbersome, designed with or without traps. Many multi-band models have a 20m-reflector which wingspan exceeds 11 m with a boom length that sometimes reaches 12 m ! Hopefully some short multi-band beams come with boom no longer than about 2 m (e.g. Mosley Mini-33).
In fact beams are as bulky as quads and probably much heavier due to the tubing size ! But here also, as the profile of a beam is arranged in the horizontal plane, at some distance a Yagi can be quasi invisible, other than through its tower and maybe its long boom.
Wind noise and white noise
Made of tubing, a beam constituted of four or more elements placed on the roof generates also more noises than a quad under high wind. At some distance the problem goes unnoticed. But many amateurs having installed their beam 4-6 m above their bedroom complained that they were disturbed by the noise of the antenna vibrating under the wind. The problem must be still worst when there is ice on the elements. In the same conditions, a quad will be more silent as it is mainly constituted of thin wires and tubing of shorter section.
About noise on frequencies, Micke, SM5JAB, sent me a note telling that a Swedish amateur using a 5-element monoband Yagi and a 6-element monoband quad (lucky he is !) noticed that on the 20m band his beam picks-up a noise 5-10 dB above the threshold where it is barely audible on his quad. The direction seems to be irrelevant, the white noise is everywhere. It has always the same strength independent of the time of day, possibly a bit weaker at sunrise. This noise does not come from static discharges that are completely different. The noise looks like to a "white noise" that we hear when listening to a band without activity.
The origin of this so-called white noise is unclear but amateurs think that it could be related to man-made interferences emitted by a nearby town. Other explanation, the quad's takeoff angle is different than of the one of the Yagi's and so picks up anything from another angle.
In all cases, very few amateurs have noticed this effect and it should be interesting that other users of both Yagi and quad confirm it, and mainly under what conditions of use (band used, time of day, weather, noise background level, etc). Your feedback is welcome.
Another element to consider is that a quad, contrarily to a beam moves in the 3 dimensions (a way of putting it), and specially in the vertical plane. If you place it on top of a pylon requiring guy wires, while rotating these wires can be an obstacle for the spreader arms. You will have already less problem in using a diamond quad. The best solution is to build a lift system that might lift the antenna and its rotating system of 0.5-1 m during rotation and that should place it back in position once in the right direction. Another solution, easier to install, is to place the top guy colar just below the lower base of the quad, thus about 2.7m below the boom, or to place the mast supporting the boom at least 2m higher than expected.
This problem can however be completely ignored if you install the quad on a freestanding mast, and it has even not to be very high (5-10 m) as the quad radiation pattern is not altered when the antenna is placed a few meters above ground.
Then, quads are less sturdy than beams. Made of metal or fiber-glass, the spreader arms in form of square (X) or diamond (+) on which the wires are tight are not more fragile than elements fit together of a Yagi. But the wire section (minimum #12 AWG) and the way that they are fixed along the arms (channel drilled in the arms section) is the weakest point of the installation. Some quads like the Degen BBQ are even portable but I bet that being regularly folded back, they will never endure years long such a stress.
Other drawback, during the winter season, a quad mades of too light and not resistant material (mast, spreaders or wire) will bend and will probably not support long time such a treatment, specially if there is ice on wires. So, if you live in a tempered and windy or a nordic area, think about that twice before buying a quad.
If designing a quad with wood or bamboo sticks seems to be an economical solution, one must know that a 3-element quad made of bamboo weigths about 6 kg vs. 3 kg only for a fiber-glass version. This latter is not only twice as light but much more resistant too. Hopefully, whatever the material used, by design the vertical poles of a diamond quad (about 4m long for the 20 m band) are sufficient strong to hold alone all the structure.
What can impress some newcomers is that made of wires, quads look as delicate to handle as a spider web. Recall all the same that the weight of a short quad like the 2-element Degen BBQ 101520 is 19 kg being made of stainless steel. It is much heavier than a beam made of titanium (5 elements Titanex, 9 kg) or aluminium (4 elements Mosley, 13 kg). Its wires and fixings are also very resistant. Other positive point, these wire antennas are easy to install even if the first assembly can last some hours if you are not used to assembly quads. With some habits you can do the job in half an hour.
At last, when a quad is erected in diamond, at winter the rain or the snow can slide along the wire what reduces the accumulation of ice on the antenna much better than using a square quad on which the ice can easily accumulate on all wires horizontally tight.