
This page is in 2 parts,
all about....... The Reflection Section The purpose of this section is to explain what happens when unused 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. Electrical energy moves forward and backward in a coaxial cable and in ladder line. (Everything I tell you about Coax is also true for ladder line, except that ladder line has far less loss.) Electrical energy moves forward because the generator (your rig) pushes it toward the antenna. It moves backwards because the antenna can not absorb all the energy, so the unabsorbed energy goes back down the coax. (The absorbed energy is converted into ElectroMagnetic energy and is transmitted out into space.) The reflected energy will be rereflected when it reaches
the tuner or the tuned circuit in the output stage of the transmitter. NO
LOSSES happen at the reflection points, and your rig will not blow up
because reflected energy got into the tuned circuit. 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 are:
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 rereflected 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% rereflected. Coax #2 still has reflected waves because of the mismatch between coax #2 and the antenna, but those reflections will be rereflected 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 rereflection. 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 rereflections 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 rereflect 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 offresonant antenna, thereby tuning it to resonance." An even better way to describe what happens is to point out that the specific spot called the "matching point" is where the impedance is 50 Ohms with zero reactance and it exactly matches the impedance of coax #1 at that point. There is really no need to claim that coax #1 or coax #2 have been tuned, because it is the "matching point" that is connected to coax #1, not the complete length of coax #2. 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. This is the end of
the simple truths.
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. 1) Reflections happen at the coax  antenna connection, and again at the coax  tuner connection.
2) These reflections do not cause energy loss.
3) Energy moving in a coax will have losses due to leakage and ( I^{2} * R ) heating.
Follow the zig 
zag path of power! Where did the rest of that power go? 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.
Notice that 2.323 Watts gets to the tuner and the rest was
lost to heat and leakage. 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 ? 

SWR =1.4 
SWR= 3.0 
Input Power 
100W 
100 W 
Loss of power going up Coax 
8.55W 
8.55W 
Power reaching Antenna 
91.46W 
91.46W 
Power Radiated by Antenna 
88.91W 
68.59W 
Reflected Power returned to coax 
2.6W 
22.86W 
Loss of Power back down coax 
0.217W 
1.95W 
Power that arrives at tuner 
2.32W 
20.9W 
Radiated power eventually settles out at > 
91W 
86.7W 
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.
Belden
9913 Belden
8216
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 have come to the very last subject on this page.
Because
that can happen, but it is not due to the reflected power!
There is a totally different reason.
A high SWR on an antenna probably means that the antenna is not tuned
to the frequency that is being used. This, in turn, means that the antenna
has some inductive or capacitive reactance that is detuning the final
amplifier. Detuned final amplifiers draw far too much current and can
burn up. The rig or linear amplifier will have to be retuned to avoid
creating too much heat.
Many linears and nearly all tube amplifiers
have some tuning knobs that allow you to "dip the plate current" or adjust
the SWR by adjusting something on the front of the device.
Transistor rigs usually do not have any
tuning adjustments. To avoid the extra heat created when running a
detuned amplifier, there is a protection circuit that will significantly
reduce the output power if the SWR is high.
Finally we are at the conclusion section. I hope you have seen that . . . . .
Antenna SWR
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 ungrounded 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 ungrounded 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) ungrounded 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:
Questions:
1. Will a high SWR blow up my
transmitter?
2.
What should the SWR of a simple antenna be?
Answers:
1.
It should NOT have been 1:1
2. You will not blow
up your transmitter!
NOW
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
QST:
"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.
A note 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.
In my
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
nonperfect 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!!!!!.....