This page contains lots of material. Expect a long,
This page contains lots of material. Expect a long, facinating read!
This page is in 2 parts,
---The Reflection Section---
The purpose of this section is to explain what happens when un-used energy comes back down the coax from the antenna.
Here are some simple truths that you probably knew before you got here, but when they are all put together, you will have 7 different things happening.
It is a little difficult to keep track of all 7 things that are happening, but this section will try to help you do that. Please go slow here. Take breaks if you would like to.
It helped me to draw diagrams of all this. Please feel free to stop and grab some paper to draw a diagram or two, or more.
This page is the most difficult page to understand of all the pages in this site. It uses high school algebra but I show you every move. Please feel free to skip all the math stuff, but please read the discussion parts so you can learn what is really going on.
Your SWR meter reads the reverse energy in a coax, and converts that number into a value called the "Standing Wave Ratio". That number has very little meaning. The value is when you convert that number back into what it measured in the first place, which is the percent of returning energy. That is why you need a SWR meter.
You should always use an antenna tuner. It goes near your rig, in the shack. Its duty is to match your antenna and coax to the impedance of your rig, not to change the SWR in the coax that goes from the antenna down to the antenna tuner. Many radios have tuners built in. Some tuners are automatic.
OK, nothing is perfect, and there will be a very very small amount of resistance in the coil and capacitor in the tuner which will create a very small loss, but it is truly tiny. (0.01 dB is a good estimate) This is absolutely true, Honest!
Typical station setup using swr meter and tuner.
Note that everything between the transceiver and the "air" in drawing above
is considered as your "antenna system" referenced later in the article.
Coax #1 in drawing above is usually quite short, and coax #2 is far longer because it goes from your desk (tuner) up to the antenna.
The following information is absolutely
correct, no matter what you have heard from your engineering professors or
your favorite ham radio magazine.
The two most important people who agree with me
L. B. Cebik, W4RNL who has written many articles for the ARRL on transmission lines and antenna tuners.
This is the last, but long, simple truth.
The antenna tuner adjusts the electrical length of the antenna and coax #2 so that the reflected energy has the exactly correct phase to be re-reflected at the antenna tuner. When the tuner is correctly tuned, no energy gets back into coax #1. An SWR meter is usually placed into coax #1 as a tuning aid, to measure the reflected energy. That meter will show an SWR of 1:1 when the reflected energy has been 100% re-reflected.
Coax #2 still has reflected waves because of the mis-match between coax #2 and the antenna, but those reflections will be re-reflected at the tuner and they will add to the transmitter energy output. It may seem strange that the system is resonant and still has reflections due to mismatched impedance, but the coax and antenna are not the same impedance.
Actually, except for the losses in the coax, 100% of the energy that leaves the transmitter will be radiated out of the antenna, no matter how high the SWR, because of the re-reflection. A high SWR will create a higher loss in the coax because a higher amount of energy travels backwards in the coax. This energy going backwards is subject to the same losses as the forward moving energy.
The tuner provides a conjugate match (equal magnitude but opposite reactance) for the system from the antenna tuner, through coax #2, to the tip of the antenna ends. This makes the antenna appear to be resonant, and coax #2 becomes the correct electrical length for re-reflections to happen.
Many authors have stated that an antenna tuner tunes coax #1, but has no effect on coax #2 or the antenna. That is not a good explanation. A much better explanation is that when the antenna and coax #2 are tuned, the tuner can re-reflect the reflected energy from the antenna. That is one important reason reflected energy does not get into coax #1. The other reason is that since coax #2 is now without reactance at the matching point, the impedance of coax #1 (50 ohms) exactly matches the impedance of coax #2 (50 ohms) so no reflections happen at the front end of the tuner and all the transmitter energy gets through to the tuner and into coax #2.
This is a very sticky point. According to M. Walter Maxwell in his book Reflections:Transmission Lines and Antennas, published by the ARRL, on Page 13 - 4, he says " The antenna tuner really does tune the antenna to resonance, in spite of opinions to the contrary of those who are unaware of the principles of conjugate matching. The tuner obtains a match, by which all reactances throughout the entire antenna system are canceled, including that of the off-resonant antenna, thereby tuning it to resonance."
Please be patient here. This explanation has lots of steps, and each one is critical to understanding what really happens in the coax of an antenna system that is not perfectly matched.
The explanations are below.
The antenna tuner can not change the SWR of your antenna, or its coax, so you will need to follow these 7 steps to see what actually happens with a higher SWR than the SWR meter in coax #1 says is there. The SWR meter is reporting on that very short connection between the tuner and the rig, not the coax that goes between the tuner and the antenna, but that is "where the action is."
