From Long wire to Yagi
Basics of aerials (I)
When I started to listen to the ham traffic I was rapidly confronted to technical considerations that pushed me to learn basics of propagation and aerials : how to improve the signal received from far stations, how to suppress QRM (man-made noise), what type of aerial to install, how high must it be installed, what length and what feeder to use, does it create interferences while transmitting, etc, as many questions and many others that were unsolved for some times (and mostly solved if you read all pages of this web, Hi!).
It quickly appeared that the sensitivity, selectivity and strength of signals was not only depending on the receiver quality but also on the antenna design and other factors (circuit, propagation, etc). In short, at the two extremes of performance there are at one end long wire antennas and verticals, that are usually omnidirectional with practically no gain in direction and EM field intensity, and at the other end array antennas like beams that are very directive and display a high gain in a specific direction. However, we will see that all these designs have advantages and drawbacks. So, we cannot say without more accuracy that the first design is less performing or efficient than the last one. This is already a good news !
To help you selecting the antenna the best suited to your needs and space available, you will find hereunder some useful comments about most used aerials. In other pages we will take the time to compare the Yagi with the quad and the log-periodic and try to extract the best elements of each design.
Resonance, standing waves, and harmonics
When sound and radio waves meet... they speak the same language. First of all correct a rumor often heard or read here and there. If an aerial is correctly fed that does not mean that... it is at resonance on its working frequency, or say in other words that it displays standing waves between its nodes.
To be in resonance means that the impedance displays no reactive component; the feeding impedance is identical to the radiation resistance. It is what occurs when you replace your antenna with a pure resistance.
The state of resonance only represents the matching between the antenna impedance and the one of the feed line or the transceiver. Your antenna will perfectly works if it is not resonant, shows a high SWR on the feedline, and coupled to an external antenna tuner. Try it using a simple long wire or a dipole if you don't believe me... So do not confuse the antenna itself and the feed line.
most antennas work in state of resonance and usually display
standing waves. When a wave is reflected from the end (the
node) of an antenna, the number of standing waves
"travelling" along this conductor is equal to the antenna
length divided by 1/2l.
Thus, for an aerial 1
Thus, for an aerial 1l long, there are 2 standing waves; if it is 3/2l long, there are three standing waves, and so on. Each multiple of these 1/2l segments will be also resonant at the same frequency as the fundamental frequency l.
Such an aerial, working at one or more multiples of 1/2l is said to operate at a harmonic. The standing wave at which it operates is called the order of the harmonic : at 2l for example there are 4 standing waves, the antenna operates on its 4th harmonics. This notion is essential because most antennas work with harmonics what permits amateurs to work the same antenna on several bands, themselves harmonically related (40m, 20m, etc); these are multiband antennas.
Note that in reality standing waves do not exist. This is a concept, a mathematical model that helps us to explain how work a resonant system but it is far to reflect the reality of things, like the real nature of waves.
Half-wave and quarter-wave aerials
Powered by your receiver or your transceiver, an antenna in nothing else than an electric conductor able to radiate waves into space (of pick them up from in the case of a receive system). As the aerial is powered, electrons are moving inside the conductors, generating current and inducing voltage. These principles obey to laws of electromagnetism.
To work properly, a transmission antenna must display a low SWR. If your antenna is not tuned on your working frequency, current can drastically increase in your system, and most of your signal will be lost, transformed in heat and will never reach the antenna radiator. Conversely, it will dissipate in your system where it may damage wires, coils and some other components...
To prevent this phenomenon you must cut your antenna so that it is resonant on the working frequency (so that there is a resonant state between current and induced voltage). This state of resonance depends on the type of aerial and some technical considerations as the conductor diameter and length, the feeder impedance, the use of insulators and distance from nearby objects.
For wavelengths longer than 20 meters, a 1/2l antenna can rapidly become huge, requesting several tens of meters of free space to tighten a dipole in good conditions. So, to avoid huge installations, there is another way of working, using an aerial twice as short, 1/4l long, and taking advantage of the earth properties to create the mirror-image using radials. The antenna is then placed vertically above ground.
To cover all HF bands from 10 to 160 m, a quarter-wave vertical displays a maximum length of about 40 m or four times as shorter (10 m long) if you are not specially interested in the low bands of 160 and 80m. We will take the time to review all these designs.
The vertical antenna
To reduce the necessary space to wire antennas, the solution is to stand up your wire and transform it in a vertical antenna. Defined as a Marconi aerial, in its basic configuration it is a quarter-wave pole which lower end is close to the ground.
But to work on lower HF bands, in theory a 1/4l vertical should reach 20m high and more, when it is not itself erected on a tower a few meter high. If some of them (poles) keep a stealth profile, they become rapidly huge and hard to handle, even if some manufacturers made them in light material like titanium (e.g. Titanex).
