(by Andrea Giorgetti)

The VHF transceiver, usually referred to as a VHF radio is, with good reason, part of the safety equipment; its reliability and ease of use have made it the instrument mainly used in short-range ship-to-ship and ship-to-shore communications.

What is the VHF band and how it works

The VHF (Very High Frequency) band is a portion of the radio frequency spectrum ranging from 30 to 300 MHz. The band is divided between broadcast stations, television channels, amateur radio frequencies, police frequencies and much more; what is of interest here is the marine VHF band, which goes from 156 to 162 MHz and is divided into channels. That is, each channel that we read on the display of our equipment corresponds to a certain frequency (two if the channel is duplex, one in transmission and one in reception). https://atc.mise.gov.it/images/documenti/03-Tabella_B.docx.pdf

In addition to the channels we use for speech, i.e. to talk to other stations, there are channels dedicated to digital transmissions, 70 for DSC (Digital Selective Call https://it.wikipedia.org/wiki/Chiamata_selettiva_digitale) and the 87B and 88B for AIS (Automatic Identification System).

Radio waves at these frequencies are generally not reflected by the ionosphere, they propagate by direct waves. Therefore, theoretically, I can only communicate if the two antennas (the one transmitting and the one receiving) can be physically seen. This is why VHF antennas are installed as high as possible and often at the masthead.

Compared to communications in MF or HF, where thanks to the reflection of the ionosphere one can connect stations thousands of miles away, in VHF the range is limited, but on the other hand it has great reliability, because communications are not affected by solar cycles, day-night alternation or other factors.

What is needed to communicate on VHF

Let's see what I need to communicate on VHF and what factors need to be taken into account to exploit the full potential of this instrument.

The heart of the system is clearly the transceiver, which is a radio capable of receiving and transmitting radio frequency signals, in this case in the marine VHF band. Simplifying, we can say that the transmitter processes the voice picked up by the microphone into a radio frequency signal that is carried by the coaxial cable to the antenna, which transforms it into electromagnetic waves that are radiated into the ether. Conversely, the antenna immersed in an electromagnetic field (receiving) produces a signal that the coaxial carries to the receiver, which in turn transforms it into an audio signal emitted by the speaker.

So what is needed?
Marine VHF transceivers are now very robust and reliable devices; if well installed and powered, they can give decades of use without giving any problems. In installations, the most frequently encountered critical issues are coaxial cables, and their connectors, and antennas.

1 - Coaxial cable

The coaxial cable consists of a central conductor wrapped in a dielectric material that insulates it from the braid (ground), which is wrapped in the outer sheath. In this way, the radio-frequency signal travelling inside the coaxial is as if trapped in the outer shielding created by the braid. Ideally, the transmission line carries all the energy from the transmitter to the antenna, but in the real world all coaxial cables have losses.
On the market, we can find a multitude of types of coaxial cables, each of which has its own field of application depending on the frequency, power and impedance at which it has to work.
The following comparison table is an excellent tool to compare the various coaxial cables and guide us in making an informed choice. You can find it at: https://www.wimo.com/coaxial-cable_e.html

We need 50Ω impedance, with low attenuation on the VHF band.

To be notedCoaxial cables do not like tight bends, otherwise they lose efficiency, which is why the minimum permissible bending radius is indicated in the product specification. This is important since during installation on board we often find ourselves running cables in tight places with sharp bends. As a matter of principle, if we have problems with space in cable ducts or have to run in very narrow places, a small diameter and very flexible cable will help with the installation.

If we focus on 144 MHz, the frequency closest to the marine band, we can see how much difference in attenuation values there is between one cable and another.
Assuming an installation on a sailing yacht of 15 m, we have to calculate the losses on the 33 metres of coax, which are required to connect the radio installed on the chart table with the masthead antenna. The attenuation values will then be a third of those given in the table, which refer to 100 metres of cable.

Let's take an example starting with RG 58, which is very popular due to its low cost and its diameter of only 6 mm, making it very flexible.
From the point of view of losses, as can be seen in the table, this is certainly not the best solution, and in practice we can calculate that the attenuation on 33 metres of cable is almost 6 decibels, which means that of the 25 watts delivered by the VHF radio only 6 will reach the antenna.
By using the RG 213U, a more expensive and bulkier cable with a diameter of 10 millimetres, the attenuation decibels drop to just under 3, i.e. the antenna will get 13 watts, a big difference.
If, on the other hand, we use an RG 213 Foam, the losses decrease even further, with an attenuation of only 1.5 dB on 33 metres of cable, as a result 18 watts will reach the antenna.

From this simple comparison we understand how much the choice of coaxial affects performance. Considering that the maximum signal range in VHF is limited by the optical range, the attenuation of the cable when transmitting with 25 Watts is not so critical. It does, however, become so on channels where the equipment is self-limited to 1 watt (bridge to bridge) or when we are talking about AIS, in fact the class B transponders that are most common on pleasure boats have an output power of only 2 watts. With such low power outputs it is important not to have line losses, so we have to invest in a good coaxial, install the connectors properly and protect the connector on the antenna side with self-amalgamating tape, so that moisture and saltiness cannot corrode the contacts. increasing losses.

2 Connectors

Speaking of connectors, the most commonly used are the PL 259. The usual recommendation is to use good quality ones, because best in marine environment. Furthermore, given the multitude of cables on the market, we must ensure that we use connectors that are suitable for the cable we are going to use. It goes from if that the connector must be properly installed, if we do not have enough knowledge and practice in this regard, we can find very comprehensive tutorials on the net.

3 Antenna

We now come to the antenna, an element that has been installed too many times and then forgotten.

In order to transform the radio-frequency signal arriving in the coaxial into electromagnetic waves (resonance), an antenna must be of an electrical length proportional to the wavelength (and consequently its frequency) for which it was designed. Usually in VHF, quarter-wave or half-wave long vertical antennas are used, but there are many variations.
As a general rule, the longer the antenna, the more gain it will have. Since what matters for gain is the electrical length and not the physical length, antenna manufacturers (in order not to cause confusion) specify the gain of each of their products. Since the antenna is a passive element and nothing is created or destroyed, the gain of the longest antennas is due to the narrower radiation lobe.

As is well shown in the figure, in practice with an antenna with more gain I can communicate further away, but with a heeled boat I risk directing my signal into the sea and space! This is why on sailing monohulls we tend to use antennas with 3 or at most 6 dB gain at the masthead. This allows a good range due to the height of the mast, without having to install long and heavy antennas so high up, which is neither easy nor productive. On the other hand, it is more difficult to install 6 or 9 dB gain antennas on speedboats or at the stern of sailboats as respect antennas or for the AIS.
When choosing an antenna, it should be borne in mind that marine VHF antennas can usually only be used at the frequency for which they have been cut, and that the bandwidth è of a few MHz, so that an antenna suitable for VHF speech channels will not always perform best on AIS channels; however, this problem is easy to remedy, since antennas for audio channels and specific antennas for AIS can be found on the market.

How can I check that my system is working properly?

The easiest way is a radio call to a Coast Guard station.

If you have doubts about the efficiency of the system, you should measure the standing wave ratio, or ROS, which is the unit of measurement for the efficiency of the antenna at a given frequency. ROS should never exceed 1:1.5 and stay below 1:1.2 for good efficiency. Although it is not common to have a ROS meter on board, it would not hurt to borrow one and measure the standing waves of our antenna at the beginning of the season. Note that some AIS transponders have a built-in ROSmeter, so that if the antenna has any problems, the transmitter stops transmitting to avoid damage.