Some questions on the construction of the antenna-feeder path of a single-channel radio base station

Options for building the HF path of the base station are described in the article

Here we will briefly touch on the questions that arise when we first get to know the problem.

The general composition of the base station equipment with one repeater is shown in the figure below.

This is a scheme with grounding and lightning protection, as it were, in stationary conditions. A simpler set of equipment in the field – repeater, duplexer, jumpers (adapters), repeater-duplexer, antenna cable, antenna. Preferably a lightning protection device.

If there are N-female connectors on the duplexer and on the lightning protection, which is more often the case, then you will need three jumpers:

– for Motorola DR, SLR series repeaters – N-male/N-male – 2 pcs., N-male/BNC-male – 1 pc.;

– for Heater repeaters – N-male/N-male – 3 pcs.

Jumpers are made from RG58 cable or similar. There is no point in a thicker cable for this.

A duplexer or two antennas

A duplexer is a device whose purpose is to combine the transmitter and receiver into one antenna and to isolate the influence of the repeater transmitter on its own receiver. For high-quality operation of the repeater, taking into account the sensitivity levels of the receivers, Tx-Rx isolation should be -70dB or more. The duplexer provides this.

In the case when there is a task to quickly change the repeater frequencies more than the duplexer band allows, which is about 400 kHz, without reconfiguring the latter, two antennas are used that provide the required band. At the same time, Tx-Rx isolation is achieved by vertically spacing the transmitting and receiving antennas. (horizontal distance, if it is not a hundred meters, will have the opposite effect). So, for example, for a range of 160 MHz and a transmitter power of 50 W, the vertical distance should be from 4 meters. More details about calculations here

It may be more convenient here to have dipole antennas that can be placed one under the other on one mast. Or vertical on top of the mast and dipole below it.

It is far from always convenient to place two antennas and pull two feeders, especially if several repeaters are placed at the same point. In addition, an antenna with a higher gain will have larger dimensions. Therefore, if possible, it is worth using a duplexer.

“Pin” or loop vibrator

There are no worse or better antennas according to their principle. Each type has its own characteristics, which will be optimal for a certain task. The main thing is that the antenna is designed and manufactured with high quality.

The principle of any antenna is the redistribution of signal energy from propagation in the spherical direction into a certain diagram in the vertical and horizontal planes. In this way, the concentration of energy and signal amplification is achieved in exactly the right directions – whether it is circular (omni) or directed in one direction. Everything that looks like this is called “common people” – a piece of wire with a length of ¼ or 5/8 of a wavelength, and J-antennas and collinear. At the same time, the latter can have a gain of 10 dBd, have a band of 500 MHz under PPRF and remain in the form of a pin.

Conventional single-band vertical antennas have a narrower band than dipole antennas and are therefore less versatile. However, if there is no need to change the frequency of the repeater, the narrowband collinear antenna has less chance of transmitting signals from nearby transmitters to its receiver. Also, they have the correct circular orientation, are cheaper, easier to install.

Dipole antennas have a band that covers the entire range, they can be increased by combining them with adders, you can form a diagram from conventionally circular to directional, you can place several antennas on one pipe stand (mast). And it will be a rigid, strong structure resistant to weather conditions. But their diagram is not a flat circle but a cardioid, they are more bulky and more difficult to install.

When used with a repeater that operates on the same frequencies, a narrowband collinear is also quite suitable. But in the case when a stationary radio station must keep in touch with several repeaters whose frequencies differ from each other by tens of MHz, a broadband antenna is needed.

Thus, “who is more valuable to mother-history”? It all depends on the task and specific conditions.

