Stacked Yagi antennas for VHF and UHF

This article is primarily geared towards the requirements of VHF/UHF amateurs who need a larger, powerful array antenna for terrestrial DX operation and special operating modes such as EME and MS. If you are a newcomer and have already gained some experience in VHF/UHF radio, you can achieve good results with just two vertically stacked 7-element Yagis for the time being.

What is the benefit of stacking VHF antennas vertically or horizontally or both, vertically and horizontally in a group? Stacking is used as a method to achieve more antenna gain in a preferred direction. Let Martin Steyer, DK7ZB, have his say on this question. He published an easy-to-understand article on this topic back in the late 1990s [1]. Read the edited version to find out what possibilities there are to increase the gain of an antenna system by stacking and what effort is required to do so.

Here you can find the article by Martin Steyer, DK7ZB, which was kindly made available to us by the amateur radio magazine "Funkamateur":

Source reference:

[1] Steyer, M., DK7ZB: Stacking of VHF Yagi antennas, FUNKAMATEUR 46 (1997), H. 5, pp. 602-605

From the standard to the exceptional

The 16-element array antenna has become the standard among radio amateurs. It is the most frequently constructed and used directional antenna in the 2 m band. Group antennas with 12 or 24, 32 or even 48 elements are rarely used. As the number of elements increases, the ratio of material expenditure to performance becomes increasingly unfavorable and the mechanical problems also increase. Larger array antennas have a high wind load and place high demands on the load-bearing capacity of the antenna mast, rotating pipe, support frame and rotor. The following questions arise during practical implementation: What is the correct stacking distance and how are the individual antennas correctly interconnected?

The dilemma of correct spacing

The question of the correct stacking distance cannot be answered right away. The correct stacking distance can only be determined in individual cases and for defined applications. Basically, there are two different limiting cases, whether for two antennas or a large antenna array. The distance required for a maximum possible gain of around 3 dB is relatively large. Furthermore, compared to a single Yagi antenna, increasing side lobes and nulls are formed in the vertical directional pattern. A large stacking distance also complicates the mechanical design. Although a significantly smaller spacing results in optimum suppression of the side lobes, the maximum achievable gain is then reduced to 1.5 to 1.8 dB. Using the example of two vertically stacked 7-element Yagis for the 2 m band, the dilemma with the different stacking distances becomes clear in three case studies. The Yagis each have a boom length of 3.0 m and a gain of 10.5 dBd.

Case 1:

• Stacking distance: 3.12 m
• Combined gain: 13.9 dBd
• Side lobes: +/- 30 degrees around main beam direction
• Suppressed side lobes: -8 dB

Case 2:

• Stacking distance: 2.46 m
• Combined gain: 13.5 dBd
• Attenuation of side lobes >12.5 dB

Case 3:

• Stacking distance 1.14 m
• Combined gain: 12.0 dBd
• Attenuation of side lobes: -40 dB

If you compare these three cases, the question arises as to whether it is worth all the effort for this marginal difference in gain.

Mechanics and physics in antenna construction

For mechanical reasons alone, one is inclined to give preference to a vertical arrangement. For example, four antennas on top of each other are far better in practice than four antennas in an H-cross, whose mechanical problems can quickly become so great that this form of construction can no longer be realized even with antenna lengths of 1.5 ʎ. In addition to the complicated design of the H-cross, the high moment of inertia can also be a problem. Here is a brief excursion into physics: During the rotary movement, the moment of inertia relates to the torque in the same way as the mass relates to the force of the linear rotary movement. The surface moment of inertia increases with the distance of the mass to be brought into a rotary motion from the center of rotation or the axis of rotation. This means that (even symmetrically) eccentrically mounted antennas have a much greater moment of inertia than vertically stacked antennas whose center of gravity coincides with the pivot point. That sounds complicated at first, but a concrete example from practice makes it clearer:

Have you ever tried to turn a large Yagi group in H-configuration by hand during a field day? Then you will know that, in contrast to a group of stacked antennas, this requires a lot of energy. With the cantilevered design of an H group, it takes considerably longer to get the mass moving in order to react quickly to changes in the antenna alignment. The rotor is subjected to much greater torsional forces (twisting in the longitudinal direction) during acceleration and braking. Consequently, the rotor must also be designed "one size larger". These are all reasons that speak in favor of exclusively vertical pitch.

