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Measuring SWR (Long)

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alorona

Measuring SWR (Long)

There's always a lot of discussion about measuring SWR, low SWR values, SWR lights not lighting up, etc. Here's something that might give you a better 'feel' for SWR.

Imagine that you measure your forward power at 100 W and your reflected power at only 1 W. You'd probably be very happy about this. Congratulations, your SWR would be 1.22 to 1.

Pause for a moment and let it soak in that an SWR of 1.22 is fabulously good. Once you get to this point, below which we're dealing with reflected power that's less than 1% of your power, or four-hundredths of a dB, it's not worth doing any more to your antenna system to improve it. No trimming, cutting, raising, lowering, hanging stuff from it, adding remote tuned things, nothing. You're done for the day and can now get on the air!

If the reflected power were 4 W, which would still look pretty small on the meter, the SWR would be 1.5 to 1.

A lot of people might be bothered by an SWR of 1.5, but really this is still very good and it's probably not worth going back up to the antenna to mess with it any more. Most importantly, the station on the other end cannot possibly hear the difference between your 1.2 and 1.5.

At this point you may be saying, "Yeah, but my rig/amplifier/other thing isn't happy unless the SWR is below 1.5 to 1, so I would still have to do more work."

Okay, that's fine, but my point is that the absolute number 1.5 nonetheless represents a system that is working quite well.

Let's allow the reflected power to increase all the way to <gasp> 10 watts! At that point, your SWR would be about 2 to 1. Sounds pretty bad, but surely if you were stranded in the desert and had to use your KX3 and a wire to get help, you'd be happy to have a match this good.

Now, let's look at a certain type of SWR meter found in virtually every HF rig out there that uses a directional bridge, a diode as a half-wave rectifier, a filter capacitor, a resistor, and a readout of some kind - analog meter, digital display, or other.

These types of SWR meters are very common. There's one in the K2 (it's actually in the KPA100), one in the K3, and probably every other rig out there. And those Birds, MFJs and Daiwas have them, too.

This type of circuit turns RF voltage into a DC current, usually using either a germanium or Schottky diode, because these diodes have the best sensitivity due to their low forward voltage drops: 0.3 V for the germanium, and 0.4 V for the Schottky.

Let's say that our directional bridge, when 100 W is in the forward direction, delivers 10 V to the forward Schottky diode. In our example above of 1 watt reflected, the reflected diode would see 1.0 volts from the reflected port of the bridge. Since this is above the diode threshold voltage, the diode would be happy and operating in its linear region where we'd get about 1 volt out for 1 volt in. The meter would correctly read 1.22 to 1.

Now say you have your Elecraft rig set for TUN PWR = 10 W, so when you tune up, forward power is 10 watts, and if the SWR is the aforementioned 1.2 to 1, the forward power would produce a little more than 3 volts at the diode and the 0.1 watts reflected would produce about 0.3 volts. Here is where we run into trouble.

Since a Schottky diode doesn't fully conduct until we exceed its 0.4 volt threshold, expecting to get 0.3 V out for 0.3 V in isn't quite realistic. The diode's output voltage won't be zero, but it'll be smaller than expected, according to the nonlinear region around its "knee". I measured a Schottky diode and got about 0.09 V at that level. The meter would indicate an SWR of... 1.06 instead of the actual 1.22. This reading is false.

There are some things that can be done to the circuit to compensate for this error but I'm not sure how many rigs go through the trouble. Suffice it to say that measurement of very low SWR has a large amount of uncertainty because the detectors get more and more nonlinear as the thing they're trying to measure (reflected power) gets smaller and smaller. Due to this effect (and also due to directivity) every meter of this kind has a point below which SWR measurements are probably wrong.

When we see posts here by folks puzzled as to why low SWR measurements from two different instruments don't agree, this undoubtedly is one of the reasons.

I chose these scenarios deviously, to illustrate a kind of corner case, but I think you get the idea that errors always exist in any power measurement like SWR. You might say, "But my SuperDuper wattmeter is calibrated by aliens in a metrology lab deep inside a secret mountain in Nevada and I would bet my life on it." Great, cool, congratulations. But I'm afraid every last power meter has errors stemming from mismatch, directivity, nonlinearity and drift.

If you simply tune for minimum reflected power you should be good to go, and don't get too hung up on if the meter says "1.2" or "1.5" or "1.0". Remember, once you get down into this zone it's very likely good enough.

