3D printed holder

Here is a view of the three parts to be 3D printed in PLA (it would also work with ABS or PC, but the antenna shown in the photo above has been installed outdoors for two years, is still working, and has survived two summers).

The AMF files are attached in the files projet.

From left to right: passive element holder, driven element holder, and the "case" used to secure and protect the SO239 connector.

You can see the recesses for M4 countersunk screws and the matching hex slots for the nuts, which allow precise and secure positioning of the holders on the boom.

On the "case," you can see the two openings for cable ties, used to attach the enclosure to the boom, between the driven element and the reflector.

1:1 balun and driven elements

Measurement and Tuning of the Balun

The photo above shows the setup for feeding the radiating elements via the SO239 connector.
The system uses two lengths of RG58 coaxial cable, measuring λ/4 and 3λ/4, respectively, to achieve both:

• impedance matching close to 50 Ω,
• and out-of-phase feeding of the two dipole arms.

This arrangement is a quarter-wave balun (“balanced to unbalanced”), which provides two key functions:
• It transforms the unbalanced signal from the coax into a balanced signal suitable for the dipole,
• and suppresses common-mode currents that could distort the radiation pattern, improving VSWR stability over time.

Quarter-wave Split Balun Case (two λ/4 lines, one per dipole arm)

Each dipole arm is fed by its own (n*)λ/4 line, grounded via the coaxial cable shield.
The impedance “seen” by each line is thus half that of the full dipole:

Z L = Z dipole / 2

Discussion on Dipole Impedance

• A free-space half-wave dipole has a characteristic impedance of 73 Ω (theoretical value, ref. ARRL/RSGB).
• However: in a Yagi-Uda, proximity to directors and reflector (“accompanied dipole”) lowers this impedance, typically to around 60 Ω (even down to 50–55 Ω with strong coupling), depending on geometry.
• This reduced impedance is what is used in all real-world matching calculations.

Example for Z_dipole = 60 Ω (common for a well-designed 12-element Yagi, Rothammel/DL6WU):

Z L = 60 Ω / 2 = 30 Ω

Each λ/4 line transforms this load as follows:

Z in arm = (Z0^2) / Z L = (50^2) / 30 ≈ 83 Ω

The two λ/4 lines (one per arm) are brought together in parallel at the feedpoint (coax hot pin):

Z in total = 1 / (1/Z in arm + 1/Z in arm) = 83 / 2 ≈ 41.5 Ω

In practice, the dipole’s impedance depends on coupling: The closer the dipole is to reflector/directors, the lower Z_L gets (sometimes as low as 45–50 Ω). With stronger coupling, the combined input impedance comes close to 50 Ω, which is ideal for RG58.

Thus, this setup typically achieves a VSWR of 1.1–1.2: a robust and experimentally validated compromise.

Key takeaways:

• Never trim the coax lengths after initial accurate cutting.
• Fine-tune only the dipole length (±0.5 to 1 mm) to optimize the match.
• Always leave 1–2 cm of slack at the connector end for proper soldering, then trim to precise length.

Feeding in Opposite Phase: λ/4 and 3λ/4 Sections

In this “split balun” design, the two arms of the dipole are each fed by a separate length of coax: one λ/4 and the other 3λ/4.
The λ/4 and 3λ/4 sections are wired in such a way that a signal from the transmitter reaches one dipole arm one quarter-wavelength ahead in phase compared to the other arm, and because a λ/2 line adds 180° of phase but does not affect impedance, the total phase shift between the two arms is 180°.

This means that when the RF signal reaches the feedpoint, the two arms are driven in perfect opposition of...