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SWR
- Thread starter nd2023
- Start date
A high VSWR doesn't actually cause the transmitter output to go down [except in the case of transmitters with automatic power reduction systems - see below].
What is actually happening is the power which should be radiating from the antenna is now actually being returned back down the coax, into the final output device(s) and turned into heat.
So, you may have for example 1000 watts output from your transmitter but only 800 watts being radiated from the antenna - the other 200 watts are returned to the transmitter.
If you increase the power output, you might increase the power being radiated, but you also increase the amount of power being returned down the coax and turned into heat in the output device - which will, in time, cause the demise of said item!
When you have a high VSWR, the thing to do is cure it. Find out what is causing it and fix it, so that all your power is being radiated. A transmitter that isn't running efficiently is using more power than necessary and will not last!
In the case of transmitters with automatic power reduction systems, a control system actually reduces the drive to the final stage in the case of high VSWR. This is done primarily to protect the output device(s). A high VSWR as mentioned will cause the device(s) to become abnormally hot, and can cause premature and sometimes catastrophic failure.
What is actually happening is the power which should be radiating from the antenna is now actually being returned back down the coax, into the final output device(s) and turned into heat.
So, you may have for example 1000 watts output from your transmitter but only 800 watts being radiated from the antenna - the other 200 watts are returned to the transmitter.
If you increase the power output, you might increase the power being radiated, but you also increase the amount of power being returned down the coax and turned into heat in the output device - which will, in time, cause the demise of said item!
When you have a high VSWR, the thing to do is cure it. Find out what is causing it and fix it, so that all your power is being radiated. A transmitter that isn't running efficiently is using more power than necessary and will not last!
In the case of transmitters with automatic power reduction systems, a control system actually reduces the drive to the final stage in the case of high VSWR. This is done primarily to protect the output device(s). A high VSWR as mentioned will cause the device(s) to become abnormally hot, and can cause premature and sometimes catastrophic failure.
A
AMX
Guest
Also, just to add to what Studio1 said, running a transmitter into a higher SWR than what it is rated for in it's specifications, can cause the amplifiers to become unstable.
As the poster above says, if the VASW is high, it's broke. Fix it. And learn to keeop a regular eye on the reflectomer. It has no inherent reason to change, therefore if the reading has changed, something in the system has changed to cause it. Changes in antenna systems tend to get worse, not better.
Cold Reality
That may be true if you don't live where antennae ice up in the winter. Severe icing can raise the SWR temporarily, even when antenna elements are heated. In any event, keeping an eye on SWR is always a good idea, and changes not due to weather should be a concern. In severe icing, you may need to reduce power or even go to a backup transmitter temporarily until conditions improve.
littlejohn said:As the poster above says, if the VASW is high, it's broke. Fix it. And learn to keeop a regular eye on the reflectomer. It has no inherent reason to change, therefore if the reading has changed, something in the system has changed to cause it. Changes in antenna systems tend to get worse, not better.
That may be true if you don't live where antennae ice up in the winter. Severe icing can raise the SWR temporarily, even when antenna elements are heated. In any event, keeping an eye on SWR is always a good idea, and changes not due to weather should be a concern. In severe icing, you may need to reduce power or even go to a backup transmitter temporarily until conditions improve.
Re: Cold Reality
If you monitor reflected power down to the tenths of a watt even rain will cause readings to change. The VSWR meter in the transmitter may not appear to ever move compared to a digital or analog bird.
SirRoxalot said:littlejohn said:As the poster above says, if the VASW is high, it's broke. Fix it. And learn to keeop a regular eye on the reflectomer. It has no inherent reason to change, therefore if the reading has changed, something in the system has changed to cause it. Changes in antenna systems tend to get worse, not better.
That may be true if you don't live where antennae ice up in the winter. Severe icing can raise the SWR temporarily, even when antenna elements are heated. In any event, keeping an eye on SWR is always a good idea, and changes not due to weather should be a concern. In severe icing, you may need to reduce power or even go to a backup transmitter temporarily until conditions improve.
If you monitor reflected power down to the tenths of a watt even rain will cause readings to change. The VSWR meter in the transmitter may not appear to ever move compared to a digital or analog bird.
The idea is to keep the ice off so the reflected doesn't get out of hand. And yer right, ice will temporarily upset the system. Some elements are more subject to ice problems than others, a matter of some concern to those of us who work in the ice belt of the southeast.
As regards the tenth of a watt, that's often spurious accuracy. The guys at Bird used to would explain to you why they only needed to see 25dB or so of directionality in a wattmeter sampling section. This is why antenna arrays are properly tuned using an admittance bridge. In the event of a fault, the reflectometer may go up or down in a minor fault. The salient point is, if the reflected value changes, you ought to go find out why. It shouldn't change.
As regards the tenth of a watt, that's often spurious accuracy. The guys at Bird used to would explain to you why they only needed to see 25dB or so of directionality in a wattmeter sampling section. This is why antenna arrays are properly tuned using an admittance bridge. In the event of a fault, the reflectometer may go up or down in a minor fault. The salient point is, if the reflected value changes, you ought to go find out why. It shouldn't change.
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