RF Foward and Reflected Power

Forward and Reflected Power

In this article I will be covering forward power, reflected power, loss (dB), and SWR. Let’s start with forward power. Forward power is the power (voltage / currents) generated from a transceiver that are transferred thru the coaxial lines, meters, etc., that are connected to the antenna. We also have a reflected wave that occurs due to the mismatch from the line to the antenna. For now we will forget the characteristics of train waves, skin effects, coax lengths, etc., to keep this article on track with the basics.

The reflected waves are the waves (currents) that are returning to the transceiver due to a mismatch (antenna not being 50-ohms Z). The ratio of the forward vs. the reflected power is called VSWR or SWR. This ratio is calculated by comparing the forward to the reflected waves. A meter will display this reflected power as SWR on the readout. I have a chart that will show (in percentile) of how much power is being lost or absorbed from the actual transmitted power going to the antenna.

To make use of this chart we will take an example of 4 watts of forward power and having an SWR reading of 2.0:1. If you follow the bottom (VSWR) and find 2.0 (drop down-lines) we move across to the left and where the two points intersect is 11% power loss. We take 4 watts multiplied by the 11% we get at total of 0.44 watts or 440 milli-watts of power being lost or absorbed. This means we have a total of 3.56 watts of forward power going to the antenna. For simplicity sake we will not be using effective ERP which involves more decibel calculations.

If you look at the chart below, you will have the calculated values for loss, which will break down the values into smaller SWR numbers. Usually an SWR of 2:1or less is very acceptable, which is about a loss of 11 percent or 0.44 watts.

Table 1

In table 1 we will be using the VSWR, Reflected power (%), and transmission loss (in dB) columns. The return loss was calculated for a power meter, which we will discuss in a later topic. With the forward and reflected explained in reference to the SWR calculations we also have attenuation from coax, connectors, coax switches, and meters. The power lost thru these devices can be combined and subtracted from the antenna gain to give what is called effective radiated power or ERP. This (ERP) is the actual wattage or power that will be delivered to the atmosphere.

Coax has attenuation that is calculated with length, transmitter frequency, dielectric constants, and velocity factors. This may sound confusing but the main concern is that we have losses in the coaxial cable itself and within the connectors. To make up for this attenuation or loss we will need to install an antenna with enough gain to overcome the losses that are added by coax, connectors, meters, etc. This is the main reason why the antenna is the most important part of your setup. It also helps to have a receiver with a good front end and has great image rejection (adjacent channel splatter).

For insertion loss we will also have to consider the wattage lost along with our SWR chart values. This along with attenuation in coax, and meters will also add up rather quickly. This is why the antenna gain is so much more important than adjusting a transmitter for an extra watt. (See power vs. distance article). Every watt helps, but the most powerful gains are within the antenna and its height. If you have made it through the article so far you are on your way to understanding the importance of a good antenna. There are more in-depth issues that come into play with antenna gain and insertion loss but in this article we are only covering the basics to help understand some of the basic principles that hamper a radio wave.

After an SWR of 2:1 or more is obtained you can see in the chart how rapid the percentage in loss occurs. You can still use an antenna that has a reflected 3.0:1 SWR but you will have some loss. On receive it won’t make much difference in received “S” units as long as the antenna is fairly close to being of a resonant length for that frequency range. So hooking up a receiver to an antenna of 3:1 SWR should be fine and the received strength should not be hampered if at all. The antenna gain will actually determine the received “S” units; of course a fine tuned receiver front end is always helpful. Insertion loss is basically an attenuation of the forward power. If you look at the chart you will see for example 0.5dB loss is about 11% lost power or same as an SWR of 2:1. The insertion loss is measured in dB and can be measured using a spectrum analyzer. Most power meters, coax switches, pre-amps, etc will be rated accordingly. The insertion loss is very critical and it should be observed with caution. Let’s say we have a power meter that is rated for 0.02 dB insertion loss (remember insertion loss is frequency dependant, or varies by frequency) that gives us a power loss of 0.488 percent.

Most coaxial cable manufacturers rate their cable in loss by dB per 100ft. We must also add this attenuation to the meters, switches, connectors, etc. As you can see this will add up very quickly and will add more loss. You will want to keep your coax lengths short as possible, and use the best quality cable such as 9913 or the best you can afford! The goal of this article is to make known forward, reverse power, SWR ratios, insertion loss and the importance of quality cable, low insertion loss equipment, and high gain antennas.

73

Technician Times ©2011