Antenna/Power Gain


with a db here and a db there, everywhere a db


With a few exceptions, decibels or db is a relative power measurement. Originally developed to express the response of human hearing to changes in audio levels, it has been found to be useful in many other areas as well. Most commonly used in HAM RADIO to express relative power gain or lost in RF amplifiers and/ or antenna systems, it is a handy and easily understood tool. Most ham radio receivers S or Signal meters are calibrated using 6db per S-unit. Since almost all of us are familiar with S-meter readings I will refer back to it from time to time. For the most part an increase of less then 3db has minimum value to a ham radio operator representing only 1/2 of a S-unit increase, not justifying a major outlay of time or money. However, while 3db is not a major improvement it may be the difference between making a contact or not making that contact.

What's it worth to you?

Each 3db increase is equal to doubling the reference power.

(3db = X2) ( 6db = X4) (9db = X8) (12db = X16) (15db = X32) (18db = X64) (21db = X128)
other useful levels are:
(10db = X10) (13db = X20) (20db = X100)


Referring back to the S-meter you can see that it takes 4 times the power (6db) for an increase of 1 S-unit.

If you are receiving a station at S7 and that station is running 100 watts of power, to increase to S8 would require that the power be increased to 400 watts and for S9 1600 watts. Since this operates both ways maybe this will help you to understand why that legal limit amplifier that you took out a second mortgage to buy did help but didn't do as much for you as you had hoped.

The area most often misused when antenna gain is discussed is the reference. Saying that an antenna has10db gain doesn't mean anything unless you know what it is being compared with. The most common reference used in ham radio is the simple 1/2 wavelength dipole antenna and is referred to as dbd (db over a dipole). Another less common reference is the isotropic antenna referred to as dbi (db over an isotropic). The isotropic antenna only exist as a theory and radiates equally is all directions. It has a negative gain of 2.1 db when compared with a dipole. The gain of an antenna stated as 10dbd could also be stated as 12.1 dbi both being correct. Many people buy by the numbers, since an antenna rated in dbi has 2.1 added to it's dbd rating some manufacturers have switched. An antenna rated at 10 dbd has more gain then one rated at 11.5 dbi.

Watch out for that reference.


Power gain for amplifiers is sometimes described in db's. An amp. having 10db gain and rated for 1kw will require 100 watts drive. Likewise an amp. having 10db gain and rated at 100 watts will require 10 watts to drive it to full power. Both amplifiers have 10db gain or increase the power by 10 times, that is the relative part, while the input and output power levels are vastly different. Your greatest asset when comparing RF amplifiers is YOUR RF power meter and YOUR dummy load. Knowing the readings you get normally, either an increase or decrease in power will be readily apparent.

The majority of Power meters are not accurate and each will give a different reading.
Measuring power with anything other than a dummy load will give unreliable readings.

 

Tiny was here!