There are hopefully solutions to reduce their height from 30 to 50% in using one or several trap coils, quarter wavelength stubs and radials. But shortening this way their length, they become sometimes bulkier, but worse, all these artificial reductions alter the theoretical performance of the aerial, mainly its radiation pattern and its ability to pick up weak radio waves.
Theoretically, to work on its harmonics a vertical needs a perfectly conducting ground that produces a mirror-image of the aerial under the ground, which simulates an half-wave aerial; this is the ground system. It can be elevated (in the air) or buried. Of course a vertical can work without any radial, but you will quicky note that its performances will be far below the ones of a vertical using a good radial system.
The soil being rarely conductor as we want, in practice an artificial ground is built adding some tens of 1/2l radials that are laid evenly on or just under the ground surface to avoid any accident.
The other solution, very appreciated, is erecting the vertical between 1/2 and 1.5l above ground to reduce the ground effects (50% less at 1/2l high) and to connect to its base four or more radials 1/4l long called in this case a counterpoise to simulate a ground plane, hence its name.
In all cases, and whatever can write some incompetent manufacturers, remember that a vertical antenna shorter than 1/2l always requests a ground system to be efficient.
The subject being very interesting to understand the earth influence on antennas, I suggest you to read the article the mystery of radials, in which we will explain what are the effects of the ground, the utility of radials and what are their specifications.
In analyzing the radiation pattern of verticals, we note that compared to a l/4 vertical, a vertical cut at l/2 does not show much improvement. For short, in HF it is not necessary to use a vertical longer than about 8 meters high.
Set up at ground level, a vertical antenna shows a radiation pattern in the elevation plan close to 0° : its fires horizontally all around the radiator with a radiation pattern in torus or ring shape. If it is good for DXing, RF signals will bump into nearby obstacles, and it is one of the reasons for which it is recommended to erect a vertical high above ground to avoid signal loss. The other reason is to reduce ground effects and the need of installing many radials.
But what means "high" over ground ? Is 2 meter enough or do we deal with 10 meters high and even more ? The question is maybe innocent but very interesting.
Indeed, as we see below, we can improve the directivity of a vertical antenna in elevating it substantially above ground. So, erected respectively from 0.5m to 6m, 15m and 25m high, up to 5 narrow radiation lobes gradually appear below 75° of elevation.
Radiating in all directions, of course and contrary to a directional antenna, the F/B signal ratio will not change but these additonal lobes will improve the operator abilityy to contact DX stations, specially via the F2 layer.
Note that a 5/8l vertical performs better than any vertical cut at 1/4lor even at 1/2l. Their gain is also higher than the one of a dipole.
A 5/8-wave antenna displays two symmetric lobes (view in elevation) and has 1.2 dB more gain over a 1/2l vertical, so about 3 dBi. Hustler 27J, Sigma 5/8 or Maco V 5/8 are all 5/8l verticals. These models are however discontinued nowadays but are probably available on the secondhand market. Today most 5/8l verticals are designed to operate on 2 m and 70 cm bands.
Among surviving HF models, Zero-Five Antennas sells three monoband freestanding 5/8-wave antennas. Their gain is 3 dB over a dipole.
Mobile vertical antenna
If you have a limited outdoor access and want to work on HF, you can also install a vertical of about 1.5-2 m high (5-6'). Not those small and fragile hamsticks and other screwdriver antennas offering a low efficiency and mainly dedicated to local (QRP) QSOs but the rugged and larger models that weight up to 5 kg (~10 lbs) offering a high unload Q.
High-end models offer an overall Q up to 580 thanks to the use of a large conductor, an air-wound construction, large spacing between turns and the best insulating material. Of course their price is proportional to their performance.
Made of aluminium (light) or stainless steel (heavier) tubing, such antennas are resistant, weatherproof and can be fixed on your car with a "quick disconnect" kit, on a tripod outside your van or on the hardtop, on a balconery or even on the building structure (on the roof or on the chimney and upwind in this latter case). You can also build a removable support to fix temporary the base on a window or on the frame. Some models are fixed, others have a spring mount or are turnable like a rotary dipole.
These small antennas are first designed for mobile installations and are center loaded. However we cannot ignore that in restricted places amateurs will try to use them for portable operations or at home. Used with a base station at ground level, you can place the antenna in a tripod or on top of a small mast.
You can also place it in an adjacent room or in the attic (not recommended), on the external wall, on the chimney or on a balconery. In all these conditions far to represent the center loaded system of a car, you may need to install at the base a counterpoise made of several 1/4l radials (at least one per band). Without radials you will probably experiment difficulties to fine tune the antenna, you will get a high SWR and loss up to 50% of your ouput power.
