The SWR. That
(Last update 25/08/00)
By Miguel R. Ghezzi (LU 6ETJ)
(To read this article correctly
you must have installed the "Symbol" font)
Very few topics in the vernacular amateur radio are so polemic and they are as infested of conceptual errors as the SWR and their family. That would not be so bad, if it was not because the errors come winning the battle...
So far the confusion that it won't lack who say that the matter is a mere "question of opinions". An unfortunate and lapidary sentence synthesizes the bewilderment: "Each teacher with their booklet... "
It doesn't care that no serious textbook endorses the misunderstandings explicitly, like Don José Ingenieros would say:
"They are as the nails, the more it hits them, deeper they are sunk..."
(He said that about the prejudices). Frequently they derive
from a superficial, incomplete and/or inattentive reading but other, more comprehensible, of certain technical complexities that the matter
Since the topic is long and difficult of explaining without mathematical developments that the Ham could not understand, I will begin the article with a series of statements that I will expose without demonstration with the healthy intention of developing them some day. Then:
- It is not true that it is necessary to
match the antenna to the line so that the system is an effective radiator.
- It is not even true that it is necessary in VHF, UHF or microwaves.
It will almost always be more convenient and more comfortable to match the line to the radio "below" and not "up."
- In general it is not true the callled "Reflected Power" gets lost.
- It is not true SWR produces ITV, harmonic, spurs or interferences to other services.
- It is not true reflected power
"reenter" to the transmitter (at least not in terms intuited by
the majority of hams) and destroy the final transistors or tubes.
- It is not true that varying the
the line lenght you can modify its SWR.
- In general it is not true that the feeding line should be cutted to some multiple or submultiples of the wave lenght
(unless it is using it as transformer in the foreseen cases, see
explanations later on).
- It is not true the SWR makes
"the line irradiates."
To keep in
The incident power and
the reflected power they don't represent what their name intuitively
In what concerns to the security and
correct operation of the equiment, SWR it doesn't have any
importance, waht it cares it is the load impedance on which it operates.
The previous list will surely be considered
a "absurdity " for many Hams (and not few professionals), but it is correct and
|How to face the
The correct way of developing this great
topic it would be to begin with the basic theory, but in that case it
would be enough to read the numerous and more solvent authors of radio
technical books and radio engineering that, they are
plentiful in the libraries (some reflected in the bibliography that
offers on the end of the article), but the medium ham it will be more
anxious to have some answer to these statements right now, that
which takes not less authoritarian statements, so that we will
attempt in an almost disordered way to go advancing in the
demonstrations with numerous redundancies and "turns to the same
Surely with the time and the critic I shall be able to
modify this article to be more comprehensible and structured.
The losses and the line mismatch, first
Many Hams usually think that high SWR it is responsible for important losses in the
radiated power. This
misunderstanding arises almost naturally by reading a directional wattmeter
indicating a certain Direct Power versus a
Reflected Power for a given system. This last number, always it is
sufficiently big as to worry the man. Few notice the
importance of a small graph that is published in the ARRL Handbooks from
long time agvo. In it we can see a scale that indicates "Additional losses
caused by Standing Waves" that it is function of the
losses for perfect match and actual SWR.
A glance to this interesting graph shows
that the losses to due SWR are, in general, very lower to those that are deduced from reading the
"Reflected Power" indicated by
the wattímeter. This apparent contradiction between wattímeter reading
and the graph results should be enough to convince to
anyone that "something is bad in their intuition" (or that something it
is bad in the handbook"), but although it shows clearly that the
resulting losses 5: 1 SWR on 80 m can be worthless, the
force of words "Reflected Power" almost always wins the game...
To use it we should know which those are line losses in
dB when the line is correctly matched to the load, either for the manufacturer data
provided or by mensuration. We enter with that
information to the horizontal axis indicated as "Line Loss in dB When
Matched" tracing one vertical line until intersecte the
corresponding for SWR measured near the load (the curves are those
that have the SWR = xx" legend) and in that point we trace a horizontal
line toward the left until to reach the marked vertical axis as "Additional loss, etc, etc" that
it will tell us which it is the
additional loss that we should add to the losses of the matched line to
find the total loss in presence of SWR.
