Questions about reflected power on a transmission line

[JR Richter]

TL;DR Summary

If a reflected signal is redirected back to the transmitter and then back to the antenna feed point, why can't we measure this reflected power at the transmitter? Our forward power remains the same even with a SWR.

This could be a typical setup: We're transmitting a signal of 100W measured directly out of our Pa / Matching network into coaxial feed line. The feed line has a 52.2 -J0.31 Ohm impedance. The length of this coaxial feed line is 100'. The matched line loss of this feed line is .709dB/100' at 14.1 MHz. The feed line is connected to an antenna with a feed point Z of 200 + J0 Ohms.

We know from calculations that our SWR at the feed point is ~ 3.83. There will be an additional loss of .562dB due to impedance mismatch at the antenna. The total line loss will be .709dB + .562dB is 1.271 dB. Calculations show that we have ~ 74.6W at the antenna feed point. The remaining power of 25.3W is fed back to the transmitter. With line loss of .709dB the reflected power seen at the transmitter is 21.54W. This calculates out to ~ 2.98 SWR at the transmitter output.

Sorry for the calculations but just wanted to show where I am at with this example. Many authors have said that this power reflection is then sent back to the feed point of the antenna. I feel this reflected power is absorbed because of the perfect match at output of our transmitter.

My question is, If this power is truly sent back to the antenna, then why can't we see this reflected power on our forward power measurements? I can't imagine the final Pa throttling back or increasing its power to maintain the 100W output. Thanks Jr Richter

 

[Baluncore]

Welcome to PF.

You have;
1. A final PA stage; 2. A transmatch; 3. An SWR forward and reflected power meter; 4. A transmission line; 5. An antenna matching unit; 6. An antenna.

You will adjust your remote antenna match as best you can, to minimise reflection of power from the antenna, at the frequency of operation.

You will then adjust the transmatch to maximise forward power. The phasor from the PA, is then being added to a delayed phasor, (reflected from the antenna), and that vector sum is being matched forward into the line by the trans-match.

The reflected energy is not really absorbed, not reflected, but is added vectorially to the transmitter output, to become the forward power.

Line loss is minimised when your antenna is correctly matched to the line by the antenna matching unit, because then there is no reflected power, being attenuated twice by the line.

To confuse things, amateurs (14.100 MHz), often call the transmatch an antenna tuner.

 

[tech99]

You will find, if you connect the VSWR meter after the transmatch, that the forward power is indeed more than 100W. To find the power radiated, we deduct reflected power from forward power readings. For example, a 1W transmitter, when adjusted to deliver maximum into a mismatched feeder, can give readings of, say, 10W forward and 9W reflected. Power radiated = 10 - 9 = 1 W. In this case the VSWR of the feeder is very high. We have a reflected power ratio of 9/10, or a voltage reflection coefficient of SQRT 9/10 = 0.95. The VSWR will then be 1+0.95/1-0.95 = 39.
If you have a transmatch, you will see these numbers if you place the VSWR meter on the antenna side of the transmatch. In the case I have mentioned, 9 W are stored by bouncing back and forth along the feeder. When you switch on the transmitter, it will take a few cycles for the energy store to fill up, and when you switch off, radiation will continue for a few cycles as the stored energy is delivered to the antenna.

 

[JR Richter]

Welcome to PF.

You have;
1. A final PA stage; 2. A transmatch; 3. An SWR forward and reflected power meter; 4. A transmission line; 5. An antenna matching unit; 6. An antenna.

You will adjust your remote antenna match as best you can, to minimise reflection of power from the antenna, at the frequency of operation.

You will then adjust the transmatch to maximise forward power. The phasor from the PA, is then being added to a delayed phasor, (reflected from the antenna), and that vector sum is being matched forward into the line by the trans-match.

The reflected energy is not really absorbed, not reflected, but is added vectorially to the transmitter output, to become the forward power.

Line loss is minimised when your antenna is correctly matched to the line by the antenna matching unit, because then there is no reflected power, being attenuated twice by the line.

To confuse things, amateurs (14.100 MHz), often call the transmatch an antenna tuner.

I believe you covered the majority of our transmitting scenario nicely but left out the closed loop output sensing or ALC that maintains the desired power level.

Welcome to PF.

You have;
1. A final PA stage; 2. A transmatch; 3. An SWR forward and reflected power meter; 4. A transmission line; 5. An antenna matching unit; 6. An antenna.

