Everyone involved with radio transmitters needs some instruments
to assess basic antenna functionality. Among these instruments, the
best-known and most-used one is the Standing Wave Ratio meter. Some
radio amateurs develop a cult for these little gadgets, having them in
line all the time and watching the needles bounce while they chat. I
have seen some guys owning 5 or 6 SWR meters, and no other instrument
relating to antenna testing! While it's unfortunate that some people -
specially amateurs - assign so much importance to SWR and so little to
other parameters, it's also a fact that SWR needs to be known, so if you
use transmitters, you need an SWR meter.
What is SWR?
Transmission lines have a certain characteristic impedance,
typically 50 or 75 Ohm for coaxial cable, and about 300 to 450 Ohm for
open-wire balanced feedline. Such an impedance stating means that the
cable is naturally suited to carry a ratio of voltage to current
according to this impedance - for example, a 50 Ohm coaxial cable should
carry 1A for every 50V applied to it, and the phase of voltage and
current should be the same. Only in these conditions will the cable
transfer the most power while loosing the least, and more importantly,
only if this ratio is adhered too, the ratio will stay the same along
the cable! That means, if to a 50 Ohm cable you connect a load (antenna)
with a different impedance, then the cable will transform this
impedance, so that at the input to the cable you will have an impedance
that is neither the load impedance nor the cable impedance!
It's outside the scope of this article to explain in full the
impedance transformations along transmission lines. You can look that up
in many electronics textbooks. For now, let's go back to the theme of
SWR! The fact is that the impedance transformations along a transmission
line lead to sections with higher and lower voltage, and higher and
lower current, called standing waves because the highs and lows are
spaced in wavelengths at the operating frequency, and do not move along
the cable. The voltage standing wave ratio is simply the ratio between
the voltage at the highest and lowest point of such a standing wave!
This ratio happens to be equal to the ratio between the highest and
lowest current, and also is equal to the ratio between the cable
impedance and the load impedance! Which means that it is also equal to
the ratio by which the current-to-voltage ratio departs from the correct
value it should have for that cable!
Examples always help to clear up misconceptions. So here goes an
example: Let's say that you connect 10 meters of 50 Ohm coaxial cable to
an antenna that is perfectly resonant on 146 MHz , but has a feed
resistance of only 25 Ohm. This will be an SWR of 2:1, because the
antenna will be taking 2 A for each 50V applied to it, twice as much as
the cable likes. The cable will transform this impedance along its
length: A quarter wave away from the antenna the impedance will be 100
Ohm! Another quarter wave further, it's back to 25 Ohm. This cycle
repeats: Every half wave from the antenna, the impedance will be 25 Ohm,
in the midpoints between them it will be 100 Ohm. At all other points
along the cable, the impedance will be reactive, even if the antenna is
perfectly resonant and thus has no reactance! And now, the most
confusing statement for novices: Even while the impedance varies through
a large range along the cable, the SWR along it stays totally constant
at 2:1! If you don't believe this, you have several choices: Try it, or
study it in a book, or think about it, or close your eyes to the fact
if you prefer; but the fact will not change!
In practice, a lossy coaxial cable will tend to make SWR lower as
you get away from the antenna, but you need really lossy cable to notice
this, and you should not use such bad cable!
Instead of this approach, you can also think of SWR in another way:
Imagine the transmitter sending a wave up the cable, in the proper
voltage to current ratio. This will be a traveling wave. Now, the load
will reflect a portion of this wave if it is not of the same impedance
as the cable. This reflected wave will travel back to the transmitter,
creating interference patterns along the cable, resulting in the
standing wave of voltage and current maxima and minima. While one wave
travels up and the other travels down the cable, the interference
patterns stay fixed. This is just another way to look at exactly the
same phenomenon.
Common SWR meter topologies
It's easy enough to go into a radio store and buy one. You can get
them in a wide variety of sizes, shapes, power ratings, frequency
ranges, in analog and in digital versions, with a single meter, with two
meters, with a crossed-needles meter, and I would not be surprised if
you even could choose the color! But there is one department in which
almost every factory-made SWR meter falls short: Frequency coverage.