Al W6LX
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Elecraft mailing list

Re: Measuring SWR (Long)

My friend Al has described some interesting things about SWR measurement here. But not all power/swr meters behave this way. In fact the Elecraft W2 meter is in this class. The designer added an ingenious little circuit to the diodes to bias them into their linear region. What the smeans is that even at low power the W2 is giving accurate measurements not just of reflected power, but for forward power as well. this is important since the effect that Al describes is valid not just for reflected power, but also forward power at low levels.

There is one other things to watch for that actually negates Al’s suggestion to always tune for lowest reflected power. As an ATU tunes, it provides different loads to the driver. This will greatly affect not just the reflected power, but forward power measurements as well, to the point that we may measure very low reflected power, but the forward power is very low as well. Thus when we tune we really do look for both maximum forward power in conjunction with minimum reflected power.

One last point. The directional coupler measurements in the KPA500, KAT500 and KPA1500 are designed to handle large power levels since the devices are meant for high power. We don’t really try to be accurate at low levels. Indeed the KPA500 won’t even indicate power below 25 watts and the KAT500 needs something like 10 or 20 watts to work properly. This allows us to be accurate at high power levels and also avoid having to be concerned with the diodes workin in their non-linear mode. In other words, on these products you can pretty much believe the power levels the device is showing when it is showing them.

Al, thanks for a nice write-up.

73!
Jack, W6FB

> On Apr 9, 2019, at 12:42 PM, Al Lorona <[hidden email]> wrote:
>
> There's always a lot of discussion about measuring SWR, low SWR values, SWR lights not lighting up, etc. Here's something that might give you a better 'feel' for SWR.
>
> Imagine that you measure your forward power at 100 W and your reflected power at only 1 W. You'd probably be very happy about this. Congratulations, your SWR would be 1.22 to 1.
>
> Pause for a moment and let it soak in that an SWR of 1.22 is fabulously good. Once you get to this point, below which we're dealing with reflected power that's less than 1% of your power, or four-hundredths of a dB, it's not worth doing any more to your antenna system to improve it. No trimming, cutting, raising, lowering, hanging stuff from it, adding remote tuned things, nothing. You're done for the day and can now get on the air!
>
> If the reflected power were 4 W, which would still look pretty small on the meter, the SWR would be 1.5 to 1.
>
> A lot of people might be bothered by an SWR of 1.5, but really this is still very good and it's probably not worth going back up to the antenna to mess with it any more. Most importantly, the station on the other end cannot possibly hear the difference between your 1.2 and 1.5.
>
> At this point you may be saying, "Yeah, but my rig/amplifier/other thing isn't happy unless the SWR is below 1.5 to 1, so I would still have to do more work."
>
> Okay, that's fine, but my point is that the absolute number 1.5 nonetheless represents a system that is working quite well.
>
> Let's allow the reflected power to increase all the way to <gasp> 10 watts! At that point, your SWR would be about 2 to 1. Sounds pretty bad, but surely if you were stranded in the desert and had to use your KX3 and a wire to get help, you'd be happy to have a match this good.
>
> Now, let's look at a certain type of SWR meter found in virtually every HF rig out there that uses a directional bridge, a diode as a half-wave rectifier, a filter capacitor, a resistor, and a readout of some kind - analog meter, digital display, or other.
>
> These types of SWR meters are very common. There's one in the K2 (it's actually in the KPA100), one in the K3, and probably every other rig out there. And those Birds, MFJs and Daiwas have them, too.
>
> This type of circuit turns RF voltage into a DC current, usually using either a germanium or Schottky diode, because these diodes have the best sensitivity due to their low forward voltage drops: 0.3 V for the germanium, and 0.4 V for the Schottky.
>
> Let's say that our directional bridge, when 100 W is in the forward direction, delivers 10 V to the forward Schottky diode. In our example above of 1 watt reflected, the reflected diode would see 1.0 volts from the reflected port of the bridge. Since this is above the diode threshold voltage, the diode would be happy and operating in its linear region where we'd get about 1 volt out for 1 volt in. The meter would correctly read 1.22 to 1.
>
> Now say you have your Elecraft rig set for TUN PWR = 10 W, so when you tune up, forward power is 10 watts, and if the SWR is the aforementioned 1.2 to 1, the forward power would produce a little more than 3 volts at the diode and the 0.1 watts reflected would produce about 0.3 volts. Here is where we run into trouble.
>
> Since a Schottky diode doesn't fully conduct until we exceed its 0.4 volt threshold, expecting to get 0.3 V out for 0.3 V in isn't quite realistic. The diode's output voltage won't be zero, but it'll be smaller than expected, according to the nonlinear region around its "knee". I measured a Schottky diode and got about 0.09 V at that level. The meter would indicate an SWR of... 1.06 instead of the actual 1.22. This reading is false.
>
> There are some things that can be done to the circuit to compensate for this error but I'm not sure how many rigs go through the trouble. Suffice it to say that measurement of very low SWR has a large amount of uncertainty because the detectors get more and more nonlinear as the thing they're trying to measure (reflected power) gets smaller and smaller. Due to this effect (and also due to directivity) every meter of this kind has a point below which SWR measurements are probably wrong.
>
> When we see posts here by folks puzzled as to why low SWR measurements from two different instruments don't agree, this undoubtedly is one of the reasons.
>
> I chose these scenarios deviously, to illustrate a kind of corner case, but I think you get the idea that errors always exist in any power measurement like SWR. You might say, "But my SuperDuper wattmeter is calibrated by aliens in a metrology lab deep inside a secret mountain in Nevada and I would bet my life on it." Great, cool, congratulations. But I'm afraid every last power meter has errors stemming from mismatch, directivity, nonlinearity and drift.
>
> If you simply tune for minimum reflected power you should be good to go, and don't get too hung up on if the meter says "1.2" or "1.5" or "1.0". Remember, once you get down into this zone it's very likely good enough.
>
> Al W6LX
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
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> Please help support this email list: http://www.qsl.net/donate.html

