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To get the maximum power output from a transmitting aerial, it must be properly matched to the transmitter. For that matter, serious mismatch can spell disaster for the (expensive) output stage of the transmitter. For these reasons, amateur radio operators often make use of a so-called voltage standing wave ratio meter - an 'SWR' meter. in this article, we will not only describe the theory - what it is, what it does, how it works; a practical circuit for an SWR meter with a very wide frequency range is also given. With a PCB board, of course.
There is, however, a group of electronics enthusiasts that rightly considers impedance matching to be of prime importance, Amateur radio operators For them, an impedance mismatch can have disastrous results. At best, their range will be drastically reduced; at worst, they may blow up the output stage of their transmitter. Fortunately, it is not too difficult to avoid mismatching. Provided you know what you're doing, that is if you buy a transmitter ready-built, the output impedance is usually specified. The same is true for transmitting aerials: the manufacturer will normally specify the impedance. The idea is that you use an aerial with an impedance that matches the output impedance of the transmitter, and that you use connecting cable with the same characteristic impedance. For instance, if the transmitter output is specified as 75 Ohms the obvious thing to do is to use 75 Ohms coax cable and a 75 Ohms aerial, However, life is not always so simple. Many amateurs not only build their own transmitter; the transmitting aerial, too, is often the result of personal experiment, in this case both impedances are unknown, so that optimum energy transfer from transmitter to aerial can only be obtained by experimenting.
Fig 1. the simplest equivalent circuit for a transmitter working into a load, for best power transfer, the load impedance. Zo should be equal to Z.An ideal AC voltage source, U, supplies power to the Ioad, Zo, via the internal impedance, Zi. The power supplied to Zo can be calculated as follows:
Po = Ս² * Zo / (Zi+Zo)²
In this formula, U is the (open-circuit) voltage supplied by the voltage source: Po is the power supplied to the load Zo For a given value of Zi, the maximum power is supplied to the load when Zo equals Zi. In other words, if the output impedance of the transmitter is 75 Ohms, a cable with a 75 Ohms characteristic impedance should be used. Similarly, power transfer from the cable to the aerial is highest when the aerial impedance is equal to that of the cable. In that case all the power supplied by the transmitter is pumped into the aerial (the losses in the cable will normally be negligible). What happens when the aerial impedance does not match the characteristic impedance of the cable?
The energy transfer from cable to aerial is not ideal in this case, and a standing wave appears along the cable. This can be clarified as follows. Normally, the aerial is not mounted on top of the transmitter. It will be on the roof, or at the top of a high mast, or in some other suitably high position. In most cases, it will be some distance away from the transmitter - the latter being at a more comfortable location inside the house, The transmitter and aerial are then linked by a cable, in other words, the power output from the transmitter runs down the cable to the aerial. All very straightforward, you would think, but let's take a closer look at what happens in the cable. The electrical signal moves down the cable at high speed: 200,000 to 300,000 km/s (i.e. well over 100,000 miles per second). This seems very fast, but let's see how long it takes the signal to pass down a 60-foot cable (20 metres). A simple calculation (using the metric figures it's simple, anyway) shows that it will take about 100ns. For a radio amateur, 10 MHz is not such a high frequency - but the period time at this frequency is also 100ns. This means that the AC voltage source in figure 2 will have produced one complete period before any signal appears at the other end of the cable, 20 metres further on! This means that the voltage source can't see" whether or not the Cable is terminated with the correct impedance, The transmitter only 'sees a small part of the cable, and recognises its 'characteristic impedance'. The current pumped into the cable is therefore determined by this cable impedance.
Fig 2, in practice, the aerial will normally be connected to the transmitter by means of a cable. At 10 Mhz, a 60-foot cable (20 metres) is equivalent to one full period of the high frequency signal.
When this current arrives at the aerial, the results depend on the aerial input impedance. if the whole system properly matched, the complete power output from the transmitter goes into the aerial: the same voltage across the same impedance corresponds to the same current - i.e. the current coming down the cable. If the aerial impedance is too high, however, it will 'reflect some of the power. Put it this way; the same voltage across a larger resistance corresponds to a smaller current. Some of the current coming down the cable is left over', and it bounces back down the cable towards the transmitter. The same sort of effect occurs if the aerial impedance is too low, The power that is reflected back to the transmitter interferes with that coming the other way, The result is a standing wave. This can actually be detected by passing a field-strength meter along the cable: at some points a maximum is found, and at others the field strength is at a minimum. The maxima and minima occur at regular intervals, if the aerial is incorrectly matched, the field strength at that point will below-corresponding to low output from the aerial. If the aerial impedance is unknown, the degree of mismatch can be determined by measuring how much energy is reflected. By using directional couplers that only pass power in one direction,
the reflected power can be separated from the power fed to the aerial. The ratio between these two is a measure for the accuracy of the impedance match. To be more precise, the ratio between sum and difference of the forward going voltage (Uf, towards the aerial) and the reflected voltage (Ur) is taken, as follows:
VSWR = Uf + Ur / Uf - Ur
VSWR is the Voltage Standing Wave Ratio. it will be obvious that the VSWR equals 1 if the reflected voltage is zero; it becomes infinite if the complete signal is reflected. This would occur if the aerial impedance is zero or infinite. Note that the aerial impedance referred to is that at the transmitted frequency. If the aerial is correctly designed for this frequency, it will be in resonance and its impedance will be real.
