10 Element Yagi-Uda antenna for the 23cm Band (1250MHz to 1300MHz) . April 2026 Under development

Exploring further practical and straightforward construction technique for VHF and UHF Yagi antennas and ensuring satisfactory performance. Additionally, there was a particular interest in building a compact 10-element Yagi antenna for local operations, while keeping the option open to incorporate additional elements for potential future performance enhancements and modifications.
 
To achieve these goals, a key focus was placed on developing an element mounting technique that facilitated easy installation and replacement of the elements. It was also essential to enable effortless adjustment of the element attachment point on the boom without the need to drill holes into the boom itself. Moreover, careful consideration was given to designing the antenna in a manner that would facilitate its recycling into another antenna when it is no longer required.

Photo 1  Complete 1270MHz 10 element Yagi antenna including a Series Section coaxial impedance transformer.

Antenna details

Frequency:                  1270 MHz, (useful from 1250 to 1300)

Wavelength:                233 mm (23.3 cm)

Element width:            10 mm

Boom Diameter:          20 mm

Boom Length:             430 mm (Plus additional length for mounting).

Elements:                   10

Gain:                          11 dBd (approximately).

Table 1 Yagi antenna dimension details.

Fig 1 general Yagi antenna layout in relation to Table 1.

Folded Dipole  

The dimensions of the folded dipole for 1270MHz are detailed in Table 2, denoted in millimetres. The fold spacing, set at 20mm, was determined based on the bending tool used for the internal part of the element material.

 
Because the fold spacing is relatively large in comparison to the wavelength, the folded dipole exhibits characteristics that resemble a loop antenna. The length of the loop is closer to a full wavelength, while the overall end-to-end length is significantly shorter than the conventional half wavelength commonly associated with a dipole antenna. Photo 1 presents a comprehensive view of the complete 1270MHz Yagi, showcasing the relatively compact size of the folded dipole in comparison to the other elements.

 
To achieve the desired impedance matching, a 75ohm coaxial cable is employed as a Series Section Transformer in series with a 50ohm feed line connected to the radio. The length of the Series Section Transformer is determined to be 255mm, with a detailed explanation of the reasoning behind this specific length provided in subsequent text.

Table 2 Folded dipole dimension details. All dimensions are in mm.

The Series Section coaxial quarter-wave impedance transformer

For a typical two-wire folded dipole (with equal-diameter conductors), the feed-point impedance at resonance is approximately 300Ω.

Where:

• N = Number of conductors
• Z dipole = Half-wave dipole impedance ≈ 73 Ω

However, in real-world builds, the final impedance is affected by conductor spacing, element diameter, nearby parasitic elements, and mounting hardware, all of which tend to lower the impedance. Typically, 180 - 240 Ω is very common for Yagi driven elements.

A quarter-wave impedance transformer is an elegant RF technique where a section of transmission line performs impedance matching without requiring discrete components such as coils or capacitors.

This works because a transmission line not only carries energy but also transforms impedance along its length. When the line is exactly one quarter wavelength long at the operating frequency, it transforms the load impedance to a different value at its input.

By selecting the transmission line with the correct characteristic impedance, the transformed impedance can match the source, allowing efficient power transfer.

This technique is inherently frequency-dependent. If the frequency changes, or the physical length is not precise, the impedance transformation will no longer be exact. As a result, quarter-wave coax transformers are simple and low-loss, but not suitable for broadband applications.

Fig 2. Quarter-wave impedance transformer configuration.

Zin​ = impedance seen looking into the matching section (50Ω coax feedline).

Zo​ = characteristic impedance of the matching section.

ZL​ = load impedance (200Ω antenna impedance).

What this means is that a λ/4 section of 100Ω connected to the feed point of a folded dipole with a nominal impedance of around 200Ω will present a near 50Ω impedance at the end of the normal coax cable to the radio with a 1:1 SWR.

While 100 Ω coax is ideal in this case, it is not commonly available. However, 75 Ω coax (such as RG6) is widely available and provides a practical compromise. Even commercial antenna manufacturers often use 75 Ω sections to match folded dipoles.

Reorganizing the formula to make Zin the subject allows evaluation of 75Ω (RG6) coax as a quarter-wave impedance transformer.

