Maybe a word about SIMPLEX and DUPLEX operations might be of interest. When we talk to another ship on the FM VHF radios we are using Simplex. Both of us are on the same frequency and one talks then says 'over' and then the other will talk. This is called simplex.
In the past we have all used the old 'high seas' radiotelephone to call home and had to say 'over' to mom or the girl friend and it was hard to get them to cooperate and we would miss some of the other end message. In order for us to be able to listen as we talk (like a normal telephone) we need to have one frequency or channel for listening and another frequency or channel for transmitting. Then we could use DUPLEX as we were taking and listing at the same time.
You may have noticed that when using SITOR or TELEX to communicate with a coast station we usually set up the transmitter on a different frequency than the receiver. Our radio listens on one and transmits on the other, but not at the same time. This is called SIMI-DUPLEX.
We have given the radio frequencies names or channels so that we can set up the equipment faster than having to figure out the frequency and change it in the radio. Its easer to say "come up on channel 6" instead of saying "please tune your radio to frequency 156.300 Megahertz". Even on the MF and HF radios we have channel numbers, ITU Channel 1603. Meaning the third frequency on the chart of the 16 MHz frequencies. Much faster and more accurate that trying to tell the other operator the exact frequency of the channel.
On some frequencies we can hear radio signals most of the day and a lot of the night times. On some frequencies, like the broadcast band (AM) we can hear station several thousands miles away during the night but in the daytime we can only hear the local stations. On some of the HF channels we can hear stations from the other side of the world and on other frequencies of the HF receiver we cannot hear anyone. How can this be?
Maybe now it time for a word about Radio Propagation. A big word, I guess. Radio Propagation is a way to tell how the radio signal (wave) gets from one place to the other. What is this "radio wave"? The radio wave is that energy in the form of radio electromagnetic radiation that leave the antenna and travels out into space at the speed of light (186,280 miles per second). The technical types use the metric measurements and its 300,000,000 meters per second. (Three Hundred million meters per second.) Its easer to calculate the length of the antenna if we use meters instead of miles.
We can compare radio wave with the wave in a pond. When we throw a rock into the pond we see waves leave the point of impact in a circular movement from that point. The water waves have an up and down motion and we could count the waves that pass us in a minute or second. These would be cycles per second. And we used to call this cps or if a lot of them then Kilo cycles per second or for short KC. Some time ago we wanted to acknowledge all the work a German physicist had done in transmitting radio signals for the first time we named the cycles per second after him. His name was Hienrich Hertz and he did his experiments from 1885 to 1889. Thus the term Kilo Hertz or Mega Hertz came into being. Back to our waves and of them passing our point of view in Hertz (cycles per second) when we talk of radio waves were much faster than the ones in water we talk about Kilo or thousands or Mega, million of these Hertz.