Why vertical arrangement is the best choice

• Vertical stacking results in a wider horizontal directional lobe and a narrower vertical diagram.
• Horizontal stacking results in a larger vertical directional lobe and a narrower horizontal diagram.

Coaxial cable vs. two-wire line

In most cases, matching on V/UHF frequencies is done by using quarter-wave transformers. We will discuss only coaxial technique, although it is not necessarily advisable to use coaxial cables as connecting lines when interconnecting very large groups with long antennas. In their entirety, they can quickly reach a length of several meters with corresponding additional attenuation. In addition, low-attenuation cables are heavy and add a considerable amount of mass. For these reasons, specialists in large EME systems use self-made two-wire cables, which are considerably lighter and have less attenuation. However, you have to be prepared for an increase in SWR in damp conditions and nothing works at all in icy or hoarfrost conditions.

The technology of efficient power distribution

Coaxial matching splitter, also known as 3 dB couplers or -3 dB combiners, divide the supplied power equally (50%) between two antennas while maintaining the impedance. They make use of the fact that the characteristic impedance of coaxial lines depends on the ratio of the diameter of the inner and outer conductor. The properties of the dielectric, in this case air, also play a role. Basically, the cross-sectional shape of the conductors is arbitrary, which is why the cross-section of the outer conductor can be square even if the inner conductor is round. The advantage of this is that coaxial flange sockets can be easily attached with screws, which is not so easily possible with round conductors. The ratio of D/d, outer diameter (D) to inner diameter (d), determines the characteristic impedance Z of the arrangement.

From industrial quad distributors to DIY coaxial solutions

Industrially manufactured quad distributors designed for the 70 cm or 23 cm band are quite compact. As a rule, however, coaxial cables are more likely to be used for groups of two or four in the 2 m and 70 cm band, which result in fairly simple mechanics. DK7ZB also prefers cable adaptation, at least for 144 MHz, because of the unwieldy length of tube constructions. A little mathematics and commercially available cable standards make it possible to realize extremely inexpensive solutions in do-it-yourself construction.

Vertical stacking of two antennas

At the connection point, an impedance of 100 Ω must be present on each of the two lines coming from the antennas so that the impedance of 50 Ω required for the combined coaxial cable is achieved when connected in parallel. For this purpose, the characteristic impedance of the transformation cable is calculated according to the relationship $Z=\sqrt{Za*Ze}$ to 70.7 Ω.

A characteristic impedance of 70 Ω therefore results in a perfect match. However, it is now difficult to procure such coaxial cables, which are no longer manufactured. If you accept a slightly higher SWR of 1.13, it is easily possible to use 75 Ω cables. The length must be an odd multiple of ʎ/4 so that the transformation condition is fulfilled. The shortening factor, which varies depending on the dielectric, must also be taken into account. For solid polyethylene cables, V = 0.667, for cables with a high air content (H500, H100, Aircom, etc.) it is higher, usually between 0.78 and 0.85. The manufacturer's specifications must therefore be observed. For these reasons, cable lengths of 5x ʎ/4, 7x ʎ/4 or 9x ʎ/4 are used in practice, depending on the stacking distance.

For the wiring, it is important to keep the connecting cables as short as possible and to take the shielding (with the shielding length of any connectors used) into account when calculating the length. In any case, the usable bandwidth exceeds the limits of the 2 m or 70 cm band, so that dimensioning for the middle of the band is completely sufficient.

Precise cabling and careful mechanics

Another important point deserves attention: the antennas must be arranged in such a way that all radiating elements are excited in phase. This means that gamma lines, inner conductors of half-wave balun cables and coaxial cables for the DK7ZB feed must be on the same side (and at the bottom) of all antennas. Coaxial splitters are not worthwhile for the 2 m band if you can get clean solder connections for the matching cables. For 70 cm, cable transformation elements should have slightly higher but still acceptable additional losses. Only coaxial matching splitters are recommended for 23 cm.

In principle, the theoretical values for the gain are always higher than those actually achieved, because cables, plugs and connecting cables with soldered joints always involve unavoidable losses. Special attention should therefore be paid to them. When it comes to the mechanics of the connection points on the tubes of an H-junction, you can avoid high costs by doing it yourself. After final assembly, additional corrosion protection should be provided by spraying several times with plastic spray.

