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Radio Frequency Gains and Losses

Feb 17, 2006
5,077
Lancer 27PS MCB Camp Pendleton KF6BL
Recently there was a discussion about changing one's coax cable going to a VHF antenna on the mast. During that discussion, there was a brief discussion of gains and losses associated with the antenna and the cable being used.

Nine paragraphs down in this brief discussion one will find a link to an online gain and loss calculator. Before using this calculator, a brief discussion as to what is happening with one's radio, feed cable, and antenna would be in order. This will be very simplistic and is not intended to insult anyone's intelligence, but merely to help those who are not well versed in Radio Frequency equipment. This discussion holds true for all modes of RF communications, to include radar, AIS, VHF, and SSB/HF.

The radio is considered the source of RF. It generates a carrier wave that is used to transmit information, whether voice or data. The means to carry that information is the coax cable. There are other means of transport but for us mariners, the coax cable is the prime method. At the end of the coax cable is a radiator that will electrically vibrate and produce waves of energy. With all things electrical, there is a source and a load. The antenna is considered the load. However, the coax cable also has load characteristics. This cable can be a means of attenuation to the electrical energy produced by the radio.

Let's narrow our discussion to the VHF realm as that is mostly what us mariners use. This will also include AIS as that uses the same VHF spectrum as our shipboard radio.

Our radios work at a frequency range of 156MHz to 174MHz. Conversely, ham radio VHF uses 144MHz to 148MHz (this range constitutes the 2 meter band). So the first question is can Marine VHF radios use ham 2 meter antennas? Simplistically speaking, yes, but there will be consequences.

Now that we know the actual frequency range, we can determine which coax to use. Coax cables have what is known as an impedance. This impedance is the result of capacitive reactance and inductive reactance at a given frequency. Impedance is measured in Ohm (Ω) but is not DC resistive. It is AC resistive since the RF wave is an Alternating Current. The two standard impedances are 50Ω (radio) and 75Ω (video). Question: can a 75Ω cable be used in place of a 50Ω cable? Simple answer is yes, but with consequences.

There are several key defining factors when it comes to coax cable. Weight, diameter, attenuation at RF, connectors, ease of installation, and so on so on and scooby dooby doo... oh... sorry.

Of these factors, attenuation is the one that will cause your signal to degrade even before it hits the antenna. There are many factors within the cable that create attenuation. This includes the materials used to build the cable, the diameter of the center conductor and diameter of the shield, the material of the shield and center conductor, and the capacitive and inductive nature of the cable. Remember impedance? That too is a factor.

So what is attenuation? It is, in simple terms, resistance. But it is not DC resistance. It is AC resistance. Remember we talked about AC resistance a few paragraphs ago. The properties of attenuation is to reduce the output of the RF energy by a value of log(10)x. Technical term there. Basically attenuation is based on a power or intensity ratio logarithm table.

If you are interested in the math, then read on else just skip this paragraph. No harm, no foul. The power or intensity uses two math formulas. dB =10*log(power in watts), and Watts=10^(dB/10). Note: dB must have something to reference, so dBV=dB Voltage or dBW=dB Watts, and so on. As an example, we will use the standard Marine VHF radio that produces 25 watts of RF power. To determine the power in dBW (or dB Watts): dB=10*log(25) dBW=13.97. So 25 Watts is about 14dBW. To convert that back to watts: W=10^(14/10) or W=10^(1.4) 14dBW=25 Watts (rounded).

To determine the attenuation of one's RF coax cable, one can visit: https://www.qsl.net/co8tw/Coax_Calculator.htm. Here one will be able to simplify the process of getting the value of attenuation based on the coax they choose. Plus as an added bonus, they will also determine how much of the RF signal is lost on its way to the antenna.

Antenna gain is theoretical. This means that the manufacture has tested their antenna and determined, based on theoretical measurements that their antenna will produce a gain of x dBd (dB with reference to a dipole antenna). It used to be gain dBi (dB with reference to isotropic radiator). The new gain figure adds 2.15 to the old value. So it looks like the antenna is performing better than expected. Go figure.

What antenna to select? That is of personal choice. There are so many different designs to chose from. I cannot recommend any one brand or type of antenna except for a mast mounted vertical antenna. But then again, which mast mounted vertical antenna? Ask on the forums for what other use. A consensus is usually the best route.

To properly calculate gain and/or losses, we have to ensure we are speaking the same language. Either work in watts or work in dB. Do not cross the two as the results will be invalid. So convert watts to dB, add/subtract dB, convert back to watts. That is the safest method to use.

I know this was long but I hope that it opens a door for you. Selecting the right coax cable for your radio/AIS system is important. We want to make sure we have the best signal possible.

All this based on my past experiences with different antennas from AM broadcast to large earth station satellite antennas. Obviously this is all my opinion and there could be errors. It is not intended to mislead anyone.
 
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