What directive antenna to select ?
Pros and Cons Quads & Yagis (II)
To close on more objective data, the table displayed below takes into account several items common to both quads and Yagis. Values of each item (4th column, left for quads, right for beams) have been balanced according their importance over the other items and over their overall importance on the final choice.
To make a choice between two or several products, there is nothing better than using a balanced table to get an objective result. The weight assigned to each item is of the uttermost importance to get significant and accurate figures. It takes more time to determine than to fill in boxes. For example, if you consider that the furtivity of your antenna is not important, you can reduce its value to very few thing or simply remove this line. If its gain is very important, gives it a maximum value. But in the same time you must also assign to each item a value compatible with the others, to which they can be somehow compared. The turning radius for example can be in your mind as important as the weigth of the antenna, thus assign to both items the same maximum value, etc.
Select also a range compatible with your estimation knowing that it is useless to use a scale between 0 and 100 for example if you are unable to fix the value of an item within 10% of accuracy; in this case use a scale between 0-10.
It is as much hard to work with decimal figures; 8.9 and 9.1 will probably give results on par in the field, but first of all that means that you can estimate each item with an accurate between 0-100... So here also I suspect that you can't do it and it is advisable to use a scale between 0-10 too. This preprocessing is mandatory because at the end, you will have to consider these terms on a equal footing to calculate them.
When your estimation is well level-headed, the result is final and irrevocable, you must select the highest value... If you are not agree with the result, that means that you did a mistake, that you underestimated one or several items or worse, that you forget to include a new variable, which weight could be predominant over all others, maybe is it your bias and subjective feeling ... But if you make this estimation seriously your subjectivity must be rejected at least 20 dB down, Hi ! Here is the table.
Cubical Quad Antenna Calculator, QSL.Net
Let's briefly analyze these results. According to the values assigned to each item, as amazing as it is, globally these facts incline to be in favor of quads that win with 84% of points vs 57% only for Yagis (168/200 for the quad vs. 115/200 for the beam), although we known that most amateurs prefer beams... The difference exceeds 25% and I do not see how we could reverse the balance. I suggest however to the reader having a long experience of both types of antennas to make the same or a more accurate test. Feel free to give me your feedback if you do it or if you have any other relevant information that could complete this review that visibly interest many hams.
Put back in its context, this result, based on objective technical data and field experience demonstrates already that both antenna systems have advantages and drawbacks but that quads are globally "better" than Yagis. The difference looks objective but it does not confirm the subjective feeling reported by users. That could mean that once setup in the field, amateurs changed of opinion; the weight of some items exceeded the values fixed a priori or another information omitted in this list took the step on the others.
It is not any doubt that an amateur living in a residential area, an appartment or having only at its disposal a small estate has to accept some rules of coexistence or co-ownership that will affect his or her choice.
The fact that the quad is a true 3D object, far to be stealthy, and that sometimes can be easily damaged by ice, are so many factors that impact probably negatively most amateurs wishing to install this kind of antenna on their roof or in their backyard.
If I had to make a choice, without taking into consideration the price and sizing, I should select a quad with 4 or 5 elements for its overall performances, this is an indisputable fact reading its specifications, not an a priori.
However, taking into account the stealth, easy of assembling, sturdiness, availability, and price, my good intentions fall back, and surely if I had to live with the opinions of a co-ownership.
On its side a beam is easy to assembly, can be stealthy if it is of small size, light and not too bulky. They are also available to many manufacturers at very attractive prices.
But in addition to financial and available space considerations, to decide on which antenna you will bear your choice, in theory you should need to test a quad in the field to appreciate its performances vs. a Yagi.
Log periodic vs. Yagi
Why don't we see much "logs" in our countries ? Most of them are used by institutions, including embassies which roof often displays a huge log periodic. But not many hams use the log periodic excepting the one maybe charmed by their design and why not... the difficulties of building.
Indeed, it is a beam hard to design and to build due to its numerous elements and wire segments. Confronted to the accuracy required to calculate and assemble all segments as well as to respect the logarithmic spacing between elements, more than one amateur moved back in front of the task. However we must also recall that today the computers and their spreasheets offer a great assistance in (re)solving such problems.
In the field some amateurs built successfully their log periodic to name K4EWG (12 elements 13-30 MHz) and K8CU (2x 13 elements stacked 14-30 MHz) who shared their experience with QST readers.
