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ContinuousWave: Small Boat Electrical
|Author||Topic: VSWR Measurement|
posted 01-15-2013 09:53 AM ET (US)
Boaters often inquire about the value of owning their own VSWR meter or directional wattmeter for the purpose of testing or checking or diagnosing their radio installation. I do not see much value in a boater investing in a VSWR meter for the VHF Marine Band frequencies. My reasons are as follows:
--most of the inexpensive meters are not very accurate;
--to know the VSWR is only marginally useful.
To make reasonably accurate measurement of VSWR at 160-MHz you will need to spend some money. While you can get some device that says it will measure VSWR for $35, it does not really measure VSWR very accurately. A reasonably accurate meter will cost more like $135, and base level professional meters more like $500. Since a new antenna is likely to only cost $100, it does not make sense to invest $500 in equipment to check the existing antenna. It is less expensive to just get a new antenna. To make an analogy, suppose your car engine were running roughly. You could buy a sophisticated best-in-class diagnostic station for $100,000, or you could get a new car for $25,000. Which would you buy?
There is a general principle in measurement of standing waves on a transmission line that requires the sensor to be able to properly differentiate forward and reverse waves. This property is known as the directivity of the sensor. The directivity limits the value of VSWR that can be accurately measured. In order to measure low values of VSWR, say below 2:1, the sensor has to have increasingly better directivity. As the difference in magnitude between the forward and reverse waves increases, that is, as the VSWR approaches 1:1, the sensor has to be able to accurately detect a reverse wave of smaller and smaller magnitude, in the presence of a much larger magnitude forward wave. To make a meaningful measurement of a VSWR of 1.1:1, the sensor has to be very directive. The reflected wave is going to be 1/500-th of the magnitude of the forward wave. This is a very large difference in magnitude. It is easy for the sensor to become confused. Some of the forward wave leaks into the detector. Since it is out of phase with the reverse wave, it tends to cancel. The resultant measurement shows an incorrectly low magnitude for the reverse wave.
In the cheap VSWR meters, the sensor is not particularly directive, and this means that the reliability of their readings is limited. It is also particularly problematic that the error tends to indicate the VSWR is better than it really is. For example, a VSWR meter with poor directivity might indicate a VSWR of 1.5:1 when the actual VSWR is higher, perhaps 2:1 or more.
With the typical $35 VSWR meter, the VSWR measurements obtained are likely to only be roughly accurate, and are likely to show the VSWR to be better than the actual VSWR on the transmission line.
There is a phenomena in transmission lines known as line loss. The transmission line itself attenuates the signal traveling on the line. This affects VSWR measurement. Consider a transmission line with a loss of 3-dB. We connect a transmitter providing 20-Watts of power to the line. Only half this power reaches the antenna, or 10-Watts, due to line loss. The antenna has a VSWR of 3:1. This means it will reflect 25-percent of the power. A reverse wave of 2.5-watts now is sent back on the transmission line. This wave also undergoes a 3-dB loss. At the transmitter, only 1.25-watts arrive. If a VSWR meter is inserted at the transmitter, it will read foward power of 20-watts, reflected power of 1.25-watts, and show a VSWR of 1.17:1. The attenuation of the transmission line has masked a very bad VSWR at the antenna. The real VSWR is 3 and the indicated VSWR is 1.17, a very significant error.
There is a further problem with transmission line loss. As the VSWR on the line increases, the line loss also increases. This further tends to throw off VSWR measurements made at the transmitter end of the line.
Let's take this to the extreme: we measure the VSWR on a transmission line with no antenna connected at the far end. If the transmission line has 3-dB loss, and if the loss increases with high VSWR to 4-dB, we could see a VSWR measurement as follows:
We put in 10-watts, but only 4-watts reaches the end, where it is 100-percent reflected, that is, an infinite VSWR. Now the 4-watts heads back down the transmission line, again attenuated by 4-dB. This means only 1.6-watts reaches the VSWR meter. The VSWR is now calculated from forward power of 10-watts, reverse power of 1.6-watts, and found to be 2.3:1, but the actual VSWR is infinite! Also, in this example we assume a meter with perfect accuracy. If the meter suffers from poor directivity, the indicated VSWR may be even lower. This has led to a rule of thumb in VHF antenna work that says if the VSWR is 2:1 or higher, there probably is no antenna connected at the far end.
