In amateur VHF antenna designs, a split half-wave vibrator is often used. The manufacture of the power supply raises questions.

The advantage of a split vibrator is that its production does not require special devices or mechanic skills. All you need is a hacksaw, a file, a powerful screwdriver, pliers, screws, nuts, washers M4 and M5, a piece of durable plastic plate for making a central insulator and an anodized aluminum tube 1 meter long with a diameter of 8 mm.

I usually buy all the parts for manufacturing at the Maxidom building supplies store. Plastic is suitable from a cutting board for food (from 75 rubles =~ $1).

First you need to make a plate measuring 16 by 8 cm. If you have a jigsaw, use it.
ЗатLet's make 6 holes along the axes of symmetry, retreating 2 cm from the edges. The tubes of the vibrator halves are attached to the plate using pull rivets. The power unit assembly is attached with screws to a 20 x 20 square cross-section. Connect the cable through the petals and an M4 or M3 screw with washers, with a Grover washer and a nut (or use a self-locking nut with a polyethylene sleeve, a Grover washer is not needed).

Questions of interest that are rarely covered:

Question 1) what should be the air gap between the ends of the tubes in the power supply unit?
Answer 1) the minimum distance is determined by the spark gap and breakdowns in the dielectric. As is known, charges of different signs are supplied to the halves of a dipole using a generator - plus and minus, which changes to the opposite during half the oscillation period. If we discard the possibility of discharge through air (since air has a high breakdown voltage: 4 kV with a gap of 1 mm and 800 V with 0.1 mm), and take into account only the leakage of charges on a wet surface between the terminals in the central part of the dipole - voltage up to 20 kV at a distance of 2 centimeters (10 kV per 1 centimeter), in the worst case - if the electrodes do not fit tightly to dielectric (source - V.V. Solovyov - Insulation of electrical installations, textbook, Blagoveshchensk, AMG Publishing House 2014).

The resistance of fresh water from precipitation is 10 μS/meter or 100 kOhm/meter or 1 kOhm per 1 cm. However, the aluminum surface is anodized, the supply points must be protected from precipitation -sealant, varnish, plasticine, placement of the power supply inside the installation box, which will protect against the formation of a continuous film of water on the surface of the insulator and between the “electrodes”.

Power was calculated using the formulas:

P=U x I
I=U/R
P=U x U / R
R=50 Ohm

For DC:

For alternating current 1-300 MHz, select a voltage of 1-6 kW per centimeter
(For high-frequency alternating voltage, the gap must be larger due to various effects in the dielectric - heating due to RF voltage, the formation of conduction channels, aging of the material, the formation of arc discharges, etc.).

For high frequency alternating current:

We can say that for a maximum permitted power of 100 W on VHF, any gap from 1 to 20 millimeters is suitable. It is chosen for design reasons: when connecting a coaxial cable, the power points should not be too far from each other, and the cable leads should be as short as possible (for example, in Ethernet, leads outside the cable are allowed within 20 mm). With a gap of 5-10 mm and a distance of 1 cm from the edges, the distance between the power points will be 25-35 mm, which is quite convenient for installation and does not cause much parasitic reactivity. A gap equal to the tube diameter of 8 mm is required. For design reasons, it can be increased to 10 mm.

Question 2) is it permissible to supply food not to the edges of the tubes, but with an indentation towards the edges, say, 4 cm from the center?.
Answer 2: Acceptable. Explanation using simulation:

Initial antenna (ideal feeding of a split dipole into the edges of the tubes. gap 10 mm): SWR=1.01
(Z = 50 Ohm + j0).

When power is not supplied from the edge of the tubes (distance between power points is 20mm): SWR=1.13 without adjustment
(Z=50 Ohm +j6).

When power is not supplied from the edge of the tubes (distance between power points is 30mm) SWR=1.2 without adjustment
(Z=50 Ohm +j9). After tuning by changing the dipole length SWR=1.1 (Z=47 Ohm + j0)

An alternative way to configure: not by changing the length of the dipole, but by changing the last director:
SWR=1.01 - I had to move the far director and change its size.

Original value 4 directors

• length 0.877
• position 0.994

In this way, the antenna tuning frequency is shifted upward until it is in the center of the required range - 144.3, 145 or 145.5 MHz. In multi-element antennas, the farthest director can be moved.

Since a 4-element antenna is taken, after setting up 4 directors

• length 0.867 (became 10 mm shorter)
• position 1.055 (displacement along the traverse further by 61 mm).

Thus, increasing the distance between power points leads to the formation of reactance of a positive sign (inductance) and increases the SWR to 1.2.

To set up, it is advisable to make a reserve of 70-80 mm at the far end of the traverse to move the last director to the edge,
in this case, it is possible to achieve an SWR of 1.1-1.0 without deteriorating the radiation pattern.

The directors are fastened in the same way as with a split vibrator - using insulating pads and rivets. Only the sizes of the platforms can be taken smaller, because... the load on the elements is weaker than on the dipole. It is also possible to install elements through the traverse, provided that the tubes are insulated from it (insulated elements, for example, in the centers of the tubes in a heat-shrinkable tube (“heat-shrink”). The most profitable option is to manufacture the cradle on a 3D printer, which is not available to everyone.

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