The positive thing using a log periodic is that the method of feeding the antenna is rather simple (at least compared to its design !). It consists in using a balanced feed line for each element, all adjacent elements being fed with a 180° phase shift by alternating element connections. Saying that, with all its elements and the long boom required to support them (8-12m long for a 12-element beam), the log periodic is bulkier and heavier than a Yagi offering the same performances.
Contrarily to what state some authors, all elements of a log periodic are not active at all frequencies. They should say that all elements are active forward of the one most active at any given frequency. In practice only 3 elements are really active at a time (the radiator at resonance on the actual working frequency, its reflector and the first director) although all elements show some current which amplitude decreases with the distance. We can thus compare the gain offered by a log periodic with a 3-element monoband Yagi, practically as long and as wide but of course much more stealthy with its 3 elements.
Justly, Bill Jones, K8CU, works with an homemade 12-element log periodic and uses also a 3-element monoband Yagi. Where the log periodic displays a maximum gain of 5.7 dBd the beam exceeds 7 dBd, on par with a 2-element quad. We can however reach 6.9 dBd using a 4-element log periodic and optimizing the elements spacing and length in modifying the traditional log periodic design.
But worst, the maximum gain of the log periodic "falls" quite often out of the band : the 5.7 dBd gain for example is recorded on 22 MHz and it is of 5.4 dBd on 21 MHz; there is also a peak of 5.5 dBd at 27.5 MHz right in the CB band instead of 28.5 MHz (5.4 dBd gain). The 14 MHz is the only frequency where the gain is maximum (5.1 dBd gain) to fall down just after. But all theses values are ridiculously low.
The log periodic bandwidth is also broader than the one of a Yagi, and as many variables on this antenna, it varies depending on the values of the design parameter t and the relative spacing constant s.
In the same way, its gain is generally 30 to 50% lower than the one of a Yagi using the same number of elements, with a free-space forward gain that never exceed 9 dBd for a 12-element log periodic. For your information, such an lof periodic has a boom length of 8m (26.5') and with all its tubing and its large boom of 75 mm diameter (3"), its weight reaches 53 kg (116 lbs.) ! It offers a half-power bandwidth of 43°, a F/B ratio of 14.4 dB at 14 MHz and up to 21 dB on 28 MHz. Any 5-element beam exceed these values and many wire beams add to that a featherweigth. But what should be the performances of a 12-element Yagi used in the same spectrum of frequencies ?
Of course there are very few if not any Yagi of such a sizing in the field. We know however that on any kind of beam the antenna gain is proportional to the length of the array, provided the number, lengths, and spacings of the elements. We can thus estimate that a 12-element Yagi should display a 14 dBd gain; it leaves the log periodic decibels behind it, Hi!
We can also calculate the gain of long Yagis as a function of overall array length, itself being a function of the numbers of elements. So, if a 12-element Yagi gives an optimum array length of about 3.2l, the expected gain is ranging between 12.5-15.8 dBd. In all cases the Yagi wins against the log periodic.
Adding more than 15 elements on the log periodic does not significantly modify the behaviour of the circuit. However it is possible to get a higher gain tilting the elements towards the sky what increases the gain of 3 to 5 dB. But we don't reach yet the gain of the equivalent Yagi.
The fight of log periodics against its competitors is thus lost in advance.
Log periodic vs. Quad
The status of a log periodic is still more critical if you compare it with a quad. Even placed close to the earth (say the base 2m above ground), a 2-element quad will be a good DX performer and will reach far countries much easier than a 3-element Yagi for example. This comes from the quad design itself that takes advantage of both a very low takeoff angle and to its more "powerful" loop measuring a full wavelength long instead of 1/2l in the case of a beam's radiator.
A 2-element log periodic does not exist, it is called a parasitic array ! It is thus difficult to compare designs having so much differences. In all cases, erected 10m high (30') a quad will give performance to make a Yagi green. So imagine adding 10 more elements to this quad... The Yagi is under cover but the log periodic gave up the fight, Hi!
Log periodics have thus still an hard life in front of them if they want to fight on par against directive arrays like quads or even Yagis.
For more information
Cubical Quad Antenna Calculator, QSL.Net
The ARRL Antenna Book and related books
RSGB has also published tens of books about antennas
Antenna manufacturers and dealers