Small mythology about radiation patterns
By Miguel R. Ghezzi (LU 6ETJ, Argentina)
Diverse antennas radiation patterns are well descripted in conventional technical literature, but in spite of it, an erroneous interpretation of explanatory figures, which frequently accompany theory, produces errors on conclusions.
With regard to the horizontal radiation pattern everything seems go well, but on the vertical radiation pattern is where the wrong concepts seems appear. See a typical handbook illustration, for example:
Above we see a half wave dipole vertical radiation pattern showing a main lobe with radiation angle of about 30 degrees, but usually is also accompanied by the pattern for of 1/4 and 1 wave lenght over ground heights (left figures).
Paying attention to the corresponding drawing, it is clear when increasing height over ground the only one lobe becomes two for 1 wavelenght height.
Simple book examples finish there and it is not made enough effort showing that, as you increases height over ground this phenomenon repeats in such way that the lobes go multiplying, until, for "many wavelenghts" heights, these lobes becomes very numerous.
In which cases the antenna is "many wave lenghts" over ground?, clearly in VHF or UHF, it is not so common on 80 or 40 m, but equally there are some cases where hams can spread their dipoles among great height buildings.It is "Vox Populi" that "a low radiation angle makes possible long distance contacts" but this statement without those accessory considerations that should have, simply become a error, rather a great error...!, especially in HF, although also in VHF.
It doesn't exist "one lobe of radiation"...
We have seen many lobes exist, so many as 1/2 waves lenghts height there are between antenna and ground, in this case, what is the sense of sentence "the radiation lobe of the antenna"? or this other one: "to lower the radiation lobe", or such manufactured antenna "has a lower radiation lobe..."
Evidently they lack sense, on the other hand it is correct to think of some method by means of which we can make those interesting radiation lobes be increased to expense of those that don't interest us. On this way, is licit to affirm that "such antenna concentrates its radiation in those lobes of smaller angles" or understand perhaps we have thinking on "Free space radiation lobe" and not "Real" lobes.
In VHF-UHF like in HF the more important factor to get low lobes it is the antenna height, if we carefully see the examples we shall notice when increasing antenna height, more radiation lobes not only appear, but the first one and lower of all they have a smaller elevation angle, a lot smaller as much as higher antenna is.
Up to where I know, this phenomenon is very little well-known in my local radio amateurs circles (in fact, I have never seen it in my amateur literature). All of us know that increasing height in VHF, for example, allows us to reach longer distances, especially by getting a more distant radio-horizont, but it is not so well-known that it makes possible the irradiant system aims energy in lower angles.
This happens because the antenna vertical radiation lobes became of the vectorial composition of the direct field with reflected field on ground. It is an error to believe that ground reflection doesn't exist at high frequencies, this misconception is generally derived of the association with the propagation phenomenon for terrestrial waves, that only takes place in low frequencies. Reflection obeys different laws and takes place between two different dielectric constant material boundaries, this way, the rays reaching ground are reflected. Making one analogy, we all have seen that, for certain incidence angle of the light, the more trasparente glass behaves as a mirror.
You can see a detained analysis of this principle in the article: Analysis of the normal propagation conditions on FME-FUE systems. in this web site.
A better approach
Given a certain height it is possible to design antenna systems that concentrates the available energy on those lower radiation lobes. For example, in VHF/UHF is common to use in phased dipole arrays. This system has the property of dimishing energy of high "real" lobes and to take advantage of channeling it toward those having lower angles, something like placing a plate above a lamp for not wasting light directing it toward the ceiling.
In the following figure we can see the differences between one vertical dipole pattern and four vertical colineal dipoles which were modeled on CAD for a frequency of approximately 100 MHz at 40 meters height at time to carry out the one I design for broadcasting FM use.
In this case the plane of real ground has been substituted by ideal ground plane to legibility of the radiation lobes. We see, in passing, the certainly and confiability in the results we can predict the behavior of these standard antenna systems. Notice decreased intensity lobes higher than 30 degrees elevation .
It is convenient that the is antenna of HF to great height?
From a general point of view we can affirm that it is right. The fact of having many radiation lobes directed toward different angles increases the probability that some of them coincides with the good one for a certain skip distance. But only "from a general point of view". Now, the directive properties of an trasmitting antenna is identical to its properties like receiving antenna. This means that our antenna also has one bigger probability of receiving signals coming from areas that don't interest us, and, if over one or more of them it is developing an electric storm, what we will receive of there it will be noise of those storms which, naturally, interfere signals we are attempting receive by the lobe that favored the contact.
It is convenient the antenna has a pattern with preponderance of the low angles?
This is another common place. "generally speaking", to achieve more distant jumps it can to be affirmed that yes, but the statement "generally speaking" quickly becomes "a law" and it is common to listen that the "best antenna" is that has "lower launch angles". This neither is right, because although it is considered that angles between 3° and 25° are better for long distance communications it doesn't mean always that is true. Sometimes it is possible that the best angle for a communication is 5°, while another one could be 20°, 45° in a third one, or even more.
