What do you call this, SWR or what?

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What do you call this, SWR or what?

Post by Rodney » Mon Oct 23, 2006 7:10 am

When you have an antenna connected directly to a low powered transmitter such as those used in Radio Control, and the antenna is disconnected or collapsed, the output stage seems to heat up more than it does with the antenna extended. I am told that this is due to the missmatch or improper loading on the output stage. I am told that SWR is not the proper term for the measurement of this missmatch, what is the proper term then. thanks

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Chris Smith
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Post by Chris Smith » Mon Oct 23, 2006 8:24 am

It is a SWR or signal to wave ratio of loading. [standing wave ratio]

SWR, Return Loss, and Reflection Coefficient.

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Standing Wave Ratio (SWR)

Post by radionut8888 » Mon Oct 23, 2006 10:28 am

The Standing Wave Ratio is a number representing the ratio of RF power travelling down a transmission line (coax) to a load to that travelling back from the load to the source. Generally, no antenna is SWR free but most accept and radiate the power sent to it well. If an antenna collapses or the coax breaks, then the SWR goes high and most (if not all) of the power sent down the coax line is reflected right back to the source. This explains why your RF transistor produces more heat with a bad swr, since the power must be absorbed in the source rather than radiating from the antenna


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Post by cato » Tue Oct 24, 2006 3:06 pm

The SWR is sort of a side effect of the, and therefore an indication of the resonance mismatch and apparent impedance mismatch caused by the collapsed antenna.

If you think of the antenna as a pipe being filled and drained with/of water you might be able to envision the fill pump turning on and off and then the drain pump turning off and on such that just as the first slosh or wave of water get to about the middle of the pipe, the drain pump turns on...so that by the time that first fill slosh gets to the end of the pipe (where the pipe cap is) all the water behind it is going in the other direction and the size of the first slosh is almost zero....and the cycle repeating... thats resonance....

if you leave the pumps cycling as before but shorten the pipe.... now a pretty big slosh is going to smack up against the pipe cap and get reflected back toward the pumps... accordingly, the pressure that the pumps see will be much higher or much lower that normal and the water flow will be much lower or higher than intended...this is like the situation with the collapsed antenna..... if the flow or current is much higher...jumping back to electronics, the power P dissipated in the transistors is P=VI ...so if the pressure V increases, the power can increase, if the flow increases, P can also increase....alternatively, if things break the other way, the voltage can get so high that the transistors break down and get so hot that the magic smoke that makes them work escapes.

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Post by Robert Reed » Tue Oct 24, 2006 4:16 pm

In the laws of electric/electronics, optimum power transfer occurs when the generating source impedance equals the load impedance. Generally speaking for RF that would be a Final Tx amplifier with an output 'Z' of 50 ohms fed into a transmission line with a 'Z' of 50 ohms driving an antenna that has a 50 ohm 'Z' at point of inception. . Now the normal power flow is steady and in one direction only (towards the load). When any of these vary in 'Z" from one and another, a mismatch occurs, and depending on the severity of the mismatch, some power or a lot of that incident power will reflect from the load (antenna) and start flowing back to the source (final amp).The result of this produces standing waves on the transmission line and can be measured with the right equipment. Now, instead of a nice flat power measurement across the line, we have a series of peaks and valleys as we move down the line. This is known as the Voltage Standing Wave Ratio (VSWR). The higher the ratio of peaks and valleys, the higher the VSWR . Your problem is produced by a mismatch, and the VSWR is the result of that mismatch and not the reason for that mismatch. So in describing your problem, you were told correctly - it is a MISMATCH.
BTW- the most respected wattmeter in the feild, the BIRD wattmeter, will measure power flow in both directions and by simple computation or using a nomograph supplied by them, a direct correlation can be made. Also I should mention that that power flowing back to the final ampl. has to be dissipated by it as it now becomes a partial load and will generate more heat due to that.

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