There are 7 things you need to know. First, I will list the 7 things, and then each one will be explained in detail.
The reason this following information is not
well known is because most people do not take the time to understand each
step that follows.
This means that energy will zoom up the coax between the antenna and the tuner and some of it will return down the coax. The "lost" energy is both lost in the coax, and radiated out into space by the antenna.2) These reflections do not cause energy loss.
Energy losses are caused by heating ( I2 * R ) or radiation, but not by reflection. The law of conservation of energy tells us that what ever goes into a reflection will come out if there is no radiation and no heating.3) Energy moving in a coax will have losses due to leakage and ( I2 * R ) heating.
These losses are well documented by the companies that make the coax. Here is an excellent calculator to find the losses in different kinds of coax Off Site Calculator . This calculator will help you convert the dB losses into actual Watts for a better understanding of what is happening.
Follow the zig - zag path of power!
Do it now.
If you have done this correctly, the calculator will tell you that Belden 9913 has a dB loss of only 0.388 dB and that calculates out to 91.461 Watts output from the coax if you put 100 Watts in to it.
Where did the rest of that power go?
It was lost to leakage inside the coax and to ( I2 * R ) heating.
4) How much of that 91.461 Watts will be used by the antenna and how much will be reflected?
The reflection coefficient is a number that tells you the percentage of reflection at the antenna - coax connection. The symbol "p" is used to represent this reflection coefficient. The math is easy to do.
p = ( SWR -1 ) / ( SWR + 1 )
We started by assuming that the SWR is 1.4 to 1. Use that 1.4 value to fill in the formula.
p = ( 1.4 - 1 ) / ( 1.4 + 1 ) = 0.4 / 2.4 = 0.166
The reflection coefficient is used for voltage, current, and when squared, it is used for power.
Since the reflection coefficient is 0.166 in this example, the voltage reflected will be 16.6% of what arrives from the generator, and the current reflected will also be 16.6% of what arrives from the generator. The power that is reflected will be the square of the reflection coefficient.
To find out how much power is reflected, you will need to use the following formula.
Reflected Power = p2 times the Power available
Reflected Power = (.166)2 times 91.461 Watts.
Reflected Power = (0.02775) Times 91.461 Watts
Reflected Power = 2.54 Watts
This means that 2.54 Watts of the forward power will be reflected back down the coax toward the tuner, and the rest ( 91.461 W - 2.54 W = 88.921 Watts) 88.921 Watts will be used by the antenna and be radiated into space.
Try drawing a picture of this. Be patient. Go slow.
Is it break time yet?
The power that reached the coax - antenna connection was 91.461 Watts and 97.25% of that power will be radiated into space, leaving 2.75% to be reflected back down the coax. Both of these percentages come from the Reflection Coefficient that has been squared.
(Reflection Coefficient)2 = (0.166)2 = .0275, which means that 2.75 % will be reflected.
Power used by the antenna = 100% - 2.75 % = 97.25%
How much power will be radiated by the antenna?
The antenna will radiate 88.921 Watts into space.
This number will get slightly larger after the reflected power is returned to the antenna, but for now, during the first cycle, only 88.921 Watts are transmitted.
How much power is headed toward the tuner?
Only 91.461 Watts was available at the antenna - coax connection, and 2.75 percent of that will be reflected back down the coax toward the tuner.
( 91.461 Watts times 2.75% = 2.54 Watts ) 2.54 Watts will be returned to the coax to go back to the tuner.
How much power gets to the tuner? http://www.ocarc.ca/coax.htm
We must use the calculator again. Put 2.54 Watts in the place of the 100 Watts just above the "calculate" button. Press the "calculate" button.
Do it now please.
Notice that 2.323 Watts gets to the tuner and the rest was lost to heat and leakage.
5) How much power is re-reflected at the tuner?
100 % of the reflected power that gets to the tuner will be re-reflected. In this case, the power that is re-reflected is 2.323 Watts. This 2.323 Watts now starts its way back to the antenna.
6) The re-reflected energy will be in phase with the generator so the two signals will add. [Note: If the two signals were not exactly in phase, the addition still happens, but the method is messy, and the result is not the same. This would be the case if the antenna was not exactly tuned to the operating frequency as it is in this example or if an antenna tuner was not correctly adjusted.]
The generator is producing 100 Watts and now it will have an additional 2.323 Watts added to it, for a total of 102.323 Watts heading for the antenna.
This is the official end of the first cycle of the generator. This first cycle started with a 100 Watt signal leaving the generator, but only 88.921 Watts was transmitted. The total loss so far due to heating and leakage was
That's a lot of information. What is the actual result ?
