To help you selecting the antenna the best suited to your needs and space available you will find hereunder a practical data summary 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.

Standing waves, resonance and harmonic

When sound and radio waves meet... they speak the same language. If an aerial is built to perform on specific wavelengths that mean that it also accepts to be at resonance at many other wavelengths, shorter, called 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 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 4^th harmonics. This notion is essential because most antennas work with harmonics what permits to amateurs to work the same antenna on several bands, themselves harmonically related (40m, 20m, etc).

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 up them from in the case of a receiver). As the aerial is powered, electrons are moving inside the conductors, generating current and inducing voltage. These principles obey to the laws of electromagnetism. To work properly without interferences an antenna must be in resonance with the current and induced voltage, the current representing arbitrarily its radiation pattern. If your antenna is not tuned on your working frequency most of your signal will be lost, transformed in heat and will never reach the antenna radiator. To prevent this phenomenon and made your antenna resonant the aerial has to present an accurate length calculated from the working wavelength, the type of aerial and some technical data 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 longwire can rapidely become huge, requesting between 20 m and more than hundred meters of free space. So to avoid huge installations, there is another way of working, using a quarter-wave (1/4l) aerial. To cover all HF bands from 10 to 160 m, its maximum length is about 40 m or twice as shorter (20 m) if you are not specially interested in the top band (1.8 MHz). The other solution will be to fold the dipole on itself or to form a loop. We will review these designs on next pages.

To work properly on a frequency the physical length of an aerial must be in resonance with the current (I) and induced voltage (V). The left drawing displays an example of harmonic antenna cut at 1/2l. When erected vertically this aerial becomes a basic Marconi as displayed at right. It is usually a quarter-wave vertical. But deprived of its "second half" it will work properly only associated to a perfectly conducting ground (radials or ground plane) producing a mirror-image of the aerial under the ground to reproduce an antenna working at 1/2l.

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 or pratically underground. 

When working on the lower HF bands, a 1/4l vertical reaches with ease 10 to 20m high, when it is not itself erected on a tower a few meter high. If some of them (poles) keep a stealth profile they become however harder 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 trap coils, quarter wavelength stubs and radials. But shortened this way they become sometimes bulkier and, worst, 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 be resonant and 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.

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 surface of the earth to avoid all accident. The other solution, very appreciated, is erecting the vertical over 1/2l 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.

The subject being very interesting to understand the earth influence on antennas, I suggest you to read the next page devoted to "mystery of radials", in which I explain what are the effects of the ground, the utility of radials and what should be their specifications.

Note that a 5/8l vertical performs better than any vertical cut at 1/4l or 1/2l. It displays 2 symmetric lobes (view from the side) and has 1.2 dB more gain over the 1/2l vertical, so about 2.1 dBd. 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 2m and 70cm.

Radiation pattern in the vertical plane of a 5/8l ground plane compared to a 1/4l vertical. The 5/8l displays a main lobe splitted in two parts, one firing at 60° the second one horizontally with 1 dB more gain than its challenger. Document Cebik.

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 or stainless steel 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. 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!

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 longwires 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.

Next chapter

Longwire