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Bob McGraw - K4TAX

Re: Measuring SWR (Long)

In reply to this post by alorona

Thank you Al Lorona!

73

Bob, K4TAX

On 4/9/2019 2:42 PM, Al Lorona wrote:

> There's always a lot of discussion about measuring SWR, low SWR values, SWR lights not lighting up, etc. Here's something that might give you a better 'feel' for SWR.
>
> Imagine that you measure your forward power at 100 W and your reflected power at only 1 W. You'd probably be very happy about this. Congratulations, your SWR would be 1.22 to 1.
>
> Pause for a moment and let it soak in that an SWR of 1.22 is fabulously good. Once you get to this point, below which we're dealing with reflected power that's less than 1% of your power, or four-hundredths of a dB, it's not worth doing any more to your antenna system to improve it. No trimming, cutting, raising, lowering, hanging stuff from it, adding remote tuned things, nothing. You're done for the day and can now get on the air!
>
> If the reflected power were 4 W, which would still look pretty small on the meter, the SWR would be 1.5 to 1.
>
> A lot of people might be bothered by an SWR of 1.5, but really this is still very good and it's probably not worth going back up to the antenna to mess with it any more. Most importantly, the station on the other end cannot possibly hear the difference between your 1.2 and 1.5.
>
> At this point you may be saying, "Yeah, but my rig/amplifier/other thing isn't happy unless the SWR is below 1.5 to 1, so I would still have to do more work."
>
> Okay, that's fine, but my point is that the absolute number 1.5 nonetheless represents a system that is working quite well.
>
> Let's allow the reflected power to increase all the way to <gasp> 10 watts! At that point, your SWR would be about 2 to 1. Sounds pretty bad, but surely if you were stranded in the desert and had to use your KX3 and a wire to get help, you'd be happy to have a match this good.
>
> Now, let's look at a certain type of SWR meter found in virtually every HF rig out there that uses a directional bridge, a diode as a half-wave rectifier, a filter capacitor, a resistor, and a readout of some kind - analog meter, digital display, or other.
>
> These types of SWR meters are very common. There's one in the K2 (it's actually in the KPA100), one in the K3, and probably every other rig out there. And those Birds, MFJs and Daiwas have them, too.
>
> This type of circuit turns RF voltage into a DC current, usually using either a germanium or Schottky diode, because these diodes have the best sensitivity due to their low forward voltage drops: 0.3 V for the germanium, and 0.4 V for the Schottky.
>
> Let's say that our directional bridge, when 100 W is in the forward direction, delivers 10 V to the forward Schottky diode. In our example above of 1 watt reflected, the reflected diode would see 1.0 volts from the reflected port of the bridge. Since this is above the diode threshold voltage, the diode would be happy and operating in its linear region where we'd get about 1 volt out for 1 volt in. The meter would correctly read 1.22 to 1.
>
> Now say you have your Elecraft rig set for TUN PWR = 10 W, so when you tune up, forward power is 10 watts, and if the SWR is the aforementioned 1.2 to 1, the forward power would produce a little more than 3 volts at the diode and the 0.1 watts reflected would produce about 0.3 volts. Here is where we run into trouble.
>
> Since a Schottky diode doesn't fully conduct until we exceed its 0.4 volt threshold, expecting to get 0.3 V out for 0.3 V in isn't quite realistic. The diode's output voltage won't be zero, but it'll be smaller than expected, according to the nonlinear region around its "knee". I measured a Schottky diode and got about 0.09 V at that level. The meter would indicate an SWR of... 1.06 instead of the actual 1.22. This reading is false.
>
> There are some things that can be done to the circuit to compensate for this error but I'm not sure how many rigs go through the trouble. Suffice it to say that measurement of very low SWR has a large amount of uncertainty because the detectors get more and more nonlinear as the thing they're trying to measure (reflected power) gets smaller and smaller. Due to this effect (and also due to directivity) every meter of this kind has a point below which SWR measurements are probably wrong.
>
> When we see posts here by folks puzzled as to why low SWR measurements from two different instruments don't agree, this undoubtedly is one of the reasons.
>
> I chose these scenarios deviously, to illustrate a kind of corner case, but I think you get the idea that errors always exist in any power measurement like SWR. You might say, "But my SuperDuper wattmeter is calibrated by aliens in a metrology lab deep inside a secret mountain in Nevada and I would bet my life on it." Great, cool, congratulations. But I'm afraid every last power meter has errors stemming from mismatch, directivity, nonlinearity and drift.
>
> If you simply tune for minimum reflected power you should be good to go, and don't get too hung up on if the meter says "1.2" or "1.5" or "1.0". Remember, once you get down into this zone it's very likely good enough.
>
> Al W6LX
> ______________________________________________________________

