There are some things you need to understand when building your ham station whether it's a base station or mobile.
Terms you will eventually learn and need to remember is "gain", "loss", "efficiency" and ERP.
An antenna has a specification called "gain". It refers to the difference between the power at the input to the antenna vs the power you get out of the antenna into free airspace. Coax has a rating specified as "loss". This refers to the difference between how much power is introduced to the coax at the transmitter and the actual power you get out of the coax going into the antenna. It will go a long way when you understand how to use these numbers. Primarily the term you will use in both is "dB".
Let's talk first about antennas. There are various types of antennas. Verticals, Yagi beams, horizontals, directional and omni directional (all directions). We will discuss antennas primarily for VHF/UHF omni directional operation even though these rules and formulas are the same for all frequencies. Typically these are verticals. They are used mostly on vehicles and base stations for "local" use. Horizontal antennas are used for DX (long distance) and AM work but that is for another discussion.
A typical reference antenna is a 1/4 wave vertical or ground plane. It typically has a "gain" of 1. In other words, putting 10 watts into the antenna will give you 10 watts out in all directions. All antenna specifications you will see, refer to gain as referenced to a "1/4 wave isotropic radiator". So an antenna with a gain of 6 dB is 4 times the input based on the increase of power over a 1/4 wave radiator, meaning 40 watts ERP or "effective radiated power" or "equivalent power" with 10 watts into the antenna.
|dB Value||Power ratio (gain)|
Looking from the above the antenna, sometimes called a polar pattern, your "pattern" has the same "gain" in every direction as shown below on the right.
Directional antennas have their place. They are used in point-to-point remote sites, direction finding, or if your house is too far away from a station like a repeater and you still need to work the repeater. A directional antenna will push all or most of the power in one direction. It is also useful on a tower with a motor controller to point it in a specific direction or azimuth. The gain can be very high, from 3 to 15 db or more. Stacking and phasing these antennas can obtain huge gains in excess of 25 db or more. These antennas are used by some hams to bounce signals off the moon. The pattern from above the antenna is shown below.
Coax is used to connect your radio to the antenna. Coax is a special cable with a center conductor, surrounding insulation, a braided or foil shield covering, and an exterior insulation jacket, usually black. Coax is used to connect your radio to your antenna. All coax is "lossy" meaning whatever power you place into the coax, a small amount will be absorbed by the coax and/or reflected back to the radio. No coax is 100% efficient. If the coax is relatively short, 30 ft or less, this shouldn't be an issue. At higher frequencies and longer lengths you can suffer huge losses so it is important to calculate and specify the best coax for your ham station.
There are a number of websites with tables so you can input your coax type, the frequency you desire to use, and the length of the coax. The site will give you a "dB loss" for your parameters and you can decide to shorten the length or use a more expensive coax or take some other action.
100 feet of RG-8 at 147 mhz is 2.26 db loss (sometimes you may need the number in efficiency so it would be 59.43%
100 feet of RG-58 at 147 mhz is 5.9 db loss or 25.6% efficient. This means you loose 3/4 of your power in the coax. Not good.
To understand this in another way, let's say you had a length of coax at a given frequency with a loss of -6 db. Let's say you had an antenna with a gain of +6 db. Then (-6 db) + (+6 db) = 0 db gain/loss so let's say your transceiver power output was 25 watts, your actual equivalent power output from antenna would be 25 watts.
Said another way, if your coax had a loss of -6 db and the antenna only had a gain of +3 db, your equivalent power out would be 12.5 watts.
Shockingly if you had a coax with a loss of -10 db and an antenna with a gain of 0 db (unity gain) and your transmitter had an output of 10 watts, you'd only have an equivalent power output of 1 watt!
With a Prepper mindset, obviously you can see that having as much gain in an antenna with low loss coax means you won't need to waste precious electric power from a higher power transmitter especially from a battery, in a disaster scenario while achieving good communications.
Copyright ©2021 Rick C. Ver 0.03 Nov 20 , 2021