How can you maximize the gain of portable antennas on 144 MHz? Unlike a single Yagi, which would require a long boom and significant height above the ground, the group consisting of OK2VZE, OK2LOL, OK1ZHS used a 4×6-element Yagi array in the 2008 Alpe Adria Contest. OK2R also uses this type of antenna.
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Single Yagi / Antenna Array
The longer the antenna, the higher it must be positioned above the ground. If we consider a gain of approximately 16 dBi, this is achieved by a single Yagi antenna with a boom of about 8 meters. This requires a greater torque for rotation and, of course, a taller mast.
Advantages of a single Yagi antenna
clean radiation pattern
no need for antenna combining
quick assembly of a segmented antenna
Disadvantages of a single-element Yagi antenna
Narrow radiation pattern, particularly noticeable in the horizontal plane
Requires a taller mast, at least 7 meters
Difficult to raise and lower the mast with the antenna
Requires greater torque to rotate the antenna
Directional pattern and gain of a single Yagi antenna in open space
Directional pattern and gain of a single Yagi antenna at a height of 7 meters
An antenna array with a gain of approximately 16 dBi can also be achieved using an array of two, three, four, or more antennas. A larger number of antennas is used as a fixed antenna system with a wide radiation pattern.
Advantages of a two-antenna system stacked vertically
shorter antennas with a boom of about 3.5 meters are sufficient
the radiation pattern in the horizontal plane is wider than that of a single Yagi
the torque required to rotate the antenna is lower
a taller mast is required, at least 7 meters
Disadvantages of a system with two antennas stacked one above the other
The two antennas must be coupled
Raising and lowering the mast with the antennas is difficult
Directional pattern and gain of two YU7EF Yagi antennas in open space
Directional pattern and gain of two YU7EF Yagi antennas at a height of 8 meters
Advantages of a system with three or four antennas stacked vertically
shorter antennas with a boom of approximately 2.2 meters, or 1.5 meters (for four antennas), are sufficient
the radiation pattern in the horizontal plane is wider than that of a single Yagi
Disadvantages of a system with three or four antennas stacked one above the other
A taller mast is required, at least 8 to 10 meters
Raising and lowering the mast with the antenna is labor-intensive
The need to combine three antennas
Antennas longer than 1.5 meters must have a split boom for transport inside a car
Advantages of a four-antenna system in a 2×2 (H-shaped) configuration
shorter antennas with a boom of about 1.5 meters are sufficient;
such antennas can be transported inside a passenger car;
A shorter mast, approximately 5 meters tall, is sufficient;
easier to raise the mast;
acceptable torque for rotation
Disadvantages of a four-antenna system in a 2×2 (H-shaped) configuration
Use of an H-shaped structure
The need to combine four antennas
The radiation pattern in the horizontal plane is narrower and less defined than in other antenna configurations or a single Yagi
Directional pattern and gain of four DK7ZB Yagi antennas in an H configuration in free space
Directional pattern and gain of four DK7ZB Yagi antennas in an H configuration at a height of 5 meters
TIP – Countersunk-head nuts
Petr OK1FIG used countersunk nuts in his design, which allowed him to achieve a boom with no major protrusions at the bottom. In addition to looking better, this prevents the antennas from getting caught when loading them into a car
Choosing from antenna options
I assemble the antennas by myself. The examples of antenna installation demonstrate whether or not even larger antennas can be lifted. Sometimes, however, the wind is blowing, and even a taller mast can still be difficult to raise. I opted for a design consisting of four shorter Yagi antennas in a 2×2 (H-shaped) configuration, partly because I can transport these antennas inside a passenger car without disassembling them.
YU7XL, S53WW, DG7YBN, G0KSC, DK7ZB Yagi
YU7EF does not describe antennas this short; the shortest model is a 6-element Yagi. The S53WW design is interesting in terms of gain, but it requires a loop dipole, which is more difficult to build. The antenna is also more sensitive to its surroundings and the manufacturing process.
The short oblong YU7XL antennas are interesting in terms of the boom length-to-gain ratio. However, the three-dimensional construction complicates both transporting the antennas and assembling the quadrant.
The remaining DG7YBN, G0KSC, and DK7ZB Yagi antenna designs come down to the ham radio operator’s individual preference. I chose the design by DK7ZB. The 1.5-meter-long antenna has 5 elements and a gain of approximately 8.4 dBd. The impedance is 50 ohms. It is fed via a so-called choke balun made from coaxial cable.