Many SWR meters are limited to just the HF range, just VHF, and often
they are accurate only over an even smaller range.
This limitation comes from the technology they use. There are a few
circuit topologies in common use. One is the toroid bridge. This is
basically a design in which a sample of the antenna line current is
taken via a toroidal transformer, and a sample of the voltage is taken
by a capacitive divider. The two samples (both converted to small RF
voltages) are combined in proper phase relation and amplitude using a
bridge circuit, and then rectified, the result being two DC voltages
proportional to the amplitudes of direct and reflected waves on the
transmission line.
This approach works well over a moderate frequency range, and even
permits accurate power measurements over such a range, but at very low
frequencies the current sample gets too influenced by the magnetizing
current of the transformer, and the capacitive divider rises too much in
impedance, so that both sample voltages become inaccurate. On the high
frequency side of the range, the inter-turn capacity of the transformer
plays a large role, and the capacitive divider affects the measured
impedance of the transmission line, again making measurements imprecise.
This circuit can be used easily over a frequency range of 1:10, and
some manufacturers push it as far as 1:100 (160 m to 2m is common), but
this compromises accuracy on both sides.
The other very common commercial SWR meter topology is the
Monimatch. It is harder to understand for newcomers, as it is a
transmission-line design employing distributed coupling between the coax
line and one or two sensing wires. It is extremely simple in design,
but needs to be properly built to work well, and it has a very large
disadvantage: Its sensitivity varies hugely with frequency! Such a SWR
meter, designed for HF, may require much more than 100W to take a
reading at 160 meters, and may be burned out by a similar power on 10
meters! If you try to use it at VHF, even 1W may drive it crazy, and you
will not get an accurate SWR reading.
SWR meter
I went back to the very basics, making an SWR meter that someone
surely has built at least 100 years ago, but I have never seen it used
in any commercial or homemade gear. It is a simple resistive bridge. It
works perfectly well over a very wide frequency range, is highly
accurate over it, and is very simple and cheap to build. Are you
interested? If so, then I have to tell you about the disadvantages too:
It can only digest low power. A half Watt is more than enough for good
measurements at any frequency, and up to 2W is OK. Above this, it will
start smoking! So, if you want to measure SWR using your 100W
transceiver, you had better remember turning the power down before
connecting the meter. Of course, measuring SWR of antennas at low power
means much reduced chance of creating interference to others! It also
means eliminating any risk of burning up the transmitter by measuring an
unknown load with very high SWR - more so as the resistive loading
provided by this meter will guarantee that your transmitter never sees
more than 2:1 SWR, regardless of what you do at the meter's output!
This meter is designed as a test instrument, to be connected, used
for taking the necessary readings, and then disconnected. It makes no
sense to leave it in the line permanently, like some people do, because
it eats up 3/4 of the transmitted power, and a similar part of a
received signal.
Here is the schematic of the SWR meter, exactly as I published it in
the Chilean magazine Radioafición. You can also get a high resolution
version for printing purposes by clicking on the image. I hope the few
Spanish words will not upset you too much. I was too lazy to edit
them...
Three pairs of 100 Ohm resistors are combined in a bridge circuit,
with the load as the fourth resistor. D1 rectifies a sample of the input
signal, while D2 rectifies the differential voltage across the bridge,
which is proportional to the square root of reflected power. In this
case the two output voltages are applied to a ganged potentiometer and
to two simple galvanometers, but I have built several other units in
which these voltages are used by digital displays, microprocessors, etc.
