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

Para Dynamics PDC 1089 TVI filter

The PDC 1089 was a TVI filter to attenuate harmonics generated by CB or Ham transceivers. The 1089 consisted of a PI network of capacitor and air-core inductors. Capacitor values of 47pf and 150pf at 6KV ratings. 

The Para Dynamics PDC 1089 works through the filtering or attenuating of frequencies above the transmitter output frequencies. These frequencies are commonly known as harmonics (whereas the original transmitted frequency is the fundamental) that are created by the transmitter often times due to ineffecient grounding of the station and or possiblities of radio modifications to boost modulation.

Typically, these harmonics can interfere with other audio/video electronic equipment. The PDC 1089 was designed to help eliminate these harmonics before they reach your antenna. The 1089 filter works best closer to the transmitter output, and if necessary a second filter can be used closer to the transmitting antenna. It is recommended to ground the PDC 1089 as well as all your radio gear.

The 1089 would handle 1000 watts PEP (50% duty cycle) and about 250 watts continuous.

The attenuation was as follows:

42.17MHz (color IF) -25dB

45.75MHz (video IF) -25dB

55.25MHz (ch. 2 vid) -.50dB

loss @ 27MHz -.13dB

Reflection @27MHz -18db

Curve @27MHz -28dB

Insertion Loss

1.8MHz	-.18dB
14.3MHz	-.05dB
7MHz	-.03dB
30MHz	-.13dB

As you can see by the table the area around 7MHz seems to have the lowest insertion loss point. Further tuning of the coils can help with harmonic attenuation while decreasing insertion loss to the circuit. 

The PDC 1089 has about -50 - 55dB attenuation for channel 2 video.

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PDC 1089 Schematic

Para Dynamics PDC 50 DL Dummy Load

The Para Dynamics dummy load could handle up to 100 watts. The oil can type dummy load had the same inards. Carefully scraping the powder coating around the input connector, drilling the rivets and scraping the powder coating where the top and bottom meet, would allow for frequency use up to 500MHz with a 1.76:1 SWR.

Here are the non-modified results.

Frequency	SWR
1.8-20MHz	1.02
21-28MHz	1.05
54-146MHz	1.09
225MHz	1.30
300MHz	1.33
350MHz	1.62
400MHz	2.06

*Here are the modified results*

Frequency	SWR
1.8MHz	1.0
28.5MHz	1.05
54MHz	1.09
146MHz	1.11
225MHz	1.30
450MHz	1.76
500MHz	1.76

Resistance vs. SWR

Resistance in ohms +/-	SWR
50	1.0
49/51	1.02
46/54	1.08
45/55	1.1
40/60	1.2
30/70	1.4
25/100	2.0
12.5/150	3.0

i.e. 49 or 51 ohms equals 1.02:1 SWR. For every ohm SWR changes in value .02 +/-

Estimating RF Power and Distance

Power vs. Distance

(Vertical antenna)

The charts below will reflect the distance an HF radio wave will travel across a flat surface. Some consideration concerning ground loss, the earth’s surface curvatures, atmospheric conditions, QRM, or any other factors. Assuming each antenna has a height of at least 25-30ft talking vertical to vertical. Horizontal polarization range will be a little further due to antenna gain on TX/RX.

There are many factors that will hamper performance in a ground wave, which is very unpredictable with soil, antenna height, gain, terrain, etc. This is dead key AM power and is a very relative measurement and you will see that the formula shows some linearity but not necessarily in the real-world due to terrain or ground loss.

For this experiment we will assume 4 watts will travel 11.78 miles across a flat plain giving 1 “S” unit without any terrain or noise interference. We will also assume that the  “S” meter has been properly calibrated and will have linearity errors, which may differ from the numbers obtained by formulas.

 

The charts above are just a relative measurement and accuracy will depend upon ideal characteristics. These charts are not compensated for frequency but I will be crunching numbers later to give more realistic data. I will also be testing the chart against real-world results very soon keep checking back on my website.

The charts do reflect the same vertical antenna base to base so to speak so the results will vary dependent on the variables. Please note that the calculations are for wave travel and not power gain needed for “S” units.

