VSWR doesn't matter? But how about "mismatch loss"?

What I gleaned from the excellent answers for the original "VSWR Doesn't Matter?" thread is that high VSWR doesn't really matter in a lossless transmission line environment between a transmitter's antenna tuner and the antenna, since any reflected RF energy will simply continue to "bounce" back and forth between the tuner's output impedance and the antenna's input impedance until it is, finally, completely radiated from the antenna without loss.

But then why does the concept of "mismatch loss" exist in reference to antennas? I have quickly calculated that if a transmitter outputs 100 watts, and the TX antenna has an impedance that will cause a VSWR of 10:1 -- using lossless transmission line -- that the mismatch loss in this "lossless" system would be 4.81dB! (Reflected power 66.9 watts, RL -1.74).

Since mismatch loss is the "amount of power lost due to reflection", and is as if an "attenuator with a value of the mismatch loss where placed in series with the transmission line", then I would think that VSWR would *definitely* matter, and not just for highly lossy lines either. But here again, I'm probably not seeing the entire picture here. What am I missing??

Confused!

-Bill

Reply to
billcalley
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If the system is Z0-matched, e.g. antenna tuner, there is a mismatch gain at the tuner that offsets the mismatch loss at the load so, in a lossless system, nothing is lost. Wave cancellation toward the source is balanced by constructive interference toward the load.

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73, Cecil, w5dxp.com
Reply to
Cecil Moore

Now my head *really* hurts! This is a VERY confusing subject, to say the least. (And I also thought antenna tuners actually had a

*loss* due to their limited Q...I think I'm going to change careers now and just become a pet groomer; or perhaps simply give up completely and work at Radio Shack).

-Bill

Reply to
billcalley

Real-world antenna tuners do have a loss but we previously specified a lossless system. Of course, real world tuners and transmission lines suffer losses but we all just live with those losses while striving to minimize them. The point is that an antenna tuner reflects most of the reflected energy back toward the load thus accomplishing a mismatch gain that offsets some, if not most, of the mismatch loss. High SWR transmission lines are indeed lossier than flat matched transmission lines of the same material.

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73, Cecil, w5dxp.com
Reply to
Cecil Moore

That's basically true but ignores the stress that can be placed on the output circuit of the transmitter. Why do you think VSWR shut-down circuits are so popular? One can see rather high voltages or currents that are potentially damaging to the transistors and capacitors.

Now, since "lossless" is an abstraction and all materials have voltage and current limits, just make this easy on yourself and always strive for a VSWR of 2:1 or less. It simply works better and is more reliable.

Reply to
Charles Schuler

But renders many all-HF-band dipoles useless. :-) I regularly run up to an 18:1 SWR on my 450 ohm ladder-line. Owen's transmission line calculator says I'm losing about 0.8 dB in 100' of line under those conditions on 40m. IMO, it's a small price to pay for all-HF-band operation.

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73, Cecil, w5dxp.com
Reply to
Cecil Moore

How many Ham transmitters have a balanced output?

How are you feeding a balanced line?

If you are using an antenna tuner with unbalanced in (50 ohms) and balanced out (variable impedance), you should be OK in most situations.

Reply to
Charles Schuler

If a tuner is placed directly after the TX and properly adjusted the TX will always see a 50 ohm load and the shutdown circuit will always be happpy. Again as long as the TX sees a match there is no unusual stress placed on it. Remember that before the invention of coax cable SWR was rarely considered. Instead the tx was tuned for proper established operational parametrs and all was right with the world.

1:1 SWR CAN MEAN YOUR COAX IS FULL OF WATER.

Jimmie

Reply to
Jimmie D

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No tuner! I don't like tuner losses. The feedpoint impedance is always between 35 ohms and 85 ohms resistive. My choke has an impedance in the thousands of ohms.

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73, Cecil  http://www.w5dxp.com
Reply to
Cecil Moore

Your "amount of power lost due to reflection" statement would be true if the line were connected to something resistive at the line's characteristic impedance. With a properly tuned tuner, that's not the case -- instead, the impedance looking into the tuner will also reflect power, and in a way that makes it all work out so that the power all ends up being radiated, which is what you wanted in the first place.

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Tim Wescott
Wescott Design Services
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Reply to
Tim Wescott

If you start considering loss in the tuner and the line then yes, a greater mismatch between the antenna and the line will result in more lost power (and more component heating in the tuner). You really want to leave that subject be until you understand the properties of a lossless system.

