Good antennas
A beginner radio amateur finds it hard to orient himself in various antenna designs. Antenna books and the internet are full of various constructions. Good guides on which antenna to choose are the results lists, which indicate the type of antenna.
If we do not buy a ready-made antenna, we must know which dimension or material has a significant impact on the resulting properties of the antenna during construction. Personally, I prefer years-tested antenna designs or antennas with direct feeding via coaxial cable because:
- every matching circuit has certain losses
- due to the absence of measuring instruments, it is easier to tune one thing (the antenna) than two things (the matching element and the antenna)
- antennas with an impedance of 50 ohms at the feed point are broader bandwidth than the same type of antennas with lower impedance and matching circuit
- antennas with an impedance of 50 ohms at the feed point are much less sensitive to surrounding antennas and other objects
- simpler construction
- … however, the disadvantage is slightly lower gain and worse F/B (front-to-back) ratio
Simple HF antennas
A simple proven antenna is horizontal half-wave dipole. When fed in the center, the impedance will be close to 50 ohms. Its value depends on the height above the ground and surrounding objects (buildings).
The length of the half-wave dipole is calculated as: l = 150/f [m;MHz]
In practice, the length will be slightly shorter, which we will use for final tuning of the antenna in the given environment (we shorten it). End insulators can be purchased or made from old thick coaxial cable with Teflon dielectric. The wire must be strong enough to withstand the load.
Given that coaxial is an asymmetric feed and a half-wave dipole fed in the center is a symmetrical antenna, their connection should be solved with a 1:1 balun. Proper construction of the balun requires suitable materials and winding techniques – if these conditions are not met, balun it will not perform its function correctly. We can also use commercially available baluns, but for low power or portable antennas, it is not recommended to use a balun.
In higher bands, we can replace the balun with several turns of coaxial cable just below the antenna.
A popular shape modification is the dipole in the shape of an inverted V.. It requires only one high mounting point and part of the energy is also radiated with vertical polarization, which is advantageous for DX connections.
Multiband dipole antennas offer a compromise option for operation on multiple bands. They are realized with traps (e.g. W3DZZ), tuned with a tuner (e.g. G5RV, Zeppelin), multiple parallel radiators or other feeding methods (FD-4).
A vertical quarter-wave radiator is an excellent DX antenna.. To function properly, it requires a system of counterweights - radials. In the case of underground (buried in the ground or placed on the ground), the more and the longer - the better. A truly effective system consists of dozens to hundreds of radials. The advantage is that the radials are unobtrusive and do not have to have a resonance length.
Unlike elevated radials, which must have a resonant length. Their advantage, however, is that we can achieve high efficiency with just one to four radials. The easiest to implement are two radials, which we first tune to the band as a dipole or inverted V at a low height (2-3m above the ground is sufficient). We then add a quarter-wave radiator and check the resonance and matching of the antenna. From the 40m band onwards, we can advantageously use telescopic poles made of fiberglass, making the antenna implementation very simple.
An excellent DX antenna for the lower bands is an inverted L antenna.. It is suitable where it is not possible to implement a quarter-wave antenna with full height. You can find a description of such single- and multi-band (160m, 80m and 40m) antennas at www.160m.de.
Material: purchased in local electrical installation shops, Amidon toroids at www.awas.sk or www.ges.cz, components or complete antennas also at www.ok5im.com
“From which antennas can not expect much?”, is a common question. These are mainly very short vertical antennas (height less than 0.1 wavelength in the given band) and various substitute antennas, such as balcony antennas.
The requirement to work on multiple bands can be solved with a multi-band antenna on a common boom. However, in home conditions, it is the most feasible directional antenna is a 2-element yagi. The material is relatively available and the construction requires only common tools - a saw, drill, measuring tape, screwdrivers, and a set of wrenches.
Such antennas were described by Martin DKZB at: http://www.DK7ZB.fox28.de/start1.htm
For the 10m band, it is possible to modify a 3-element yagi to 50 ohms with a betamatch and a 2.4m boom, which can be purchased relatively cheaply. Its impedance is about 30 ohms and can be adjusted to suitable by simply modifying the lengths of the elements. VSWR. I used this antenna for two years with excellent results.
Also structurally simple is HB9CV, colloquially known as “hábéčko”. An excellent description of the genesis of this antenna appeared some time ago on the OK2KKW website. http://www.ok2kkw.com/HB9CV/HB9CV_1969.htm . Opinions on HB9CV are varied - some radio amateurs had problems with it. HB9CV is particularly interesting in terms of high gain and large F/B ratio.
moxon beam has an advantage over these antennas in smaller dimensions, but requires thoughtful mechanical construction. It is more likely to be used by low band contesters, as it can be realized as a fixed wire directional antenna with electrically switchable radiation direction.
spiderbeam is a multi-band multi-element antenna, which excels with a number of good properties. Due to its low transport weight and dimensions when disassembled, it has immediately found favor in expedition operations, but it is also an excellent antenna for home use. Home construction is also possible, but its commercial price is not so high that it would be worthwhile to realize it 'on the knee'.
The trapped tribander is a very popular antenna and many manufacturers include it in their range. Home construction is very demanding, as the tribander is electrically and mechanically quite a complex antenna.