Loss of power going up Coax
Power reaching Antenna
Power Radiated by Antenna
Reflected Power returned to coax
Loss of Power back down coax
Power that arrives at tuner
Radiated power eventually settles out at >
This example below uses the same SWR = 1.4, as in the example above, but the COAX now has a loss of 2.5 dB using (Belden 8216) which is Rg - 174 compared with much better Belden 9913
SWR is not a killer at all.
Coax loss = .388 dB Coax loss = 2.5dB
Input Power- - - - - - - - - - - - - - - - - - - - - - - - - - - - - -100 W 100 W
Loss of power going up the Coax- - - - - - - - - - - - - - 8.55 W 43.7 W
Power reaching the Antenna - - - - - - - - - - - - - - - - 91.46 W 56.2 W
Power Radiated by the Antenna- - - - - - - - - - - - - - -88.91 W 54.6 W
Reflected Power returned to the Coax- - - - - - - - - - - 2.54 W 1.56 W
Loss of Power going back down the Coax- - - - - - - - 0.217 W 0.68 W
Power that arrives at the Tuner- - - - - - - - - - - - - - - - 2.32 W .87 W
Radiated power eventually settles out at- - - - - - - - - - 91 W 55.1 W
These losses are terrible! The coax losses have ruined the output power!!
Finally we are at the conclusion section. I hope you have seen that . . . . .
Have you ever measured the SWR of a simple antenna?
It should NOT have been 1:1
Lots of people think that a good antenna should have a 1:1 SWR as measured by an antenna analyzer. That is just not true, and this article will explain why.
We need to look at what SWR means, and how an "antenna system" is different from a simple antenna.
Unfortunately, we need some 6th grade mathematics for this explanation. I will do the math, you can just read. OK, you can do the math with me if you like.
One way to find SWR is to make a fraction of the coax and load impedance. There are other ways that work well also, but this is really simple.
SWR is really a simple fraction that puts the larger impedance in the top of the fraction, and the smaller impedance in the bottom of the fraction.
It is done this way so the answer is always equal to, or greater than, 1.
Let's try this to see what I mean. Assume that you have a 50 ohm coax and a 72 ohm dipole antenna as a load. Write the larger number on top, which in this case is the 72 ohms.
SWR =72/50 = 1.44 which means the SWR is 1.44:1
Lets try this again using a 36 ohm vertical antenna with at least three un-grounded radials.
Remember to put the 50 ohms on top this time. (50 is larger than 36 )
SWR = 50/36= 1.389 which means the SWR is 1.389:1
OK, lets stop right here and look at the results. Both answers are almost 1.4:1 but notice that neither answer is 1.0 :1. This is the whole point of this article. Antennas do not have a SWR of 1:1.
Since you now know the whole point of this article, you might think, "Why should I read the rest of this article?" The reason is to learn more, and see how to use this information.
The numbers that I picked for the characteristic impedance for the two types of antennas are actually real, and correct numbers that are found with these types of antennas. Nearly every single wire resonant dipole has an impedance of 72 ohms. Nearly every resonant vertical antenna with 3 or more un-grounded radials has 36 ohms.
The conclusion here is that these antennas have an SWR of 1.4:1 when correctly made.
Let me say that again....they do NOT have a SWR of 1:1. They are NOT supposed to have an SWR of 1:1. They ARE supposed to have an SWR of 1.4 :1.
Does it seem to you that I am getting really excited about this? Yes, I am.
Why do I make such a big deal out of this?
Because many amateurs think they have heard that antennas should have an SWR of 1:1.
What they have most likely heard is that an antenna system should have an SWR of 1:1.
I will deal with the idea of the antenna system in a minute, but before I do, it seems like a good place to tell you that there is a type of antenna that has a characteristic impedance of 50 ohms. The ground plane vertical antenna with 3 or more drooping (at 45 degrees) un-grounded radials can have a characteristic impedance of 50 ohms, BUT that is only true for the one frequency where that antenna is resonant. If you ever change frequency, the impedance of that antenna will also change which will change that SWR from 1:1 to some higher value.
Antenna systems (Remember these words!)
It is true that an "antenna system" should have a 1:1 SWR. An antenna system includes all the stuff that goes between the output of the rig and the tips of the antenna. Usually that includes the short coax that leaves the rig, the SWR meter, the antenna tuner, the long coax that goes up to the antenna, and the antenna itself.
That 1:1 SWR is measured just after the signal leaves the rig, so the 1:1 SWR is located in that short coax between the rig and the SWR meter. The antenna tuner is responsible for making the antenna system resonant and creates a 50 ohm impedance at the connection at the rig.
