SWR BRIDGES
WHY THEY DON'T MEASURE SWR
WHAT ARE WE TRYING TO MEASURE
The only thing we really want is power in the form of EM radiation from the antenna sent in the desired direction. The two things we don't want are damage to the radio and unwanted emissions that disturb the neighbours. An antenna which is perfectly tuned AND matched to the radio and feedline's prefered impedance and which radiates in the correct direction will achieve all of these things.
Note that tuning and impedance matching are two different things. As it turns out, in most cases they are close to the same but not exactly. With SWR we are trying to get the tuning correct NOT the impedance. Many SWR meter circuits on the net are trying to find 50Ω. Resistive wheatstone bridges do not measure Standing Wave Ratio, they measure impedance difference from 50Ω. This is NOT the same as tuning (finding resonance).
WHAT IS SWR
SWR is the RATIO of forward to reflected power. There is only one way to achieve this and that is work into a purely resistive load. One implies the other. In both parallel and series resonant circuits, the driven resonant frequency is the one where impedance is at a maximum (parallel) or minimum (series) and in this case it will be purely resistive. An antenna is nothing more than a resonant circuit whether it is fed at the driven resonant frequency or not.
At resonance, since the load must be purely resistive, all power will be consumed. If it is all consumed, there can't be anything reflected - simple. Consumed means either converted into heat or radiated.
EXTENSION
SWR has nothing to do with the absolute value of impedance. Impedance only comes into the picture when we start talking about mismatches in impedance.
SWR is all about the RATIO of resistance to reactance. A perfect capacitor does not consume power because the energy it stores in one half cycle is returned to the circuit in the other half of the cycle - ENERGY (POWERxTIME) IS REFLECTED 100%. The same for a perfect inductor. In the case of a capacitor the energy is stored in an electric field. In the case of an inductor it is stored in a magnetic field.
In either case, a perfect capacitor or inductor will reflect 100% of the energy it stores in each half cycle. They both have an infinite SWR because the power sent to them is 100% refelected.
A perfect resistor, on the other hand, will consume 100% of the energy it receives. Because all physical resistors have length, this energy will always be in two forms. One is heat and the other EMR. Even a film resistor 10mm long will emit 10Hz EMR if driven with 10Hz. The level will, of course, be so low as to be unmeasurable but it is still there. All of the rest of the energy it converted to heat. In the case of an antenna, most of the energy is converted to EMR but it still has resistance and so some some will be converted to heat.
In either case, a resistor consumes all the energy sent to it. There is nothing reflected (because nothing is stored) so it will have a SWR of 1:1
SWR need not only apply to antennas. It can be applied to any circuit with any impedance.
MEASUREMENT DIFFICULTIES
One must consider BOTH the antenna itself AND the transmission line. It is the combined SWR of both together we are interested in. The same principles apply whether coaxial cable or balanced feeder is used. In the case of coax, a general rule of thumb is that it should only really be treated as transmission line when it is greater than 1/10 λ. Shorter than this length and other things like connections and connectors can make more of a difference.
There are many situations where less than this length of coax is used especially on the lower bands. In one situation I have only 1½ metres of coax between the radio and the antenna (in a vehicle) which means anything from 160m to 15 metres and the coax could have a characteristic impedance varying widely from 50Ω.
Up to ¼λ is really needed to bring the impedance of the transmission line to near 50Ω which now makes the possible frequency range with a varying impedance up to 50MHz.
In order to tune such a system I want to know the SWR, not necessarily the impedance although I would like that to be close as well. Working a radio into a slightly incorrect resistance is not likely to do as much damage as working it into an incorrect SWR. Many RF transistors will die quickly with only a small overvoltage (caused by reflected power). They will also die on overcurrent (too low a resistance) but that takes a lot longer.
PROS AND CONS OF VARIOUS METHODS - (shows advantage as I found it)
FACTOR | Stripline SWR meter | Resistive bridge | ||
MEASUREMENT | A microstripline meter measures SWR not impedance. | Two requirements are needed for a nul reading. The SWR must be correct but so also must the impedance be exactly 50Ω. | ||
ACCURACY | The higher the frequency the more of a percentage of ¼λ there is so the higher the reading and thus more accuracy as far as the meter part is concerned but stray reactance also becomes more of a factor. The most significant contribution to poor accuracy is the voltage drop across the diode detectors. The pickups I use for HF though, are at least as accurate as the radio's internal meters. At VHF my radios seem to prefer antennas I have tuned with my striplne meters than the indication I get from them - they put out more power. | These devices suffer the same inaccuracy problem created by the diode voltage drop as any other meter. When I built one, I found stray reactance to be a real problem though especially on high VHF (2m) and more so on UHF. I could not tune a UHF antenna to my desired perfomance requirments using one. I found I could get a better tune by adjusting element lengths for maximum radio output. | ||
IN CIRCUIT | If the gap between tracks is kept above 1½ or 2mm, it can be left in circuit up the a couple of kW. The central micro-stripline is around about 50Ω, although this doesn't matter much, and is electrically isolated from the rest of the circuit so has very little loss. | Can't be left in circuit unless you like the smell of burning resistors and want your power down to less than 25%. | ||
BANDWIDTH | Yes- these meters don't have the best of bandwidths but so what? I use 2 pickups, one for 160m-10m then one from 6m to 70cm. The HF pickup has an opamp switched in on 160 and 80m but still provides a better tune than the radio's internal meter. | Resistive bridges can be used to tune an antenna on the lower bands. This their only advantage. Since you can't leave them in circuit, it isn't much of an advantage especially since extra pickups for the stripline meteres are easy to build. As the frequency increases, so do the effects of stray reactance. | ||
COMPLEXITY | Apart from the micro-stripline circuit boards required, this is about as simple as you can get. The meter can be located as far away from the pickups (withn reason) as you like. The PCBs can be made in minutes and do not have to be connected to a meter circuit. There is no reason why a pickup could not be attached to each radio (or output connection) and a switch with a few resistors set up to check the SWR of the radio of your choice. A bit of PCB, 2 resistors, two diodes and two capacitors per radio isn't much. | I looked at several designs and they all had more parts than the stripline SWR meter. | ||
STRAY REACTANCE | I couldn't see this as a problem since both sides of the meter are identical and any stray reactance affects them both equally. It might lead to slightly higher SWR values but I couldn't notice anything major. | I tried one these things and worked on it for days trying to get the layout right. I managed to have some minor success with 10m-6m but on 2m and, worse, on UHF it went to pieces. No matter what layout I tried and no matter what shielding method I used, I couldn't get a tune on UHF that my radio liked. |
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