Rotators are an expensive matter. The principle is very simple: a slow-speed motor with a gearbox. The problematic part is acquiring a suitable gearbox. Larger rotators are supplemented with an electromagnetic brake, which is a big problem when manufacturing 'on the knee'.
Surprisingly good results can be achieved with the AR-300XL rotator. The advantage is the very high-quality and sturdy materials from which the rotator is made. If we eliminate vertical loading and misalignment, the rotator can also turn a smaller HF antenna. This requires the use of two bearings and a lot of thought, but it is possible. Some pieces have problems with azimuth indication; it is ideal to use independent rotation indication.
Professional lattice masts are not available for novice Hams. On a flat roof, we can easily realize a mast - a tripod. We secure the structure to weights that we make by pouring concrete into old tires. Be careful with the weight so that the roof can support it.
A fixed mast with a height of about 10m can be made by connecting two long scaffolding pipes. We will dig the foundations into the ground to a depth of at least 1m, into which we will install the first scaffolding pipe. After 120 degrees, about 10m from the base of the mast, we will dig a trio of foundations for anchoring. This can go into the ground if we install short scaffolding pipes and secure the anchoring up, it won't bother anyone. After the foundations have set, call your friends, make a low scaffold, and prepare the second pipe with the antenna. We lifted such a system with three people without problems, but it would probably go even better with five.even better.
The construction of masts is a challenging thing; it's better to over-dimension everything than to wipe away tears later. Don't forget about grounding.
Simple VHF antennas
To embark on VHF to contest with antennas like ¼, ½, Slim-Jim and similar ones is pure nonsense. The four most commonly used types of antennas are: yagi, quad,Quagi and parabolas.
Yagi antennas are popular for their good electrical parameters and simple mechanical construction. Thanks to this property, they can be made detachable, which allows transport to temporary stations.
Here too, the fashion wave works - very popular are yagi DK7ZBwhich are gaining popularity at the expense of older models like F9FT, DL6WU, DJ9BV, PA0MS, Y23RD, and others. When following the construction procedure, the antennas work excellently. The most challenging part is making the DK7ZB match from coaxial cables. There are many cables on the market that are not the most suitable for this purpose; it's worth visiting a few stores to find the best in terms of solderability and flexibility. The 50 and 28 ohm DK7ZB antennas are also suitable for beginners; avoid the 12.5 ohm ones as they are very sharp and critical of the surrounding environment.
Quads despite their outstanding electrical properties, have not spread much. This is due to their mechanical complexity. A proven antenna is DL7KM "octagon, double quad". The antenna is relatively broadband, has an impedance of around 50 ohms and a gain of 7-8 dBd. At 2m, the 7-element quad according to GW4CQT was popular at one time.
quagi is a hybrid antenna that results from combining elements from a yagi (directors) and a quad (radiator and reflector). The advantage is a higher gain than that of a yagi of the same length and a higher impedance at the feed point with only slightly more complex mechanical construction than a pure yagi. Such an antenna was used, for example, by W5UN. http://web.wt.net/~w5un/w5unqu97.htm
Parabolic antennas dominate higher VHF bands.. Making a multi-element yagi with the same gain as a parabola is very demanding on accuracy. In the case of using the factory dish, or of a parabolic section (e.g. SAT, WIFI), it is enough to make a suitable irradiator and set the antenna. Proven irradiators are designs according to DB6NT, DL4MEA and others.
Due to the much lower weight, dimensions, and wind resistance, VHF antennas put less strain on the mast and rotor. Smaller antennas can also be rotated manually, which is very operational.
Since the radiation pattern (especially of longer) VHF antennas is narrower, it is worth spending time on precise azimuth adjustment. Due to surrounding objects, such as the mast, the mechanical axis of the antenna may not align. For this purpose, it is possible to directly use signals from the band, beacons, or other stations to calibrate the antenna rotation.
Coaxial cables and connectors must be perfect.Many beginner VHF amateurs underestimate this fact with the words 'it works anyway.' The RG-58 coaxial cable is unsuitable. If you need a thin cable, for example due to weight, get inspired by the coaxials used in WIFI technology. Preferably choose longer versions of connectors that better capture the cable strain. It is better to buy a quality connector for one hundred and fifty crowns than five lousy ones for thirty...
You can find many of the mentioned antennas here - at www.CQ.sk.
Glossary
impedance at the feed point - it is the impedance at the antenna radiator without any matching circuit.
half-wave dipole - a simple resonant antenna balun - impedance transformer. transformer, which additionally ensures the transition between balanced and unbalanced loads.
trap - an LC element added to the antenna for the purpose of electrically lengthening or shortening a component, thereby achieving resonance.
tuner - an antenna matching element that ensures impedance transformation.
tribander - an antenna capable of operating on three bands.
rotator - a device for rotating the antenna.
boom - the supporting part of the antenna to which individual elements are attached.
lattice mast - a mast in the shape of a pyramid, very sturdy.
Slim-Jim - a simple VHF antenna.
Match - antenna matching circuit.
Antenna radiation pattern - usually a graphical representation of the directional distribution of energy radiated by the antenna.
Next time: how to make contesting easier