Antenna feeder and jumpers

A coaxial cable for connecting antennas has the main characteristic – signal attenuation in dB per meter. The greater the length of the feeder, the greater the attenuation. Also, the greater the frequency, the greater the attenuation. For example, the average attenuation characteristics in dB at a frequency of 450 MHz of the most commonly used cable types:

Cable typeRG58Cables 3/8” type
RG8, LMR400, 7D-FB
Cables 1/2” from
tubular insulation
Attenuation dB
at 100 meters
2510 -114,75-5

Attenuation of 3 dB gives a signal power loss of 2 times. That is, when we connect the repeater to the antenna with a 75-meter length of RG8 cable, we will receive only about 10 W from the 50 W at the output of the repeater. And if there is an opportunity to organize the minimum length of the feeder, then it is worth doing it and not winding the extra cable with a bay “in reserve”.

RF devices with a resistance of 50 ohms are used in radio communication. In broadcasting – 75 ohms. What will happen if a 75 Ohm cable is used in radio communication? There will be a mismatch at the transition of 50/70 Ohms between the devices, and the reflected signal will lead to an increase in CSC and additional losses, but it will work. But if there is a choice to use a 50-ohm cable with a thickness of 3/8″ or a 7/8″ cable with a resistance of 75 ohms on a feeder with a length of 100 meters, then in numbers the losses on the latter will be significantly lower at high frequencies and it will be more efficient.

Sometimes the way out of the situation with a long feeder can be to move the repeater closer to the antenna in the cabinet for external use and lay the power cable to it.

There is a statement that the length of the coaxial feeder should be a multiple of ½ the wavelength. It is true if we align the antenna with a cable in this way to prevent signal transformation. The length of the measuring jumper is also important when setting RF devices to 50 ohms during their manufacture. But if we connect an antenna with a resistance of 50 ohms, which is matched (balanced) on its connector, to a 50 ohm duplexer with a 50 ohm cable and connect this with 50 ohm connectors, then the length of the antenna feeder in relation to the wavelength of the operating frequencies Does not matter. And it affects only the attenuation of the signal in the cable. The same applies to jumpers between the repeater and duplexer, filters and other RF devices.

The detachable connection of the feeder and the antenna must be carefully sealed. Getting moisture into the connectors will lead to a loss of communication quality. It is easiest to seal in the field with PVC tape for outdoor work and raw rubber.

Coaxial connectors

In radio communication in the VHF/UHF bands, connectors of the UHF type (other name PL259/SO259), N-type and BNC are more often used.

The UHF connector, although it has such a name because at the time when it was invented, UHF was considered anything above 30 MHz, it is not intended for use at frequencies above 200 MHz due to its design. It can be used in the VHF range, but with it, it is easy to allow poor-quality installation and connection, which will lead to high SWR.

BNC connectors work at frequencies up to 4 GHz, have a reliable contact and are convenient for frequent connection and disconnection. There are 50 Ohms for radio communication and 75 Ohms for video transmission. It is necessary to distinguish between them when choosing.

N-type connectors are designed for bands up to 18 GHz and are more versatile for a variety of radio systems. Available for most cable types.

Lightning protection

The lightning protection is designed to remove the charge of atmospheric static that accumulates in the antenna and feeder. Accordingly, it works when connected to a ground that has sufficient leakage resistance. Without it, lightning protection makes no sense. At the same time, it should be understood that no lightning protection can save the antenna from a direct lightning strike. Therefore, the electrical regulations provide for lightning arresters located on top of buildings and having a separate grounding circuit. (see the picture at the beginning of the article).

Lightning protection devices are designed for a certain frequency range and transmitter power. Lightning protection designed for high-frequency devices with low power is not suitable for radio communication with repeater powers of 50-100 W. Accordingly, high frequencies require their own lightning protection devices.

In addition to lightning protection, when the antenna feeder is long, the braid of the coaxial cable is grounded in order to remove static. One grounding device is installed on the feeder after the antenna and the other before entering the building. To install the grounding device, the outer insulation of the cable is removed, and after installation, the place of installation is carefully waterproofed.

Short-circuited antennas (circular and directional with a dipole, J-antennas, etc.) fixed on metal masts remove atmospheric static that accumulates on the antenna itself and on the cable screen, provided that the masts themselves are grounded.

High-quality grounding of the repeater or stationary radio station also removes static from the braid of the antenna cable.