By now we have some idea of what we want to measure. The next question is: how?
The VSWR meter circuit given in figure 3 can be used for transmitting frequencies between 2 MHz and 30 Mhz. The unit is connected in series with the cable, close to the transmitter. The current flowing from transmitter to aerial and vice versal passes through the primary of a transformer. Both of these 4 currents produce a current in the secondary; the direction of the current flow in the secondary is obviously determined by the direction in the primary. By combining the total voltage (Uf+Ur) with the correctly chosen and half-wave rectified secondary voltage, Ur and Uf can be obtained separately. These voltages across C2 and C1, respectively can be measured with a simple meter circuit, consisting of low-pass filters (R1/C3 and R2/C4) and a meter with preset series resistor. At frequencies above 30 MHz, no transformer is needed. The same job can be done by adding two secondary 'strips' that run parallel to the main through feed. This is shown in figure 4.
Fig 4. An even simpler circuit for a VSWR meter can be used if higher frequencies (100MHz - 300MHz) are used.
The directional characteristics of this circuit are best when the following conditions are met:
where Za is the impedance for which the unit is intended.Obviously, electrical waves can run in both directions along all the strips, however, if the above conditions are met, waves in one direction will decay quite rapidly. With the diodes only conducting in one direction (as all good diodes should do...), the forward going wave will build up a voltage across C2 and the reflected wave can be measured across C1. As before, the two voltages can be measured with a simple meter circuit. The frequency range of this VSWR meter is approximately 100 Mhz - 300 MHz.
Fig 5. The same PCB can be used for both the circuits; S1, P3 and the meter are mounted off the board. Note that the central strip must be interrupted for the circuit given in figure 3, as described in the text.
There is no need for two separate PCB board designs for the two circuits. There are so few differences between the two that the same design can be used for both, as shown in figure 5. If the circuit given in figure 3 is to be mounted for measurements at up to 30 MHz), the centre stripline must be divided into two halves. This is done by scratching away some of the copper between the two central holes. The transformer is wound on an Amidon ring core. The secondary winding consists of 30 turns, the primary is only half a turn, which is equivalent to passing a wire link through the ring this link is soldered into the two holes in the central strip, effectively fastening down the transformer at the same time. This construction is clearly illustrated in the photo.
Mounting the high-frequency HF version (figure 4) is even easier. The central strip is left intact, of course. The only important point to watch is that C5, C6 and the transformer are omitted.
For both circuits, calibration is quite easy. The unit is included in the cable, close to the transmitter. First, the switch is set to the Uf position, and P3 is adjusted so that a fairly high reading is obtained on the meter. Note that these three components are not mounted on the PCB board. Now, with the switch set to the 'Ur' position, P2 is adjusted for minimum deflection.
After this adjustment, the circuit is connected ‘backwards': the ‘aerial output’ is connected to the transmitter, and the 'transmitter input" goes to the aerial. The same calibration procedure is repeated, but this time P2 is left untouched and P1 is adjusted for minimum deflection in the ‘Ur’ position. After restoring the original connections, the adjustment of P2 can be checked; then P1 again, and so on until no further improvement can be obtained.
If a different aerial is tried, the meter will now indicate how accurately it is matched - or how badly... If the meter originally has a linear scale, it is easy to fill in the correct VSWR values: full scale is ‘∞’, three-quarters of full scale is 7, the mid position is '3’, one-quarter is ‘1.6’, one-tenth is ‘1.2’, one-twentieth is ‘1.1’ and zero is '1'. In practice, a VSWR of less than 2 is quite good.
R1,R2 = 1K
P1 P2 = 100R Preset
P3 = 10K PresetCapacitors:
C1,C2 = 330p (ceramic)
C3,C4 = 100n
C5,C6 = 10p (ceramic)Semiconductors:
D1,D2 = 0A91 or equivalentSundries:
M1 = meter, 100 uA f. s.d.
Tr1* = Amidon ring core, Type T50-6;
Primary: 0.5 turns, 1 mm CuAg;
Secondary:30 turns, 0,5 mm CuI;
S1 = single-pole change-over switch
2 connectors (BNC, SO239)