Ron Genovesi

Re: Measuring SWR (Long)

In reply to this post by alorona

Oh My God! A voice of reason! And from here of all places! How absolutely unexpected!
Thank you sir.

Roger D Johnson

Re: Measuring SWR (Long)

In reply to this post by alorona

For an interesting discussion..."What happens to the power that's reflected?"

On 4/9/2019 3:42 PM, Al Lorona wrote:

k6dgw

Re: Measuring SWR (Long)

In reply to this post by Elecraft mailing list

Why does everyone seem to think all the secret, hollow mountains are in
Nevada? I know of one in Colorado and another in Virginia. Umm ...
maybe that's in West Virginia, not sure now. [:-))

73,
Fred ["Skip"] K6DGW
Sparks NV DM09dn
Washoe County
>> I chose these scenarios deviously, to illustrate a kind of corner case, but I think you get the idea that errors always exist in any power measurement like SWR. You might say, "But my SuperDuper wattmeter is calibrated by aliens in a metrology lab deep inside a secret mountain in Nevada and I would bet my life on it." Great, cool, congratulations. But I'm afraid every last power meter has errors stemming from mismatch, directivity, nonlinearity and drift.
>>
>> Al W6LX
>>

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Roy Koeppe

Re: Measuring SWR (Long)

In reply to this post by Roger D Johnson

"...Let's allow the reflected power to increase all the way to <gasp> 10
watts! At that point, your SWR would be about 2 to 1. Sounds pretty bad, but
surely if you were stranded in the desert and had to use your KX3 and a wire
to get help, you'd be happy to have a match this good...(etc.)"

From memory now, at 2 to 1...feedline voltage nodes and current nodes can
double -- no small thing when power is 1500 watts; can stress antenna system
components.

73, Roy K6XK

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Don Sanders

Re: Measuring SWR (Long)

Since no one has answered your question. I will venture a
supposition that many know but few will speak out.
And some still remember their CB days and the erroneous
advertising saying the power is lost.
Reflected Power flows back down the feed line and is subjected
to the normal feed line loss. It then is "reflected" back toward the
antenna, again subject to the feed line loss, where some of it is
radiated and some reflected again.
This continues until the power is dissipated. The book "Reflections"
has a very good but somewhat technical explanation.
Therein also is the reference to forgetting about striving for the
"Holy Grail" of 1:1 SWR.
And concentrating on getting it low enough that the transmitter will
supply full power output. Also using the best low loss feed line and
proper matching when possible of the feed line to the antenna.