Because the length of the transmission line used in most small boat installations is typically 20-feet or less, the influence of the transmission line loss on VSWR measurement is not as acute as in my example, but there is still some loss, about 1-dB, which tends to make the measured VSWR appear better than the actual VSWR. This effect, in combination with poor directivity in cheap VSWR meters, tends to generally always make the measured VSWR appear better than the actual VSWR.
Let us assume that a boater has access to a VSWR meter that is reasonably accurate, and measurements are taken of the VSWR of the antenna. Suppose I came over to your boat and made a series of very careful VSWR measurements on your antenna using the best available measuring instruments. What would you do with the information?
As explained above, you could evaluate the information on the basis of the antenna being seen to have a VSWR below 2:1. If it does, you can at least conclude the antenna is connected at the far end of the transmission line. If the VSWR is above 2:1 something is likely very wrong in the system.
OK, let us suppose the VSWR is measured to be below 2:1 in the VHF Marine Band--what next?
Unless you are using an antenna with an exposed metal whip, the typical VHF Marine Band antenna is encased in fiberglass. There are no adjustments possible. If you have an antenna with a metal whip, I suppose you could begin to adjust the length. However, you can only make it shorter. There is no basis to assume that making it shorter will be the necessary direction the length ought to be changed. It is just as reasonable to assume the antenna might need to be longer. What is your next step?
The manufacturer has constructed the antenna for the VHF Marine Band, and we hope he tuned it for best VSWR response. If you buy a cheap, mass produced antenna, you might get one that could benefit from some length adjustment. My experience is the better antennas are tuned right on the money. And since you cannot change them, in any case, why worry about tuning them? I do not know of any reliable statistics about the tuning of VHF Marine Band antennas. The few I have tested--with good equipment--have shown to be tuned perfectly.
I suspect that most problems associated with installation of antennas for the VHF Marine Band are a result of poor installation of the connector on the end of the transmission line at the radio by the installer. Most radios use a SO-239 connector, and most transmission lines need to be terminated in a PL-259. The PL-259 can be a difficult connector to install for beginners.
If you are installing a PL-259, I recommend you follow this procedure:
--before installing, check the DC resistance of your antenna transmission line between the shield and center conductor. It should either be an infinitely high resistance (an open circuit) or a very low resistance, probably less than 1-ohm. Compare your reading with the manufacturer's specification for the antenna. The better manufacturers provide guidance on the DC resistance measurement of their various models.
--install the PL-259
--repeat the above check, checking now with the center pin and outer connector body of the installed PL-259. If you created a short circuit in the connector or you created an open circuit in the connector, you may be able to detect it with a DC resistance measurement. For example, if the initial measurement of the transmission line showed an open circuit, and following the connector installation you measure a low resistance, you caused a problem in the connector installation.
Of course, if the antenna was already showing an open circuit, you cannot verify the connector by measuring an open circuit; you could have a bad solder connection and not even be connected.
There are really only two likely outcomes with a boater-installed PL-259: it is either correct or not. Let's explore the two outcomes:
--if the connector is correctly installed, we are back to the beginning of the discussion. The antenna is going to have the VSWR the manufacturer created. You cannot really change it. The only other possibility is the antenna is completely bad right out of the box.
--the connector is faulty; we will detect this in another way
If the connector is really completely open or completely shorted, the receiver will be affected. See below to learn how to test the receiver. The transmit will also be affected. Most modern radios will not operate at full power into a short or open load. You can monitor the DC current on the radio 12-Volt line. If the radio is trying to make 25-watts of power, it should be drawing about 7-Amperes on transmit. If you key the radio and it only draws 1-Ampere on transmit (on the 25-Watt setting), you know something is wrong. The high VWSR of the short or open antenna probably shut down the transmitter. We can use a simple DC ammeter to check the power being used by the radio. Most technically adept boaters already have a DC ammeter in their tool box. This will give us the same sort of Go-NoGo indication we would have gotten from the VSWR measurement.
It is not strictly legal to make a transmission from a boat radio when the boat is not in the water. You should wait until you are in the water, then arrange a test transmission with a cooperating station. This will give you a good idea if the antenna is working.