TIP – Use of square profiles
Using a 20 x 20 mm profile for the H-frame support structure and the individual antenna booms simplifies their assembly. This can be done using flat T-connectors measuring 120 mm x 54 mm x 19 mm (purchased at OBI)
TIP – Using a cordless screwdriver
A cordless screwdriver speeds up the assembly and disassembly of the H-frame and the connection of the antennas. Stainless steel DIN 7991 M5x30 countersunk Allen screws and standard stainless steel M5 hex nuts are used.
Construction of the 5-element DK7ZB Yagi antenna
The actual construction of the antennas has been described many times. The elements are mounted on the boom in a black polypropylene holder using an M4 screw and a countersunk nut. They have a diameter of 10 mm; the boom is made of a 20 x 20 mm profile.
RG-174 coaxial cable and plastic tubes for the baluns
The radiator is secured with cable glands in a Z54 = KP 45 (Z54J) box. Inside is a balun made from Belden 8216 RG174/U coaxial cable. Its length is 34.3 centimeters, which includes the coaxial cable shortening factor (0.66). The length is measured between the ends of the braid—the leads to the radiator and the connector are not included in the length measurement, but should be as short as possible. An N-connector is used, which is attached to the boom.
Measurement of a 5-element DK7ZB Yagi antenna
Measurement of a single 5-element DK7ZB Yagi antenna
The antennas assembled in this manner were measured under provisional conditions with the following results
The third antenna was slightly fine-tuned by altering the shape of the solder loops connected to the radiator.
Measurement of an assembled DK7ZB 5-element Yagi array (144 MHz) for portable use on a 5-meter mast
SWR of 5-element DK7ZB Yagi antennas
SWR of 5-element DK7ZB Yagi antennas in a narrower range
Measurement of a 4×5-element DK7ZB Yagi array
SWR measurement of a 4×5-element DK7ZB Yagi array
Photos from the production of the DK7ZB Yagi radiator box and baluns
TIP – Filling the box with PUR foam
The idea comes from Vlad OK1VPZ. In addition to improving the enclosure’s water resistance, this also secures the radiator more firmly.
Construction of the H-frame support for four Yagi antennas
The H-frame also uses 20 x 20 mm profiles and flat T-connectors. These are also used to attach the antenna booms. It’s a good idea to use a square here to ensure the antennas are mounted precisely. Don’t forget to mark them; otherwise, you might mix up the individual antennas and end up with them out of phase instead of in phase.
Construction of the Support H-Frame for DK7ZB Yagi Antennas
Two threaded U-brackets are used for mounting on the mast.
Cost of the DK7ZB 144 MHz Yagi antenna quad for portable use
Estimated material cost
20 x 20 mm profile, 8 meters long: 80€
10 mm tubes, 20 meters long: 30€
Element brackets: 16€
T-connectors (16 pieces) 6€
Z54 junction boxes 12€
N-connectors 16€
Belden 8216 RG174/U coaxial cable 3€
PUR foam: 4€
Nuts, screws, solder lugs: 13€
Combining four antennas: 70€
RF240 phase-matching cables with N connectors: 40€
TOTAL: 290€
For the build, I used materials I mostly had at home; therefore, many of the prices are only approximate for those interested in building a similar antenna.
Practical Experience with the DK7ZB Quadruple 144 MHz Yagi Antenna
Both the installation and dismantling of the antennas in field conditions go very smoothly. Even a single person can lift the mast without much difficulty.
Measurements of 5-element DK7ZB Yagi antennas
Measurements of 5-element DK7ZB Yagi antennas under makeshift conditions
The antenna was practically compared with an 8-element DK7ZB Yagi, which has a declared gain of 12 dBd. The array has a theoretical gain of 14.2 dBd (excluding losses in the phased cables and combiner).
Four 5-element DK7ZB Yagi antennas mounted on a folded mast
In operation, it clearly outperformed the single Yagi despite the disadvantage of being fed by a lower-quality and longer coaxial cable. I also appreciated the more directional antenna for reception, since OM3CQF is 8 km away, OM3Kii is 12 km away, and OM2ALD is 16 km away.
A quad of 5-element DK7ZB Yagi antennas