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Tim Wescott
Wescott Design Services
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Reply to
Tim Wescott

"billcalley" wrote in news: snipped-for-privacy@e65g2000hsc.googlegroups.com:

Well, there will be tuner losses, depending on how good the tuner's components are.

Yes, you're confused.

If the lossless transmission line (obviously no such animal exists) were tuned with a lossless tuner, then VSWR would not matter at all.

The loss due to mismatch in any real system will depend upon frequency, VSWR, type of feedline, length of feedline, and the quality of the tuning circuits used to match the system to the transmitter.

Let's take your example. 100 watt transmitter into, let's say 100ft. of feedline at 10:1 VSWR and assume tuner losses are negligble (they often aren't). Here are the losses for some different kinds of 50 ohm coax at

10mhz:

Belden 8237 2.19db Belden 9913 1.63db Belden 9258 3.19db Belden 8240 3.71db Belden 9201 3.83db

So, what's obvious here is that different coaxes have different losses at high SWR. Why is that? Because as power is reflected back and forth in a transmission line, the losses accumulate. So line that is very low- loss to begin with will be less affected by high SWR than line that has moderate to high losses when flat.

If 10 percent of the power in a line is lost travelling from the transmitter to the antenna, and if the antenna only radiates half that power, sending the rest back down the line, then 45 percent of the transmitter power is radiated immediately, while 45 percent is reflected. But only 40.5 percent reaches the tuner or transmitter. If ALL of that is re-reflected, then only 36.45 percent of the power is available at the second reflection to the antenna. The antenna will radiate 18.225 percent of the transmitter's power at this point, making the total 63.225 percent of the transmitter's output. Another 18.225 percent will be reflected again and, of that 16.4025 percent of the transmit power will live to be re-reflected from the tuner and 14.76225 percent will arrive at the antenna on the next bounce. Of that we can expect 7.381125 percent of the transmitter's total power to end up radiated while an equal amount starts its way back to the tuner. Anyway, it becomes a pretty simple bit of limit math to predict exactly how much will be radiated and how much lost in the coax under these conditions.

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Dave Oldridge+
ICQ 1800667
Reply to
Dave Oldridge

Hi Bill,

As a short description, that is adequate.

"Mismatch loss" is a system description, not an antenna description. So your reference is wrong.

Two things wrong here:

  1. You say nothing of a tuner inline; B. Your math is wrong either way.

It does matter if you lack a tuner (in more ways that one). Most discussion of "mismatch loss" omits such matters as tuners as it is a separable issue. Combining these topics raises your chance of confusion.

73's Richard Clark, KB7QHC
Reply to
Richard Clark

I think the confusion (which *always* seems to arise) comes from the mix of the concept with the real world. The concept claims that the system is lossless, so the power bounces around until it eventually exits the "system" via the antenna. Real world, the system is lossy, so with all the bouncing around some of the power fails to leave the system via RF radiation and instead leaves the system via IR radiation. More heat, less RF. Tastes great, less filling. :-)

Ed

Reply to
ehsjr

Unless your coax decides that it had enough of all this current. I had that happen once. The SWR had gone up and I pressed on. Then a muffled boom outside and it became 100% reflective. Now I had to get on a ladder and painstakingly scrape all the molten gunk off the stucco.

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Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

It truly wouldn't matter if there were no such thing as resistance and so on. The whole circuit could be tuned, with the transmission line a part of it, and all of the power would go out the antenna.

Unfortunately, that's not the way reality works, more's the pity.

Every time those "standing waves" bounce back and forth, they warm up the transmission line, the connectors, the transmitter tank, the transmitter itself, etc, etc, and Entropy is conserved. ;-)

Hope This Helps! Rich

Reply to
Rich Grise

Taking this at face value, yes, the "mismatch loss" is 4.8dB.

73's Richard Clark, KB7QHC
Reply to
Richard Clark

Humor noted, but Entropy is not conserved.

Entropy increases

Reply to
bw

Cecil,

Please explain your antenna and radio. I am assuming you have a solid state rig with an so-239 connector on it for the basic and then you do what?

Thanks

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73 for now
Buck, N4PGW
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Reply to
Buck

Seems to me that evolution violates that principle.

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73, Cecil  http://www.w5dxp.com
Reply to
Cecil Moore

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