Dr. Don W4BWS

God Bless All
Ham Radio does not make the world go round.
Ham Radio is what makes the ride worthwhile.

On Tue, Apr 9, 2019 at 6:43 PM Roy Koeppe <[hidden email]> wrote:

> "...Let's allow the reflected power to increase all the way to <gasp> 10
> watts! At that point, your SWR would be about 2 to 1. Sounds pretty bad,
> but
> surely if you were stranded in the desert and had to use your KX3 and a
> wire
> to get help, you'd be happy to have a match this good...(etc.)"
>
>
> From memory now, at 2 to 1...feedline voltage nodes and current nodes can
> double -- no small thing when power is 1500 watts; can stress antenna
> system
> components.
>
> 73, Roy K6XK
>
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>

k6dgw

Re: Measuring SWR (Long)

Well, the reflected power is created by a "virtual transmitter" at the
feedpoint of the antenna and heads down the coax which has a surge
impedance of Z0 ohms. It meets the SO-239 at the TX and sees an
impedance of Z1, the impedance presented by the PA and output filters.
If Z1=Z0, the power is dissipated as heat in the PA and associated RF
circuitry. If Z1<>Z0, some is dissipated and some is reflected, where
some is radiated, and some is reflected [virtual transmitter again]. Ad
infinitum, and when forever is over, it is all gone and everyone lives
happily ever after.

SWR and all the associated measuring equipment and concern with it only
became an issue when: 1) Coax replaced parallel lines and; 2) Resonant
output circuits were replaced by solid state amplifiers with
non-resonant filters. When I sat for the Extra in early 1956, the only
question that involved standing waves was one about how Lecher Lines
could be used to measure transmitter frequency.

It's important today but calculating it hasn't changed. At 10 W, a 2:1
SWR will probably work ok. At 1500 W, a solid state amplifier may not
be happy with the voltages developed at its output connector.

73,
Fred ["Skip"] K6DGW
Sparks NV DM09dn
Washoe County

On 4/9/2019 5:56 PM, Don Sanders wrote:

W2xj

Re: Measuring SWR (Long)

I agree except that even at 500KW a 2:1 or greater is the norm with open wire line.

Sent from my iPhone

> On Apr 9, 2019, at 19:10, Fred Jensen <[hidden email]> wrote:
>
> Well, the reflected power is created by a "virtual transmitter" at the feedpoint of the antenna and heads down the coax which has a surge impedance of Z0 ohms. It meets the SO-239 at the TX and sees an impedance of Z1, the impedance presented by the PA and output filters. If Z1=Z0, the power is dissipated as heat in the PA and associated RF circuitry. If Z1<>Z0, some is dissipated and some is reflected, where some is radiated, and some is reflected [virtual transmitter again]. Ad infinitum, and when forever is over, it is all gone and everyone lives happily ever after.
>
> SWR and all the associated measuring equipment and concern with it only became an issue when: 1) Coax replaced parallel lines and; 2) Resonant output circuits were replaced by solid state amplifiers with non-resonant filters. When I sat for the Extra in early 1956, the only question that involved standing waves was one about how Lecher Lines could be used to measure transmitter frequency.
>
> It's important today but calculating it hasn't changed. At 10 W, a 2:1 SWR will probably work ok. At 1500 W, a solid state amplifier may not be happy with the voltages developed at its output connector.
>
> 73,
> Fred ["Skip"] K6DGW
> Sparks NV DM09dn
> Washoe County
>
>> On 4/9/2019 5:56 PM, Don Sanders wrote:
>> Since no one has answered your question. I will venture a
>> supposition that many know but few will speak out.
>> And some still remember their CB days and the erroneous
>> advertising saying the power is lost.
>> Reflected Power flows back down the feed line and is subjected
>> to the normal feed line loss. It then is "reflected" back toward the
>> antenna, again subject to the feed line loss, where some of it is
>> radiated and some reflected again.
>> This continues until the power is dissipated. The book "Reflections"
>> has a very good but somewhat technical explanation.
>> Therein also is the reference to forgetting about striving for the
>> "Holy Grail" of 1:1 SWR.
>> And concentrating on getting it low enough that the transmitter will
>> supply full power output. Also using the best low loss feed line and
>> proper matching when possible of the feed line to the antenna.
>>
>> Dr. Don W4BWS
>>
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html

Don Wilhelm

Re: Measuring SWR (Long)

In reply to this post by Don Sanders

For a slightly different answer, take a look at my website www.w3fpr.com
article on Antennas, Transmission Lines, and Tuners.
You can also find that same article preserved at
https://www.qsl.net/w3fpr/ant_article.htm.