Let's do some analysis: At first we will assume a 50 Ohm load is
connected to the antenna side, and a 5Vrms RF signal is applied to the
input (this would be 1/2W across 50 Ohm). At the anode of D1 we would
have 2.5Vrms, or 3.5V peak, which would give 3.5V DC (the voltage drop
of the germanium diode is negligible at the low current involved). D2
would see exactly the same RF voltage at both sides, and in the same
phases, so it will not produce any DC output. The forward meter will
move (we can set the pot to place the needle at full scale), while the
reflected signal meter will stay at zero, indicating a 1:1 SWR. The
transmitter will see 2 times 100 Ohm in parallel, or 50 Ohm, 1:1 SWR
too. One quarter of the power will be dissipated in each of the resistor
pairs, while the remaining quarter gets delivered to the antenna.
Now let's go to one extreme: Disconnect the load! We all know that
this is infinite SWR. D1 would still see half of the input voltage, and
as there is now no current in R1/R2, and no voltage drop across them, D2
sees "the other half" of the input voltage, and thus produces the same
rectified output as D1 does. Both meters will deflect by the same
amount, indicating that all power is being reflected, and the SWR is
infinite. The transmitter will see 100 Ohm load, a 2:1 SWR, far away
from causing danger to any transmitter. And that is the worst SWR it
will ever see through this meter.
Third test: Let's short circuit the output! We know that this too
is infinite SWR. And from the circuit it's clear enough that with the
antenna terminal shorted, both diodes see the same voltage - same
deflection on both meters, infinite SWR, and the transmitter sees 50 Ohm
in parallel with 100 Ohm, which is 33 Ohm, or 1.5:1 SWR.
Let's now run the example posed in the SWR explanation near the top
of this page: A 25 Ohm load. We know this should be 2:1 SWR. D1 will as
always see half the input voltage, while D2 will have half the input
voltage on one side and only one third on the other side. So it sees one
sixth of the input voltage, which is one third as much as D1 sees.
When the pot is adjusted for the forward meter to show full scale, the
reflected signal meter will indicate one third scale, equivalent to one
ninth power. At this point the 2:1 mark must be placed on the meter.
And if the impedance is 100 Ohm? In this case D2 sees two thirds of
the input voltage on one side, still one half at the other. The
difference still is one sixth of the input, still one third of what D1
sees, and the reflected signal meter will correctly deflect to the 2:1
SWR mark.
What happens if the impedance of the load is 50 Ohm, but with
nonzero phase angle? In that case, both the voltage and the phase of the
RF signal at D2's cathode will deviate. The funny, curious and nice
thing is that whatever values you may try, the resulting rectified DC
voltage is always correct! Try it, if you feel like doing some math! I
will limit myself to showing you an extreme example: Say, you connect a
capacitor that has 50 Ohm reactance (a 470pF one would be close to this
at 40 meters). We know that a capacitor cannot dissipate power, so the
SWR meter had better show infinite SWR! Let's see:
The compounded impedance of our 470pF "antenna" and R1/R2 is 70.7
Ohm, 45 degrees. The current through them will thus be 0.0707A,
phase-advanced by 45 degrees. Then the voltage across the capacitor will
be 3.53V, phase-lagging by 45 degrees. As the voltage at D2's anode is
still 2.5V at phase zero, the angular compounding produces another 2.5 V
across D2, phase-lagging by 90 degrees. The phase information gets lost
in the rectification, but the 2.5 Vrms magnitude is the same seen by
D1, thus the two DC outputs are equal, indicating infinite SWR. Nice,
huh? You can measure reactive parameters without needing reactive
components in the instrument!
Construction
One of the nicest things about this project is that it's easy to
build and requires no calibration. Just be sure to mount the RF-carrying
components with the shortest leads you can, directly on the antenna
connector. The input connector can be farther away, that's not so
critical. Place the 100 Ohm resistors physically close to the ground
plane, so that the parasitic capacitance cancels out lead inductance. Do
not use single 50 Ohm resistors, they are too inductive! The ideal is
either 2 of 100 Ohm like I did here, or even 3 units of 150 Ohm each. If
you use even more resistors in parallel, of higher values, the result
tends to be somewhat capacitive. The diodes work at higher impedance, so
their leads may be a little longer, and they should NOT be physically
close to the ground! The cabling of the potentiometer and meters carries
DC only and is totally uncritical.
