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ContinuousWave: Small Boat Electrical
Measurement of VSWR Greater than 3:1
|Author||Topic: Measurement of VSWR Greater than 3:1|
posted 04-24-2006 02:26 PM ET (US)
I was checking the SWR on my VHF (for no good reason execpt
that I ran across the meter in my box of electronics stuff).
It tested out a bit of 3, which is just into the red. Since
I have another radio and another antenna, I tried all four
combinations. All were in the range of 3.0 to a bit above
that (no numbers past that point).
The radio seems to work OK.
The SWR meter is a Shakespeare made for VHF. Both antennas
Should I be smelling a probem with the meter?
posted 04-24-2006 03:58 PM ET (US)
Was the same piece of coax cable between the radios and antennas used for all the tests? I have the same meter and antennas and regularly get less than 1.5-1. But that is on the water, which might play a factor, espcially if you have a ground plate, or bonded electrical system.
posted 04-24-2006 05:00 PM ET (US)
Different pieces of coax. Coax is permanently attached to
the Shake Galaxy antennas.
Boat was on the trailer.
posted 04-24-2006 07:16 PM ET (US)
How long is the feedline to the antenna?
I'd say that a reading of 3:1 VSWR probably means there is NO antenna even connected.
posted 04-26-2006 07:51 PM ET (US)
Most of the time a measurement of VSWR is made at the transmitter end of the feedline. The measurement is often made with a directional coupler which has poor directivity. Both of these factors cause the measurement to tend to understate the actual VSWR. Consider this case where the feedline has a loss of -3dB.
The power from the transmitter is 25-watts. This is conducted along the feedline to the antenna. Due to losses, only 12.5-watts reaches the antenna. The mismatch of the transmission line impedance to the antenna impedance is such that 50-percent of the power is reflected. This would correspond to a VSWR indication of 3:1 at the antenna. Now the reflected wave has to travel down the same feedline, and it experiences a -3dB loss. Thus at the reflectometer, only 6.25-watts is present. The reflectometer indicates a ratio of incident to reflected power of 6.25/25 = 0.25, which corresponds to a VSWR of about 1.5 to 1. This clearly demonstrated how any line loss in the transmission line affects the accuracy of the VSWR measurement made at the transmitter end.
When a VSWR is measured at the transmitter end and is 3:1 or higher, and the feedline has some loss, for example a loss greater than about 1-dB, you can expect to find that the real VSWR at the antenna is quite a bit higher. This progresses to the point where there is no antenna at all connected. If the line has a 3-dB loss, the reflectometer at the transmitter will still show a VSWR of about 2.5:1, even though the real VSWR is infinite!
If the directivity of the directional coupler in the reflectometer is limited, and you can be absolutely certain that a "marine" reflectometer is far, far from a precision device, the lack of directivity sets a minimum for the accuracy of the VSWR which it can measure.
For more information on VSWR measurements and directivity of the reflectometer, see:
posted 04-27-2006 11:33 AM ET (US)
Feedline is 20' RG-8/X
posted 04-27-2006 09:05 PM ET (US)
I just tried it with a third radio, my old HX350S handheld.
That also gave a 3.0+ reading, but what interesting is that
the meter also has a power setting. That came out 15 watts.
The radio only puts out 5 watts. I'm smelling a meter problem.
posted 04-28-2006 12:07 PM ET (US)
Maybe a dumb question, but are you adjusting the calibration knob for the forward power to max the meter before switching over to the reverse power reading?
posted 04-28-2006 02:39 PM ET (US)
Dumb question, but I'm glad you asked.
Yes, I'm doing the calibration thing. Set meter switch to Cal,
posted 04-28-2006 04:47 PM ET (US)
It sure sounds then as if your meter is toast. SWR meters are pretty simple and easy to fix if the meter movement itself is not the culprit. Usually it is not, and one of the diodes in the diode bridge are bad. If you can find an electronic supply house nearby, you might give that a shot as you should be able to get 4 new diodes for less than $10. Use a heatsink on the leads when you solder them into place.
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