Note: This graph represents average values, is more accurate as greater
is line length iin terms of wavelengths, or when its length is close to half
A line perfectly matched has 3 dB loss, is connected to an antenna
that gives us a reading of 2:1 SWR measured near the same one. Which
it is the additional attenuation that will have on this line for the
presence of this SWR on it?
We enter to the graph with the
value 3 in the horizontal axis and we look for the intersection
with the curve "SWR = 2". Tracing a horizontal line toward the
left, on the vertical axis we can to read: approximately 0,35 dB.
Therefore the total loss of this line will be of 3,35 dB. Hardly
0,35 dB more than being perfectly adapted...!
To know if a
system mismatch is or not important in terms of losses it is necessary
first look this graph and decide if it is worthwhile to
adapt the antenna or not, remembering that there is an usual parameter
to characterize a receiver "Minimum Discernible Signal (MDS)" and that it is considered as
3 dB above the
noise...! Don't forget this when the additional losses are 1 or 2 dB, because they won't possibly be very
Since in VHF and UHF, the losses of those
lines are more remarkable for a perfect match, those additional
losses caused by SWR will advise a better adjustment of the antenna,
but with the existence of low prices, low losses lines for V and U, it will be less important than with our old well-known RG 8 or RG
If you carefully observes the graph will
see that to improve SWR below 2:1 or something more, it is an
effort that is not justified in the practice, since still when all
the reflected power got lost in the line it would represent only 11% of
losses that is hardly 0,5 dB.
Don't forget that an
"S" unit it is equal to 6 dB, of there that 0,5 dB is a "S" fraction impossible
of detecting even with the best ones.
We have said that SWR
that we make reference it is the one that exists on the line in the
proximities of the load. If line has losses (specially if they are
of importance), that loss will make SWR measured far from the load it is smaller, for what we can to reach
mistaken conclusions of the graph. Happily, with another
similar graph we will always be able to know the SWR on the load measuring
SWR at the transmitter end. For we use it the one also coming from ARRL handbook.
We enter to
graph with SWR measured at the transmitter end to the horizontal axis
marked as: "SWR AT TRANSMITTER"
Of there one is traced vertical line until
intersect the curve that indicates the loss that would have the well
matced cable and, starting from the intersection, we go toward the
vertical axis of the left where we will be able to read the SWR at the the
Let us suppose that the cable has 3 dB loss and the SWR meter reading at
the transmitter en it is 2:1. Which is SWR on the load?.
We look for "SWR AT TRANSMITTER = 2",
in the horizontal axis. We ascend until the "3 DB LOSS"
and on the vertical axis we read: SWR AT ANTENNA =
To adapt the impedance above or
In presence of SWR the impedance that
the line presents to the transmitter can be very different from that for
which has been designed, with the problems that this carries. It is
usually necessary to present to the transmitter an appropriate load impedance, generally
50 Ohms. The question is if it is better to match the antenna to the line on
the antenna side or the line to
the rig near it.
Any impedances matching device
introduce some additional loss, why we would have
to suppose that to match the impedance with the Ringo's hoop,
the Yagi's Gamma or slicing rod of a Slim will be better than doing it with a
transmatch comfortably installed in our
transmission shack?, keeping in mind that in the shack
we can adjust the impedance in all the frequencies while the
Gamma is adjusted for only one...
It is not logical to think that a
matching device exposed to the bleakness is subject to bigger probabilities
of deterioration that the indoors adapter?. It is not easy to realize
that to adjust that antenna in the heights is far more
dangerous for the health than to adjust it from an armchair?. And all that
because somebody that doesn't know very well about what he/she is
saying, sack to glitter "The tables of the law" replete of commandments
that he/she can hardly justify with dark and contradictory
Whenever to adjust the antenna is easy and
those losses for excess of SWR are important, it will be very convenient
adjust it on the antenna side, in 90% of the remaining cases a good
or "Pi" network at the transmitter side, will provide us excellent and comfortable
Why the power
reflected doesn't get lost ...
Since we have discarded mathematics demonstrations can carry out a simple experiment that allows to see it
empirically and with the own eyes to check it immediately...
The test set is easy to build for any Ham.
Choose an antenna or load that produces a significant
SWR on the line, enough to convince you (but not
infinite). Build the outline showed in the figure, adjust the
Sorry, the file was truncated , I will have make a
new translation soon :>)