You will need to draw the meter scales. The forward meter should be
marked just with the "set" point near full scale, and if you like it,
you can subdivide it into percentile power markings (remember that power
is proportional to the square of voltage, so 25% power is at mid
scale). You may also calibrate the meter in absolute power for a
specific setting of the potentiometer.
The reflected signal meter is best marked directly in SWR. The
infinite mark goes at the same level where the set point of the forward
power meter is. 3:1 SWR is at 1/2 of that, 2:1 SWR at 1/3, while 1.5:1
SWR goes at 1/5 of the scale. If you want to add marks for the high
range too, 5:1 SWR is at 2/3 of the range, and 10:1 is at 82%. If you
want additional markings, it's good to know this equation:
SWR = (1+p) / (1-p) where p is the position on the meter scale, ranging from 0 to 1.
Performance
I have build over 10 versions of this circuit over the years. The
original one shown in the schematic appears on this photo. It uses
germanium diodes, half-Watt carbon film resistors and SO-239 connectors.
It works well from the lowest frequencies I have cared to try it
(around 1 MHz), up to 150 Mhz with high accuracy, and 500 Mhz with
degraded accuracy (showing 1.3:1 SWR for a pure 1:1 one).
Another version I built uses 1/4 Watt resistors and Schottky (hot
carrier) diodes. I built it into a copper tube of 20 mm, with one BNC
connector on each end, bringing out the DC signal leads. That one works
well from the same low frequency, up to about 1500 MHz, with good
performance, and gets kinky at around 2 GHz.
Some time ago, I got a Digital SWR meter, an Daiwa DP-830, very
cheaply. That meter has two independent sensors: One is a current
sensor/voltage divider circuit which is rated for HF and up to 150MHz at
high power. It uses SO-239 connectors and works very well at HF, but on
the 2 meter band it is not very accurate.
The second sensor is a monimatch, rated for 2 meters through 70
centimeters, using N-type connectors. It's quite usable as an SWR meter,
even if not laboratory grade, but the power measurement is nonsensical
with this sensor.
So I decided to add a third sensor, for low power work over the
entire spectrum. I used BNC connectors, in order to have an additional
choice of connectors on the meter... The third sensor was installed on
the back of the instrument, and from the outside looks like it was
factory-made. On the inside it doesn't - but it works very well,
providing high accuracy all the way from 160 meters right through 23
centimeters. This sensor uses quarter-Watt resistors with minimal lead
lengths, tucked close to the case in order to compensate for the stray
inductance by stray capacitance, and the diodes are HP-2800 Schottky
ones. The output resistors were selected so that the power indication on
the digital meter has a fixed, known relationship to the real power:
1:50. So, for 2 Watt input power the meter reads 100 Watt. That's mighty
fine for impressing your buddy with how much power your handy delivers!
Just for the fun of it, I put together one using surface mount
components and SMA connectors, with proper care for the impedance of
traces. I could not really find out its upper frequency limit, being
short on test equipment, but at least from 1Mhz to 4GHz it worked well!
I have no photo of that one, but here is a photo of a surface mount
sensor built by Alexei. He kindly sent me this photo, which illustrates
well how the meter can be built. Note that all parts carrying RF are
assembled close together, close to the output connector, on a ground
plane that helps keep the impedances down. How high in frequency the
meter can work, depends on things like the board's dielectric constant,
the sizes of the parts and the tracks, and so on. Without caring much
for these things, but building the sensor as compact as shown here, it
will work far into the UHF range. With proper trace impedances, and hot
carrier diodes, it should work well into the SHF range.
I don't think you will ever see an SWR meter with this circuit
offered commercially. It has the weakness of blowing up if you
accidentally pump more than a few Watt into it, and manufacturers would
probably not trust their customers to handle the product with care. But a
home builder is much more aware than the average user of what he is
doing, so I trust you! And if you anyway blow it up, at least you can
replace the burnt resistors yourself, and they are really cheap!
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