You will adjust your remote antenna match as best you can, to minimise reflection of power from the antenna, at the frequency of operation.

You will then adjust the transmatch to maximise forward power. The phasor from the PA, is then being added to a delayed phasor, (reflected from the antenna), and that vector sum is being matched forward into the line by the trans-match.

The reflected energy is not really absorbed, not reflected, but is added vectorially to the transmitter output, to become the forward power.

Line loss is minimised when your antenna is correctly matched to the line by the antenna matching unit, because then there is no reflected power, being attenuated twice by the line.

To confuse things, amateurs (14.100 MHz), often call the transmatch an antenna tuner.

I am in total agreement with everything with the exception, more information is needed about the sensing circuit (ALC) that actually maintains the original output level. You nicely stated, the vector phase relationship being added at the xmtr output. If I understand correctly, this vectored sum is the new forward power. I believe that the ALC will sense this new level and adjust the drive circuits to maintain the original 100W. I believe this new forward power will be adjusted quickly by the ALC and not keep oscillating back and forth as some have mentioned. Since the typical metering system is too slow it sees the original 100W set by the operator even with a ~ 3:1 VSWR at the xmtr.

Although some internal ALC circuits were not designed well and actually caused large power spiking during power up.

Your Thoughts

Thanks

 

[Baluncore]

I believe you covered the majority of our transmitting scenario nicely but left out the closed loop output sensing or ALC that maintains the desired power level.

I believe you left the ALC out of the original post, with the elephant in the room.
The ALC makes the problem intractable, since it places the transmitter inside a feedback loop with unspecified parameters.

 

[tech99]

If we use a transmatch (which might, in effect, be integral to the equipment today) then the transmitter proper is seeing a correct match. If however the transmitter is seeing a mismatched load then there is a reduction in radiated power, but this is small in most circumstances. For example, a VSWR of 3:1 corresponds to a return loss of 6dB, or 25% power, so in this case our radiated power would be 75W rather than 100W. The job of ALC is to try to regulate the drive power to the final amplifier so I agree it might cause confusion.

 

[JR Richter]

I believe you left the ALC out of the original post, with the elephant in the room.
The ALC makes the problem intractable, since it places the transmitter inside a feedback loop with unspecified parameters.

Yes, you are correct, I just assumed that most RF transmitters use some type of feed back loops to maintain a specific output, regardless of moderate VSWR on the feed line. With that being said, I guess my original question seems meaningless? Sorry for taking up your time, although you did shed light on the reflected power being vectorially added to the forward power and no absorption in the final PA. I assume that with no ALC we would see this increase output power on our metering system.

Thanks

JR

 

[JR Richter]

You will find, if you connect the VSWR meter after the transmatch, that the forward power is indeed more than 100W. To find the power radiated, we deduct reflected power from forward power readings. For example, a 1W transmitter, when adjusted to deliver maximum into a mismatched feeder, can give readings of, say, 10W forward and 9W reflected. Power radiated = 10 - 9 = 1 W. In this case the VSWR of the feeder is very high. We have a reflected power ratio of 9/10, or a voltage reflection coefficient of SQRT 9/10 = 0.95. The VSWR will then be 1+0.95/1-0.95 = 39.
If you have a transmatch, you will see these numbers if you place the VSWR meter on the antenna side of the transmatch. In the case I have mentioned, 9 W are stored by bouncing back and forth along the feeder. When you switch on the transmitter, it will take a few cycles for the energy store to fill up, and when you switch off, radiation will continue for a few cycles as the stored energy is delivered to the antenna.

I have been interacting with a local retired communication engineer and his response has been the same as yours. I know and have advocated that matching the xmtr to a feed line does not remove the VSWR from the line. Also moving the VSWR on the antenna side of the transmatch does show a VSWR right at the point where its connected to the feed line. My testing methods are limited, although I do have two fairly accurate vector watt meters. I have one on either side of a transmatch. I can create a perfect match at the input of the transmatch and monitor the forward power. In contrast, the meter on the antenna side of the transmatch does show a higher VSWR due to the mismatch at the antenna load and losses along the feed line. However the forward power measured at the input to the feed line is lower. Mostly due to the insertion loss of the transmatch. At this point I believe the ALC has aleady adusted the output of the PA due to the reflected power.

Thanks for your input.

JR

 

[tech99]

In the case you describe, the transmitter is seeing a matched load, so the ALC will not be reacting in any way to a high VSWR.