For that reason the best antennas, will be naturally those allows us to carry on contacts with the best possible signal quality. The lazy ham may be happy with an antenna that on the average favors certain long distance contacts, but the most venturesome won't hesitate one second taking advantage of the most favorable angles, which well can come of the simple and ordinary dipole.
It can also be mentioned that in many situations will be convenient a high angle (inclusive near 90 degrees), especially in 160 and 80 meters where the reflected signal from above it facilitates the local communications. In 40 m, although the critical frequency almost it is below 7 MHz, equally they can take advantage those high angles (although a little less than 90°) with the same purpose.
A simple but extraordinarily effective solution can be achieved by means of a simple pulley to vary the height over ground of our humble and fraternal dipoles. If you observe the radiation lobes for heights located between 1/2 and 2 wave lenghts will see the interesting control capacity achieved with this method. So much as of an onerous ful size directional mono band. It is not to waste...
In VHF inclusive, where is supposed that a lower angle will improve to focus the energy on receiving antenna, it is possible get benefit with high angles at certain distances. It is known the most frequent communication way to achieve contacts beyond the optic path is the one denominated "tropospheric scatter". This propagation way, very dependent of climatic conditions, makes possible to carry on frequently contacts until about 500 km and often much more, it is based mainly in that the radiation reaches areas of the sky with disturbances responsible this way of propagation. Only one antenna, with very low radiating angle will do these possibilities are wasted when distances involved are smaller than the possible maximas for this method.
Other "quasi-myth": "The antenna gain"
Listen a statement similar to the following one "the Yagi of three elements has a gain of 6 dB" is very common. If somebody say that an "ACME car has a speed of 100 km/h", we would realize immediately that the sentence it fails a little. The right thing would be to affirm that "maximun speed of an ACME is 100 km/h", for example.
We all know the Yagi gain is given for the direction aimed, but we are thinking of azimutal direction. What about direction in vertical radiation angles?. On which vertical angle the Yagi have that gain...?
For a given vertical angle, is it possible a simple dipole has more gain (in the same azimut) that the Yagi?
The quasi-myth is to believe the antenna has a front fixed gain and, if our corresponsal are on that direction, it let us to get the 6 dB's. That would be only true if remote station is aimed on both directions, the vertical and the horizontal one ...! (or that it is exactly at the jump distance to the one that the main lobe arrives).
Let us compare the vertical radiation pattern of three antennas for the 20 meters band (below figure) on ideal ground for drawings clarity. We have carried out models by assisted design with ELNEC PC program, and I have superimposed resultant radiation patterns in a single figure for comparison.
- In red the one corresponding to a three elements full size monoband Yagui, mounted half wavelenght from ideal ground.
- In green an horizontal dipole at one wavelenght of height.
- In blue an horizontal dipole at two wavelenghts of height.
It is easy to see that on an elevation radiation angle about 50° already the dipole at 1l of height has the same gain that Yagi, while in such a low angle as approximately 5° dipole at 2l of height it leads it in a dB's couple. Ok, it is unjust to compare the Yagi at low height with a dipole at quadruple heigth, but will be easier to mount an "inverted Vee" on a 40 m height tower that a "full size" Yagi...
(We don't neither forget that directional antennas presents their own additional advantages on reception due to the important decrease of the received noise from directions that don't interest).
Then, we see that a "humble dipole" can afford to the experimenter ham get goods contacts.
Phenomenons never kept in mind...
There are phenomenons that are never kept in mind when to experience or install our antennas.
For example everybody knows when they use their VHF handie, has to look for the best position for their antenna. That can mean to move it some lateral or vertically centimeters, perhaps to incline the antenna. The signal variations that take place, are dramatic, many times in order of the 15 or 20 dB ...
How it is possible that, at the time of to install the antenna in our tower, only consideration be the highest possible placement, and as only alternative installation of a directional one, leaving aside one experience so significant as which is changing the the antenna position when all of us can see it in normal handie's procedures?
Why not to try mount an omni on an auxiliary arm attached to rotor that takes charge of making that habitually we do with our hand on handies or with the clutch on the car mobile station?
Rise or lower some meters our quarter or dipole with a simple rope well would be a amazing experience...
I believe enthusiast hams will find in these paragraphs a series of provocative initiatives to test when the good spring time arrives. I also believe it is possible to deduce of them that a radio amateur station whose owner remembers that 90% of its capacity is antenna dependent, will find that to install irradiants for diverse propagation conditions are a way to operate that makes honor to our better traditions, allows us to experience and to learn and for mainly to listen well, something that any amplifier of 1, 2 or 1000 KW will be able achieves never...
73's and DX...
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