To make a test, you must work with another station that has a radio that is known to work properly. There is no point in testing with your buddy if his radio does not work very well. Two people testing with unknown radios cannot produce any reliable results. There is no point in testing with any boat within five miles. Even very bad radio systems can communicate 5-miles, due to the usually excellent sensitivity of the receivers involved. A good test is to see if you can communicate, boat-to-boat, over open water about a distance of 10 to 15 miles. Good installations of VHF Marine Band radios can do this when both stations use 25-watt power settings. Try on 1-Watt, too. Really excellent installations may be able to communicate 10-miles or more on 1-Watt. To facilitate the test, an alternative means of communication must be available. Each operator in the test needs to have a cellular telephone so communication can occur even if the radio under test fails to communicate with the remote station.
You can also make an assessment of the antenna performance by its receive function. I have written an article than explains how to do this. See
To check antenna performance on receive, you just need a NOAA weather radio broadcast station. There are over 1,0000 of them. You should be able to copy several. Testing by receive perfromance can be deceptive in some cases. On receive there is almost no limit to the gain that can be used to amplify the signal coming from the antenna in the receiver. Unfortunately, on transmit the gain is fixed, that is, the power level of the transmitter is set to 25-watts, and it cannot be increased to overcome deficiency in the antenna. On receive, however, the receiver has enormous reserve gain that can be used to make up for loss in the antenna and transmission line. This allows the receiver to appear to work normally if receiving very strong local signals. For this reason, the receiver test of the antenna must use remote signals that are already at the margin of reception and require maximum gain. It is only under those circumstances that deficiency in the antenna can be detected on receive. Or, in simpler terms, you can hear your local NOAA station using a wet noodle for an antenna; try to hear the NOAA station that is 75-miles away.
posted 01-15-2013 01:03 PM ET (US)
Testing antenna performance on transmit by actually communicating with distant stations is a more reliable test than any VSWR measurement can provide. A low VSWR on a transmission line connected to an antenna just means that there is a good match between the antenna load and the transmission line. It does not say anything about the performance of the antenna itself.
An antenna, for example, could consist of nothing more than a resistive load, encased in a radome. Such an antenna would give wonderful VSWR measurements, but would not perform very well. It would radiate almost no RF energy There are other possible antenna flaws that could harm performance, yet produce a nice VSWR. The antenna could be very inefficient at converting radio-frequency power applied to into actual radio waves radiating from the antenna. It might be just turning a lot of the power into heat from losses in the antenna conductors themselves. Ironically, antennas with resistive loss tend to show up as having broad VSWR bandwidths. (And in some cases, resistive shunts are intentionally used to limit the VSWR range of otherwise very good antennas.) Or, the antenna might radiate all of its power in a useless direction. On a boat, the optimum angle for radiation is toward the horizon, a vertical angle of 0-degrees. An antenna might have a low VSWR but direct all of its radiation upwards at a useless angle. There is no method of simply testing the VSWR that permits a prediction on the actual performance of the antenna. Low VSWR does not mean optimum antenna performance.
Conversely, a moderately high VSWR does not mean the antenna is working poorly. A VSWR of 2:1--that is the actual VSWR not a badly measured value that is misrepresentative of something actually much higher--might be the best obtainable from a particular antenna, which otherwise works extremely well. A slightly higher VSWR does not damn an antenna to have no value. Because the usual installation on a small boat uses such a short feed line, losses in the transmission line from VSWR are often negligible.
posted 01-16-2013 11:48 AM ET (US)
There is likely more value in owning a directional wattmeter than a simple VSWR meter. A wattmeter can provide a quantitative measurement of the output power of a VHF Marine Band radio. The accuracy of a directional wattmeter is usually in close correlation to its price. For about five-percent accuracy one has to spend about $500. Doubling the price will double the accuracy.
It may be useful to measure the power output of a VHF Marine Band radio, but once that has been done, there are few decisions that can be made on that outcome. It is very unlikely the radio will produce more power than its specified output; to do so would put the manufacturer at risk of violating FCC regulations. If the radio output is less than specified, there is little the boat owner can do. FCC regulations restrict modification, repair, or service of a VHF Marine Band radio to licensed technicians. The typical boat owner cannot make any adjustments to such a radio.
It is anticipated that power output will be very highly correlated with supply voltage. If the radio is operated from a source of 13.2-Volts DC, it will probably produce its rated power within a reasonable tolerance. Operation from lower voltage will tend to produce lower power output.
I have made tests on a few VHF Marine Band Radios; they have shown the power output was in the range of 20 to 22-watts, as measured into a 50-ohm load with my directional wattmeter. Periodic testing of this nature could perhaps detect a problem if the power output were measured at a substantially lower level.
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