 

[Baluncore]

I believe that the ALC will sense this new level and adjust the drive circuits to maintain the original 100W.

Does it use the forward power for the ALC, or where exactly is the sense circuit for the ALC ?

 

[JR Richter]

Does it use the forward power for the ALC, or where exactly is the sense circuit for the ALC ?

 

[JR Richter]

I believe most ALC circuits monitor forward power and control the output level. We are not talking about the upper limit of some PA's that use a -1dB compression limit but a closed loop system that maintains a set power level.

In reference to my original question, why can't we see this vector added power on our metering system? Since I didn't mention an ALC in my original example and assume there is none, we should be able to see the added vector phasors of the reflected and forward power at the xmtr output (assuming that the matching network is at the output of our transmitter)?? At this point I am in total agreement that this reflected power has to go somewhere.

Obviously, placement of the transmatch is important and ideally it should be placed at the feed point of the antenna. When we have a match at the antenna feed point, will have a ~ 1:1 VSWR and the only loss is the inherent loss of the feed line. If we place the transmatch at the output of the xmtr, we'll have again the inherent loss of the feed line + any losses caused from the impedance mismatch between the feed line and the antenna feed point.

If I understand all the great comments posted, the reason we don't see any deviation in the output with VSWR is because of the feed back loop that controls the output of our transmitters??

Thanks
JR

 

[Baluncore]

Obviously, placement of the transmatch is important and ideally it should be placed at the feed point of the antenna. When we have a match at the antenna feed point, will have a ~ 1:1 VSWR and the only loss is the inherent loss of the feed line. If we place the transmatch at the output of the xmtr, we'll have again the inherent loss of the feed line + any losses caused from the impedance mismatch between the feed line and the antenna feed point.

You have confused the transmatch at the transmitter end of the line, with the antenna tuner at the antenna end of the line.

The antenna tuner goes between the feedline and the antenna. That must be tuned remotely to reduce reflection from the antenna, back down the feedline, seen as reverse power on the directional power meter in the shack.
First, tune the antenna, to minimise reflected power.

By definition, the transmatch is always at the transmitter, and may be built into the TX final stage coupling. It is before the directional power meter and the feedline.
Second, tune the transmatch to maximise forward power into the line.

Third, with low-loss feedline, there is some interaction between the ends of the line, so check again that the reflected power is still minimised.

 

[JR Richter]

You have confused the transmatch at the transmitter end of the line, with the antenna tuner at the antenna end of the line.

The antenna tuner goes between the feedline and the antenna. That must be tuned remotely to reduce reflection from the antenna, back down the feedline, seen as reverse power on the directional power meter in the shack.
First, tune the antenna, to minimise reflected power.

By definition, the transmatch is always at the transmitter, and may be built into the TX final stage coupling. It is before the directional power meter and the feedline.
Second, tune the transmatch to maximise forward power into the line.

Third, with low-loss feedline, there is some interaction between the ends of the line, so check again that the reflected power is still minimised.

Thank you for the clarification between a transmatch and an antenna tuner. The original question still remains! Is it unreasonable to believe that the reason we don't see any deviation on our output power metering system is because the internal ALC is constantly adjusting any deviations caused from an impedance mismatch between the feed line and the antenna feed point (and using a transmatch on the transmitter output).

Thanks
JR

 

[Baluncore]

ALC would explain why the forward power does not appear to drift over time. Before adjusting the antenna tuner, or the transmatch, you should turn off the ALC. You can then see the changes you are making, without having to fight the ALC.

 

[sophiecentaur]

TL;DR Summary: If a reflected signal is redirected back to the transmitter and then back to the antenna feed point, why can't we measure this reflected power at the transmitter? Our forward power remains the same even with a SWR.

If this power is truly sent back to the antenna, then why can't we see this reflected power on our forward power measurements?

A directional coupler will show forward and reflected power, so we can see. Directional couplers come in many different forms, depending on the operating frequency and required bandwidth (and power handled. The reflected power that arrives at the transmitter output will affect the impedance that the transmitter sees and, thus the operating conditions need to be adjusted.

A transmitter needs both efficiency and to maximise the power delivered. That will not involve a conjugate match or the transmitter would be dissipating half of the power it generates. Very expensive and generating loads of heat in the output stage. Likewise, AC power generators do not dissipate the same amount of power as the thousands of consumers and neither do efficient audio amplifiers dissipate the same amount of power as the loudspeakers dissipate and produce.