These small antennas are first of all dedicated to mobile operations where they are really very appreciated. Being quite short this kind of aerial works best for ragchewing in local QSOs on 80, 75 or 40m band. If you work from a high point and if your system offers a good efficiency, at 100 W you can even work DX stations with a little luck, even if such mobile antennas are not know to be DX chasers.
Here is a example of their performances. If you work in relatively open fields mobile users fan of DX confirm that they are capable to work easily stations in a radius of 3-4000 km (New York to California, Arizona to Hawaii or Brazil or to cover all Europe) as good as a G5RV dipole tight at low height or a 6m high vertical equipped with traps. For example, many american and canadian amateurs using Hi-Q antennas contacted bare foot over 30 DX entities worldwide in SSB (K or VE to CO, PY, GB, ON, UR, TI, 5X, ZS, VK, ZL, etc). Most of them received a signal report 58 to 59 and even 59+ using 1 kW. Their correspondent usually doesn't believe they are working mobile! (Of course, due to their limited length, use at 100 W or less in SSB, these antennas show limited capabilities).
To work on multi-bands these kind of antennas use a more or less large loading coil and an optional capacitance hat. Used together they help in balancing the resistance and capacitance of the antenna to find a state of resonance at a given frequency.
The feeding-point impedance of a mobile antenna being quite low (~11 ohms), a matching network (L network or even a RF transformer) is usually necessary.
Like all long wires or verticals, such aerials do not necessary be physically the correct length for a specific frequency. It can be shorter or longer. In such cases the state of resonance will be achieved by using a matching box or an external antenna tuner. Some high-end models are equipped with an automatic tuner that changes the coil loading and adjust the top section of the antenna to get the lowest SWR on each band.
We will not extend more on this subject that requires, if we want to be complete, a dedicated article. We only tell a few more words when we will deal with portable installations.
Advantages and drawbacks of verticals
As we told, by design a vertical antenna radiates horizontally, displaying at grounbd level a radiation pattern in the E-plan like a donut or a tore around the radiator as displayed at right. However, erected a few meters over ground, low incidence lobes appear.
So, placed very high over ground, their low takeoff angle is interesting for DXing as the RF signal travels easily across long distances; it reaches immediately the highest ionospheric F2 layer located near 300 km aloft, skipping in one hop a distance of about 4000 km (6400 miles).
The vertical favors also ground waves or surface waves that propagate along the earth surface, close to the ground. The amateur works thus "at sight" or almost. These waves can reach distances up to 160 km or more during the daytime and go much farther at night. They are however subject to a high attenuation to reach distances less than 15 km at 30 MHz.
A vertical is thus mainly used by broadcasting stations and amateurs who like regional QSOs or DXing, and of course for discretional reasons when a long dipole or a directive an tenna cannot be installed.
A vertical is thus not suited to work very near stations, located closer to 150 km or so, where you need of a long wire, a dipole or a directive antenna. Personally, from Luxembourg I had many difficulties to work on the 10m band a close friend located in Belgium at 30 km away. The situation was still worse on the lower bands. His signal was very weak, barely audible, although I worked him without problem using a short dipole placed at low height thanks to its takeoff angle near 90°.
Drawback of their design, a 1/4l vertical placed at ground level needs radials (at least 12 wires of 1/4 or better 1/2l long on the lowest frequency) and thus some place left in your garden. The problem can be partly solved if you can erect the vertical over 1/2l above ground.
A 1/4-wave vertical used without radials has no ground to create its mirror-image. The ground being usually a poor conductor in terms of RF, a 1/4l vertical will perform poorly in such conditions. At least 50% of the output power will be converted in heat and lost in the ground.
Another drawback, when working DX stations and mainly pileups, due to its omnidirectional pattern, a vertical picks up all stations at 360° and not only the ones located in your fire angle, hence the feeling that there is a huge QRM on bands. This problem can be solved using a directive antenna that will reduce side lobes of about 25 dB and rear lobes of about 20 dB.
Due to its vertical polarization, this kind of antenna is much more sensitive to man-made interferences and QRM than a horizontally polarized antenna. So, if you experiment much QRM, try the dipole or the beam; this is a world apart.
The vertical keeps however some interest when a stealth or easy to setup installation is required, specially when it is made of light material, telescopic tubing, and displays a low profile, even equipped with one or several traps. Installed permanently, a vertical can be supported by only two or three set of guy-wires, and used for portable operations, it can be assembled and ready to work in 15 to 30 minutes, radials included if they are not too numerous. It is however more expensive than a dipole and of course heavier too.
At last, if you are looking for a low cost antenna to work the world in CW for example, the vertical is a good compromize. Due to its omnidirectional pattern, and its ability to sustain high power (in general) you will get easier your DXCC or even your 5BDXCC with a vertical than using a dipole.