The values represent the travel distance of a ground wave (non-space wave) and the transmitting antenna (at least 25-30ft from the ground). Due to the nature of HF waves and sporadic atmospheric conditions you can talk much further and the frequency of a waveform will also dictate how far it travels or should we say the resistance to travel.

Notice how the curve changes after 250 watts are obtained. I ran points up to legal limits for those that are allowed to run 1500 watts (10 meters). It almost seems that when the curve hits that 250-watt mark the range starts falling off in relation to power. At the 250-watt area the curve seems to be (more) linear than compared to the lower power ranges. The curvature in the earth will also change how far the wave will travel. Use this chart only as a reference since some of the real-world values have not been compensated fully at the 100 mile + mark, but it should give you an idea.

The ground wave does not necessarily travel in straight lines at all and they tend to bounce (off objects), and become refracted. I will be covering in depth all the variables that will change this plot and hopefully we can get a better understanding on how much power do we really need? 

After a certain point in distance travel the waveform will follow the curvature of the earth’s surface. The ground variables (terrain, curvature, etc) can be ignored somewhat for the first 80-100 miles of travel. After 100 miles is reached the intensity will diminish and the distance will be hampered due to the effects of the earth’s surface and curvature.

Now the question is why is it that I can reach a certain distance but when I increase power but I do not get the same results from the charts? Some of the waveform will be generated upward (depending on antenna height and surroundings) and will vary on the terrain. In Terman’s theory the waveform takes somewhat of a concave shape and then is generated into the atmosphere. To predict we can use a formula, which I will be covering later.

The power density laws seem to work nicely at higher (100MHz and up, line of sight) frequencies (using sky wave travel) but as you go down in frequency the atmosphere will have less resistance to the waveform. The waveform is not a perfect circle (unless ¼ wave from the ground) and antenna height plays an important part on the radiation lobes and how the beam is concentrated or will travel in a certain direction. A simple example is that of a mobile antenna that is radiating on a back bumper. The angle (most part) of the lobe (in theory) should be projected towards the front of the car, so you are facing, or driving in the best radiation angle. On the roof of a car the pattern will be more symmetrical (circle like pattern). The higher the antenna the more sky wave will be generated in theory.

Some rough guidelines:

2 MHz, 45 to 100 miles daytime

10 MHz, 25 miles

20 MHz, 20 miles

30 MHz, 17 miles

(See chart below)

The 11.78 miles figure I am using is based upon Terman’s short antenna principles and formulas. In theory the wave should travel around 18 miles at 27-28 MHz, but I have also taken into account the sensitivity of the “S” meter and terrain.

*Note all calculations are relative at this point you may be able to receive a signal better than 1 “S” unit at 11.78 miles away depending on your antenna height, location, and sensitivity of your radio.

 

Frequency

The lower the frequency of a radio wave, the more rapidly the wave is refracted by a given degree of ionization. The picture below shows three separate waves of differing frequencies entering the ionosphere at the same angle. You can see that the 5-MHz wave is refracted quite sharply, while the 20MHz wave is refracted less sharply and returns to earth at a greater distance than the 5MHz wave. Notice that the 100MHz wave is lost into space. For any given ionized layer, there is a frequency, called the escape point, at which energy transmitted directly upward will escape into space. The maximum frequency just below the escape point is called the critical frequency. In this example, the 100MHz wave’s frequency is greater than the critical frequency for that ionized layer. 

The critical frequency of a layer depends upon the layer’s density. If a wave passes through a particular layer, it may still be refracted by higher layer if its frequency is lower than the higher layer’s critical frequency.

Angle of Incidence and Critical Angle

When a radio wave encounters a layer of the ionosphere, that wave is returned to earth at the same angle (roughly) as its angle of incidence. The figure below shows three radio waves of the same frequency entering a layer at different incidence angles. The angle at which wave A strikes the layer is too nearly vertical for the wave to be refracted to earth, however, wave B is refracted back to earth. The angle between wave B and the earth is called the critical angle. Any wave, at a given frequency, that leaves the antenna at an incidence angle greater than the critical angle will be lost into space. This is why wave A was not refracted. Wave C leaves the antenna at the smallest angle that will allow it to be refracted and still return to earth. The critical angle for radio waves depends on the layer density and the wavelength of the signal. The figure below-Incidence angles of radio waves.