Extreme example: EME antenna at DL7APV

Array antenna consisting of 128 x 11-element Yagis for 432 MHz. Horizontal aperture angle 1.8°, vertical 3.8°, gain 33.6 dBd

www.dl7apv.de

Photo: Bernd Wilde, DL7APV (†)

Yagis for the highest demands

First of all, you need to think about which Yagi antennas you want to use to build your antenna array. WiMo Yagis according to DK7ZB are based on uncompromising electrical and mechanical technology and come from our own production ""Made in Germany"". The design principles of these Yagis are the highest possible gain with a high bandwidth and good sidelobe and return loss. The Yagis are designed for an optimum current profile. This results in a minimum required number of elements, because the antenna gain depends on the antenna length, not on the number of elements!

To avoid skin effect losses, only elements with a diameter of 8 mm made of a highly conductive aluminum alloy are used. This is a good compromise between minimum electrical losses and the possible wind load. With steel elements, you would lose more than 0.5 dB gain due to poorer conductivity. The elements are attached with long-term stability using UV-resistant polyamide element mounts, which show no change in the electrical data even after years. Direct mounting on the boom tube with metal retaining clips is associated with oxidation at the contact points and leads to a gradual change in the antenna data. The radiating element of such a Yagi is always a folded dipole with a Teflon balun, in a weatherproof, cold-welded, additionally foamed dipole box. The connection is made via N sockets.

Lightweight construction meets high performance

Flexa-Yagis have been developed and manufactured by the RS engineering office in Germany for over 30 years. The product design of these Yagi antennas takes a slightly different approach. The optimization principle according to DL6WU allows excellent data for gain, diagram, matching, bandwidth and low losses by avoiding extremely high element currents. Thanks to the use of thin stainless steel elements, Flexa Yagis are characterized by a particularly low weight. This in turn would speak in favor of their use in phased array antennas. The balun is mounted externally as a half-wave bypass from a Teflon coaxial cable.

Of course, the variety of products on the market is correspondingly large and other Yagi antennas are offered under the brands EAntenna, InnovAntennas, YU1CF Yagis and I0JXX Yagis.

Almost loss-free impedance matching

A so-called coaxial matching splitter or power combiner (3dB splitter) can be used to connect two or four identical antennas. The matching splitter transforms the characteristic impedance from the 12.5 Ω or 25 Ω of the parallel-connected antennas to the required 50 Ω with virtually no loss. Above 70 cm, phase matching should always be used for this task. In the 2 m and 70 cm band, you can also work with phase lines. You can choose the right accessories for all amateur radio bands (VHF, UHF, SHF) and a maximum transmission power of 2000 W from an extensive range of matching splitters and ready-made phase cables.

Antenna control for large array antennas

For conventional terrestrial amateur radio, you need an azimuthal rotor with which you can cover 360° in the horizontal plane. For special operating modes such as EME, MS and Aurora, an additional elevation rotor is required to be able to align the antenna vertically up to 90°. Especially if you want to move a large antenna array, the load capacity, torsional and bending moment of an antenna rotor are important criteria. Not only does the rotor itself have to be of a "larger caliber", but it will hardly work without a second, upper bearing. The lateral forces can only be absorbed by using a secong bearing support. This increases the mechanical effort and additional components are required for the upper bearing: a fixed pipe or even a smaller lattice mast, as well as a platform for the upper bearing. You can find more information on these aspects in our "Antenna rotor guide".

Essential mounting materials for antenna array

This list of mounting materials does not claim to be exhaustive. Here are the most important "small parts" that are probably needed for every antenna array construction:

Cross clamps and cross plates, mounting brackets, U-bolts, double clamps, pipe clamps, latches (counter clamps), standpipe clamps, bracing clamps, aluminum pipes, steel pipes, GRP pipes, mast caps, mast holders, mast bases and mast base clamps, half shells and clamping sleeves for GRP pipes, as well as plastic element holders for round boom pipes or rectangular profiles. Many of the metal products are available in galvanized or stainless steel versions.

Plan and implement now

If your plans for the construction of a group antenna are fully developed and the decision to implement this extensive antenna construction project has been made - then let us advise you! The WiMo team will be happy to listen to your wishes and ideas. We wish you every success with your antenna construction!

July 2024, Alfred Klüß, DF2BC