When constructing the antenna-feeder path of a repeater or stationary radio station, the choice of antennas is determined by a number of main factors:
– range/sub-range of working frequencies;
– вthe type of directional diagram in the horizontal plane – circular (omni) or directional, and the width of its directional angle;
– antenna gain, balance between the width of the directional angle and the gain;
– the bandwidth of the antenna with the standing wave coefficient (SWR) ≤ 1.5, which is due to the duplex spread of reception/transmission frequencies.
Below we will consider the features of different types of antennas and how to use them.
Pin antennas – J-antennas (AJ) or collinear (AF) provide the optimal circular pattern.
The J-antenna is easy to install, fairly rigid, but has a low gain (0 dBd/2.14 dBi) and a fairly narrow frequency band (about 5 MHz in the VHF band and 10 MHz in the UHF band). This is sufficient for repeaters using duplexers, but when changing frequency ratings, it may be necessary to replace the antenna.
Collinear antennas consist of 5/8λ×X or 1/2λ×X links, have a gain factor of ≥3 dBd and a greater bandwidth than J-antennas. But they still do not cover the entire VHF or UHF range and are manufactured for sub-bands.
Broadband collinear antennas for VHF-UHF bands have a complex design, high cost and are used mainly in military radio communication systems (Harris, Aselsan and others).
It is important to remember that vertical collinear and J-antennas must be placed either on the top of the mast or placed at a distance of at least 2λ from it. If you place the antenna at a short distance from the side of the mast, the latter will cast a “shadow” and distort the antenna pattern.
Antenna gain and bandwidth.
- The J-antenna ½ λ has a gain of 0 dBd or 2.14 dBi, a band with a SWR ≤ 1.5 – 4.5 MHz and a working length of 1450 mm for a frequency of 154 MHz. This is due to the laws of physics.
The higher the frequency, the wider the band. But the gain of such an antenna is 0 dBd. Therefore, you should not expect more from her, no matter what is written on the label.
- A 2x½ λ collinear antenna with a gain factor of 3 dBd/5.14 dBi for a frequency of 146 MHz has a length of 3 meters, a bandwidth of 5% of the nominal frequency.
Dipole antennas (AD) are characterized by a wide frequency band covering the entire VHF or UHF range, mechanical strength and durability in operation. At the same time, the directional pattern is not completely circular in the horizontal plane and depends on the installation option. Thus, by changing the distance of the dipole installation from the mast, you can adjust the diagram from maximally circular to directional. The gain parameter of the 4-element antenna 6-9 dBd just reflects such options.
The antenna dipoles are placed IN ONE PLANE at a vertical distance equal to 5/8 of the wavelength. Then the antenna is a phased array and works as a whole.
The mast of a dipole antenna is a reflector and is actually part of it. That is, the antenna WILL NOT work if the dipoles are placed on a non-metallic structure. Accordingly, the height of the mast should cover the entire height of the dipole structure, and also ensure that the antenna is raised above the horizontal surface if the bath is placed on the roof.
Dipoles are more convenient when two antennas are used without a duplexer. In this case, they can be placed on one mast, providing sufficient vertical clearance. In the same way, you can place several antennas of different bands, or a repeater and a stationary radio station.
Directional (AY) “wave channel” antennas, or Yagi antennas, are more commonly used for the VHF/UHF bands. Structurally, they are made with a dipole vibrator or with gamma tuning.
Antennas with a dipole have a wide band, short-circuited to direct current, which practically eliminates the influence of atmospheric static, if the antenna is mounted on a metal structure that has a grounding.
Gamma-matching is cheaper to implement, but has a narrower bandwidth, is less protected from atmospheric influences, and the asymmetry of currents in the cable can lead to instability of the diagram.
Formation of diagrams by directional antennas
If it is necessary to ensure the maximum range of radio communication in given opposite directions, it is possible to use for repeaters directional antennas “wave channel” (AY), placed at an angle of 180º and combined by an adder. On the perpendicular to the line of antennas, the intersection of the diagrams and some beating and weakening of the signals are possible, but in general the desired effect is achieved.
The width of the angle of the diagram in the horizontal plane will be determined by the choice of antenna. Using the mechanical inclination of the antennas in the vertical plane, it is possible to change the position of the vertical lobe of the antenna diagram. This may be relevant in difficult terrain or high-rise urban development.
But such a solution is possible precisely when the AY antennas are placed at 180º. If the angle is smaller or a scheme of 3-4 antennas is made, it is necessary to provide a screen to suppress the rear petals and take into account the angles of the antenna diagrams in the horizontal plane.
By combining 2 or 4 directional antennas with coordinated adders in a phased array, we can get an additional 3 or 6 dB of amplification, respectively. That is, if we add two identical antennas with a gain of 6 dBd, we get a two-element directional antenna with a gain of 9 dBd.
Selection of antennas for repeater operation without a duplexer
When operating a repeater with two antennas without a duplexer, it is necessary to ensure a vertical distance between the antennas so that the transmitter does not press the receiver. The calculation method is detailed in the article: https://horwin.info/statti/metodyka-rozrakhunku-neobkhidnoho-pro/
Accordingly, if it is possible to choose the frequency ratings of the repeater, it is worth choosing a pair with the maximum spread. More duplex spread means less suppression of the receiver by the transmitter, simpler and cheaper design of the receiving-transmitting RF path.
If two antennas are of different gain, the higher one is used for reception.
If one is narrowband (for example, collinear), and the other is broadband (for example, a dipole), then with the same amplification, the narrowband is received, provided that the resonant frequency corresponds to the reception. In this way, it will “cut off” the signals of the transmitters to a certain extent.
The receiving antenna should be placed above the transmitting one.