As the frequency of a radio wave is increased, the critical angle must be reduced for refraction to occur. Notice that the 2MHz wave strikes the ionosphere at the critical angle for that frequency and is refracted. Although the 5MHz line (broken line) strikes the ionosphere at a less critical angle, it still penetrates the layer and is lost. As the angle is lowered, a critical angle is finally reached for the 5MHz wave and it is refracted back to earth.

Measuring SWR

Measuring SWR

The SWR measurement is the most important part of your setup. The term VSWR or SWR stands for Voltage Standing Wave Ratio. In basic terms this is the reflected voltage or currents that travel from your transmitter to the antenna but due to impedance mismatch (not 50 ohms) you will have a reflected wave. The ratio of forward vs. reflected power is measured in a ratio, which is SWR.

Typical SWR values range from 1:1 to 2:1. Anything 2:1 and under is very satisfactory. There are several myths about having an SWR range over 1:1 and they are just that a myth. Now that we have an idea of what SWR means let’s hook up our meter and see what SWR readings are obtained. You will need a small coax jumper to connect your radio to the SWR meter. Make sure you connections are correct for input and output. Output is the antenna and input being your radio.

There should be a FWD setting this is the forward power sampling. You will want to key your transmitter, set the to the FWD setting and adjust (using the SWR cal knob) for a full-scale reading or to the set mark on your meters scale. Next un-key the transmitter and set the switch to REF this will measure the reflected power. Your meter should read this reflection as an SWR reading. The circuit components will calculate the reflected power and display this as an SWR reading on the scale.

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Resistance vs. SWR

Resistance values for a dummy load or antenna impedance value will determine the SWR. Looking at the chart represented above we can see how the SWR changes with the increasing resistance values on the “Y” axis. We can also have different values of lower resistance's for SWR levels. For example we can have a 1.5 SWR with the resistance values of 75 ohms or 35 ohms. A rough estimate would be that for every ohm of resistance +/- over/below 50 ohms you would see a drop in SWR of .02 See the chart below.

XBOX 360 Red Ring of Death

The Red Ring of Death Introduction.

One of the most common problems with the XBOX 360 is where the GPU/CPU temperatures have exceeded a safe operating temperature. I have successfully repaired four XBOX 360's by re-seating the heat sinks and replacing the X-frames with spacers and regular bolts.

The main cause can be attributed from the cooling fans not operating at the proper efficiency. First the design of the plastic duct, on the GPU portion where the DVD player resides, the duct has a near shut-off design. This shut-off is like sticking a piece of cardboard (restrictor plate) in front of the inlet. This severly hampers airflow. There is also a much smaller heat sink that is used on the GPU to accomodate the DVD player. This shaved down heat sink, coupled with the near shut-off condition of the fan really are the two main causes but not the only causes for overheating of the GPU. On the newer XBox 360 units, they did add a cooling tube and attached a smaller heatsink inline with the CPU side which did help to alleviate the cooling restrictions.

Second, the two cooling fans typically run at 5 volts, but are most efficient at the 12 volt DC ratings. The problem of course is that running these fans at 12 volts is the noise levels becoming excessive. What I plan on doing is to construct a Pulse Width Modulation (PWM) circuit with a temperature sensor to speed up the fans from a range between 5 volts to 10 volts DC. This circuit will ramp the voltage to the DC fans dependent upon the temperature increase over time.


Another option is to simply add a resistor of say 10 Ohms 5 watts to drop the fan voltage down from 12 volts to 9 volts which would allow for the fan to run at 75% of full speed (RPM). This is typcially known as a voltage divider circuit, the voltage is divided between the resistor and fan, the resistor drops 3 volts and the remaining 9 volts is delievered to the fan. With this mod typical current draw is around 270 milliams in the series circuit.


I will be posting more on this modification in the next upcoming articles.