Design and production of antenna for pocket communication device

Pocket communication machines are widely used in various sectors of society due to their small size, light weight, and ease of use. This type of machine now widely uses a spiral antenna. Generally, the finished spiral antenna is wound with metal wires with good conductive properties and sealed. Its working principle is as follows:

Figure 1 shows a schematic diagram of a general antenna structure. D is the diameter of the helical antenna, L is the length of the helical antenna, ρ is the pitch, and Ⅰ and Ⅱ are two corresponding points on the helical line.

It can generally be assumed that electromagnetic waves move at a uniform speed C along the metal spiral.

From point I to point II, a spiral is performed, and the time required is

t = πD/C

For the helical antenna, its axial electromagnetic wave only moves a pitch ρ, and its axial equivalent velocity

y=ρ/t =ρ/C (πD)

This relationship can also be explained in the form of Figure 2. From Figure 2, we can see that:

y=Csinθ=Cρ/（πD）≤C

It can be seen from the above formula that υ is always less than or equal to C. Therefore, the helical antenna can slow down the movement of electromagnetic waves and is a slow wave system. Its equivalent wavelength λ is equivalent to the working wavelength λ. For the helical antenna, it should resonate at its 1/4 equivalent wavelength, so that the geometric length of the helical antenna can be shortened.

For a communication device operating at a certain center frequency, the number of coils N required can be approximately calculated by the following formula:

Pitch:υ=L/N

Required metal wire length: ι=NπD

For general communication devices

L=20~40cm

D=10~20mm

The following table shows the design examples of helical antennas for some commonly used frequencies. Similar designs can be applied for other frequencies.

f is the operating center frequency;

D is the diameter of the helical antenna;

L is the length of the helical antenna;

N is the number of spiral turns;

ι is the required wire length.

The above N and ρ are approximate values ​​for practical production needs.

When making it, you can use enameled wire or silver-plated copper wire or aluminum wire with a diameter of 0.5~1.5mm to wind it on organic glass or other insulating materials with a diameter of D, and punch small holes at both ends of the rod to facilitate the fixing of the metal wire; weld a thicker metal rod or plug on the bottom of the rod to fix it on the rod to facilitate connection with the machine; the outside of the entire spiral antenna can be sealed with a rubber tube or other materials, and the top can be covered with a rubber cap or sealed with other materials. This is both beautiful and generous, and it is rainproof and corrosion-resistant, and durable. If there is no such metal wire, you can also use multiple strands of thin insulated wire instead, which has the same effect, but it is more difficult to fix when winding.

The above helical antenna can also be used in various small remote control devices and other similar machines.

In order to compare the slow-wave antenna with the conventional telescopic antenna and illustrate the advantage of the slow-wave antenna's smaller size, we can make a calculation for the telescopic antenna.

Set the parameters as follows:

Frequency f=27MHZ

Wave speed c=3×108M (Note: 108 should be 10 to the 8th power)

The antenna should resonate at 1/4 of the working wavelength, and the length of the telescopic antenna can be calculated according to the formula:

L=1/4λ=1/4 c/f=1/4(3×108/27×108)=2.78m (Note: 108 should be 10 to the 8th power)

According to the table above, if a helical antenna is used, the slow-wave antenna with a center frequency of 27 MHZ will have an apparent length of only 40 cm, which is about one seventh the size of a telescopic antenna.

Li Yue in February 2009

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