In the contest team OM0A, we were selecting a suitable antenna for 14MHz. Since the workstation is of portable nature, we considered smaller antennas - 2el.quad, 2el. Yagi with direct connection of a 50-ohm feeder, DK7ZB yagi, and Moxon. All are good antennas, but due to the requirement for a simple disassemblable structure and based on good experiences with DK7ZB at VHF bands, we chose the most suitable 2el. DK7ZB yagi.
It is a classic 2el.yagi with an impedance of 28 ohms and matching with DK7ZB match made from two parallel connected 75-ohm coaxial cables. The gain is approx. 4dBd, F/B ratio 10dB. The boom is 2.3m long, the longest element is over 11m. The weight of the entire antenna is approx. 10kg.
Dimensions of DK7ZB yagi at 20m
| Element (one half) | 35 x 2mm | 30 x 2mm | 25 x 2mm | 20 x 2mm | 16 x 1.5mm | Position |
| Reflector | 0.5m | 1.00m | 1.00m | 1.00m | 2.04m | 0.00m |
| Radiator | 0.5m | 1.00m | 1.00m | 1.00m | 1.63m | 2.30m |
We ordered the material through the company ALUSET http://aluset.sk/, small mechanical parts were provided by the local hardware store. The advantage was that I had a nylon block from which the element holders were made. The block was first cut into beams measuring 100 x 40 x 80mm. Holes with a diameter of 35mm were drilled into the beams, and two holes for the mounting screws were drilled perpendicular to it along the edges. The beam was then cut transversely. This way, when the screws are tightened, the pipe inside is firmly held.
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The carrying plates measure 300 x 100 x 8mm. It is very important to drill the holes for the mounting U-clips and screws holding the nylon beams very accurately. Any inaccuracy has a significant impact on the axiality of the antenna.
The radiator is divided into two halves. Due to a lack of nylon, an insulating peg was used to connect both tubes. It is beech wood from a shovel handle. It is hard enough. It has been treated with a thick layer of silicone to prevent moisture ingress.
A waterproof box is screwed onto the top of the radiator. Inside it is the DK7ZB match termination and the output connector. A metal plate ensures the conductive connection with the boom, to which the PL connector is attached. The plate is screwed onto the supporting plate.
Making a DK7ZB match for HF is much easier than for 70cm. I recommend a thicker coaxial without aluminum foil. With a coaxial shortening factor of 0.82, a length of 3.60m is required. With a factor of 0.67, it is 2.76m. The distance is measured from the points where the braid covers the center conductor. We shorten the leads to a minimum. The coaxials are twisted into a circle with a diameter of about 25cm and secured with tape above the boom. Especially at higher power, we pay attention to the gap between the coaxial and the boom.
The finished antenna was subjected to measurement with an antenna analyzer.
| Frequency | Impedance | SWR |
| 13.85 MHz | 94 + j0 ohm | 1.89 |
| 13.90 MHz | 87 + j0 ohm | 1.74 |
| 13.95 MHz | 77 + j0 ohm | 1.56 |
| 14.00 MHz | 66 + j0 ohm | 1.33 |
| 14.05 MHz | 63 + j0 ohm | 1.26 |
| 14.10 MHz | 57 + j0 ohm | 1.15 |
| 14.15 MHz | 52 + j0 ohm | 1.05 |
| 14.20 MHz | 50 + j0 ohm | 1.00 |
| 14.25 MHz | 48 + j0 ohm | 1.03 |
| 14.30 MHz | 46 + j0 ohm | 1.06 |
| 14.35 MHz | 45 + j12 ohm | 1.32 |
| 14.40 MHz | 43 + j21 ohm | 1.60 |
| 14.45 MHz | 41 + j26 ohm | 1.88 |
| 14.50 MHz | 40 + j31 ohm | 2.06 |
Measurements confirmed that the antenna is precisely tuned to the band. In the contest, the antenna performed very well. Despite the relatively wide radiation pattern, it is necessary to aim precisely.
The antenna was built in 6 weeks, with the most time taken for the production of insulating blocks. The net working time is about 20 hours, and the costs were around 100 EUR.
Recommended information
Page DK7ZB
Page www.dxbeam.com - showcase of professional mount
contestový tím OM0A