The 75Ω is a compromise and while not a perfect transformer it will result in an SWR of approximately 1.8:1 which is quite acceptable for most systems and commercially acceptable.

In practice, the physical length of the matching section must account for the cable’s velocity factor (VF). For RG6, VF ≈ 0.80.

Example of a matching section for 1270 MHz

• Free-space λ (Wave Length for 1270 MHz) ≈ 236mm
• Quarter-wave ≈ 0.59mm
• With VF = 0.8 will result in ~47mm coax matching section.

If a single quarter-wave section is not physically convenient, odd multiples (λ/4, 3λ/4, 5λ/4, etc.) can be used. However, longer sections introduce additional loss, so the shortest practical length should be used.

Interestingly it is sometimes suggested that a full wavelength of RG6 coax can be used as a matching section. In this example, a full wavelength at 1270 MHz is approximately 236mm.

Notably, this is very close to 5 × (λ/4), which equals 236mm when velocity factor is applied. This agreement is coincidental for RG6 with a VF 0.80 and should not be taken as a general rule - matching behaviour is determined by quarter-wave transformations, not full-wave sections.

Testing

Measure electrical length coaxial quarter-wave impedance transformer (phase method)

First, a standard S11 calibration is performed on the NanoVNA at the end of the test lead.

A length of RG-6 coax is then connected to the NanoVNA, with the far end left either open or shorted. One method should be chosen and used consistently. In this case, an open circuit was used.

The NanoVNA is set to display S11 phase, and a marker is placed at the target frequency, such as 1270 MHz.

The phase reading at this frequency is noted as the starting reference.

The coax is then trimmed incrementally, typically one to two millimetres at a time. After each cut, the cable is reconnected and the phase at the marker frequency is observed.

As the cable is shortened, the phase changes progressively. Trimming continues while monitoring this phase shift.

When the phase has changed by approximately 180 degrees from the initial reference, the cable is at an electrical quarter wavelength, or an odd multiple of a quarter wavelength.

If the far end is open, the phase will be close to ±180 degrees at this point. If the far end is shorted, the phase will be close to 0 degrees.

Photo 2. The NanoVNA S11 phase display (yellow trace) shows that at 1263.4 MHz the phase reaches +180° (+179.52°). At slightly lower frequencies, the display transitions to -180°, appearing as a vertical shift. This indicates that the RG-6 coax is at an electrical quarter wavelength or an odd multiple.

In this instance, the length corresponds to approximately 5λ/4 (about 265 mm including connectors and termination lugs). The additional length between physical tested length and the calculated length is in part the N connector and the lugs and tails of the completed cable.

The display spans the 23 cm band from 1240 MHz to 1300 MHz. Across this range, the phase varies smoothly between approximately -150° and +150°, indicating that the coaxial transformer remains effective across the full band with maximum efficiency at 1263.4 MHz.

Photo 3. The completed quarter-wave RG6 coaxial cable impedance transformer

Antenna Matching (SWR and Smith)

With the antenna is full assembles the SWR was measured with a short length of 50ohm coax connected to the antenna with final adjustment made by moving the antenna's reflector element for the best SWR value.

Photo 4. The NanoVNA SWR sweep from 1200MHz to 1300MHz 

Construction

The element mounting assembly shown in Figure 3 and Photo 7 consists of a stainless steel hose clamp with a 5mm stud hole drilled in the strap. A countersunk-headed set screw is then mounted with the flat head against the boom, as depicted in Photo 7. The hose clamp stud mount eliminates the need for drilling any holes into the boom and allows for infinite lateral adjustment along the boom.

 
The element mounting bracket, as seen in Photo 7, 8, and 9, is made of 12 x 12mm aluminium channel. This aluminium channel features a 'V'-cut notch that facilitates the attachment of elements of various diameters. Additionally, a lower notch is cut out to ensure the bracket mounts flat against the boom and remains clear of the hose clamp strap. It is crucial to accurately cut the 'V' notch to achieve symmetrical element mounting.

Figure 3 Element to Boom mounting arrangement.

Figure 4 Details of the Element to Boom mounting bracket. The bracket allows either round elements or flat 10mm width x 3mm height rectangular elements.

Yagi-Uda antenna dimension calculator

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