That short coax is the only place where the SWR is 1:1, and it is the only place where it needs to be 1:1. That is the place where the 50 ohm rig attaches to the 50 ohm coax. All the rest of the antenna system will have a higher SWR.
Please remember that the impedance of the long coax will not match the impedance of the antenna. The connection between the long coax and the antenna will NOT have a SWR of 1:1.
Where can I use this information?
The place where this is most useful is for amateurs who use vertical antennas with grounded radials. I have heard several hams say "I only need 2 radials for a 1:1 SWR on my grounded vertical antenna!"
First, please note that a 1:1 SWR means the antenna has 50 ohms of impedance. A vertical is supposed to have 36 ohms of impedance.
Here are some questions that need to be asked.....
Where did those extra 14 ohms come from?
The most likely place is in the ground system where only 2 radials are working.
What effect do those extra 14 ohms have on the signal?
The 100 Watts of power that are delivered to the antenna system will be divided among each individual impedance.
How much power will the antenna get?
The total impedance is 50 ohms and the antenna impedance is 36 ohms so the antenna will receive the fraction of 36/50 times the full 100 Watts.
36/50 times 100 Watts = 72 Watts
How much power will the ground get?
The ground system will have 14 ohms in it, so the ground will receive 14/50 times the full 100 Watts.
14/50 times 100 Watts = 28 Watts.
Please notice that 28 Watts is being used to heat the ground. The worms may thank you for this kind gesture, but it seems like a waste to me. I would add some radials to this antenna to reduce the impedance. Let those worms wear coats to stay warm.
A grounded vertical antenna needs all the help it can get to be good antenna. Eight radials is not too many.
My choice would be to have the full 100 Watts be delivered through an antenna tuner into a 36 ohm antenna with a good ground system (meaning at least 8 radials). Adding more radials will reduce the impedance, and increase the SWR of the antenna, but that can be tuned away with an antenna tuner so the whole system is resonant and will have 50 ohms of system impedance right where the rig connects to the short coax.
Knowing what the antenna impedance should be (without having your antenna tuner on) is valuable so you will know if the impedance is wrong. You can find out if the impedance is wrong by temporarily removing the antenna tuner and measuring the antenna impedance with an antenna analyzer.
Now you know
1) Why an antenna should not have a 1:1 SWR.
2) Why it is important to know what the antenna impedance is.
3) That the system impedance should have a 1:1 SWR.
4) And while it was not the purpose of this article, now you know that a vertical antenna with grounded radials needs lots of radials.
5) Actually, I have never seen worms wear coats.
73, Steve, WC7I August 2009 for Hamuniverse.com
Feel free to visit wc7i.com for
even more simple antenna ideas.
End of part 2! Know you know the answers to these questions:
1. Will a high SWR blow up my transmitter?
should the SWR of a simple antenna be?
1. It should NOT have been 1:1
2. You will not blow up your transmitter!
FIGURE OUT WHICH ANSWER GOES WITH THE QUESTIONS ABOVE!
(If you can't, you need to start over... and this time, read each word slower.
Stop skimming it as if your transmitter was about to blow up! It will wait for you!)
More good reading about SWR from
"The SWR Obsession"
by Steve Ford, WB8IMY
QST Editor and Publications Manager of the ARRL
(This is a pdf file download) You'll need Adobe reader.
More info on SWR here.
from N4UJW at Hamuniverse.com:
In Steve's excellent, very informative article, there is lots of mention of the terms "antenna system", tuners, coax loss, etc.
opinion, I don't believe his intention is to get you to run out and get a
tuner at all costs just to get rid of that last little bit of swr. This
article, in simple words, for those who still do not understand, means
that as long as your radio is working into a properly tuned
"antenna system" designed for the output load requirements of
the radio, you should not worry about getting that unrealistic
"perfect" 1:1 swr that you have heard so much about.
If you do show a "high" swr at the output of the radio greater than it can handle, either your connectors, feedline, or anything and everything between the swr meter and including the antenna and it's surroundings should be completely checked. This includes the inputs of your linear. If one or more of the parts of the total antenna system is "showing" the radio more or less than a 50 ohm load, then your modern transmitter will not put out it's maximum power! You will never get that "perfect" 1:1 swr unless you use a tuner due to losses in that non-perfect feed line that is 20 years old OR in that new coax due to small losses ....... remember the "antenna system" performing at or near 100% effeciency is your goal and...... your station!
I also have never seen a worm wearing a coat, but here is proof of one wearing an apple!
"What is worse than finding a worm in your apple? Finding half a worm! WC7I"
Now go build yourself a good "antenna system"
and leave the worms alone!!!!!.....
Maxwell's book and the ARRL Antenna Book mentioned in article.