73,
Don W3FPR

On 4/9/2019 8:56 PM, Don Sanders wrote:

> Since no one has answered your question. I will venture a
> supposition that many know but few will speak out.
> And some still remember their CB days and the erroneous
> advertising saying the power is lost.
> Reflected Power flows back down the feed line and is subjected
> to the normal feed line loss. It then is "reflected" back toward the
> antenna, again subject to the feed line loss, where some of it is
> radiated and some reflected again.
> This continues until the power is dissipated. The book "Reflections"
> has a very good but somewhat technical explanation.
> Therein also is the reference to forgetting about striving for the
> "Holy Grail" of 1:1 SWR.
> And concentrating on getting it low enough that the transmitter will
> supply full power output. Also using the best low loss feed line and
> proper matching when possible of the feed line to the antenna.
>
> Dr. Don W4BWS
>
> God Bless All
> Ham Radio does not make the world go round.
> Ham Radio is what makes the ride worthwhile.
>
>
> On Tue, Apr 9, 2019 at 6:43 PM Roy Koeppe <[hidden email]> wrote:
>
>> "...Let's allow the reflected power to increase all the way to <gasp> 10
>> watts! At that point, your SWR would be about 2 to 1. Sounds pretty bad,
>> but
>> surely if you were stranded in the desert and had to use your KX3 and a
>> wire
>> to get help, you'd be happy to have a match this good...(etc.)"
>>
>>
>> From memory now, at 2 to 1...feedline voltage nodes and current nodes can
>> double -- no small thing when power is 1500 watts; can stress antenna
>> system
>> components.
>>
>> 73, Roy K6XK
>>
>>
>> ______________________________________________________________
>> Elecraft mailing list
>> Home: http://mailman.qth.net/mailman/listinfo/elecraft
>> Help: http://mailman.qth.net/mmfaq.htm
>> Post: mailto:[hidden email]
>>
>> This list hosted by: http://www.qsl.net
>> Please help support this email list: http://www.qsl.net/donate.html
>>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>

David Woolley (E.L)

Re: Measuring SWR (Long)

In reply to this post by Roger D Johnson

A typical transmitter does not reverse terminate the transmission line
with its characteristic impedance, so most of the reflected power gets
re-reflected as forward power. Reflected power isn't necessarily lost
power. At least at lower frequencies, it is likely to present a much
higher impedance. A final that did accurately terminate the line would
be rather inefficient.

A more important issue with SWR is high SWRs can cause clipping (you
need a larger voltage swing if the load is higher (assuming purely
resistive for the moment), or take it outside the safe operating area of
the output devices.

Those may cause problems at quite low SWRs, but they actually depend on
the complex impedance, so on some parts of the Smith chart circle you
may be completely safe, but on others, you might kill the finals.

On 09/04/2019 22:39, Roger D Johnson wrote:
> For an interesting discussion..."What happens to the power that's
> reflected?"

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k6dgw

Re: Measuring SWR (Long)

In reply to this post by W2xj

Or 10:1 or even 20:1. However ... there's always a "however" or a "but"
... it doesn't matter. The dielectric constant of air [the insulator
between open wire conductors] is very close to 1 whereas it is much
larger for various forms of coax. Very high SWR creates very high
voltages along the line. The energy storage with high dielectric
constants is much higher than with air, and dielectric losses go up
dramatically. In prehistoric times when we used vacuum tubes and
resonant tank circuits, we coupled power to the line with a 2-3 turn
link coil adjacent or in the middle of the resonant tank.

If the line exhibited reactance, it detuned the tank which we just
retuned to resonance ["Dip the plate, increase the coupling, repeat"].
The PA tank circuit became a thoroughly mis-named "antenna tuner." It
didn't matter what the SWR was on the open wire transmission line, and
in fact no one paid any attention to it. Then flexible coaxial cable
was invented. It was much more convenient than open wire line, however
in reasonable physical sizes, it had very low characteristic [surge]
impedances ... 50 and 75 ohm impedances were the result and now, SWR
mattered. The dielectric constant inside the cable was very much
higher, and the higher voltages from high SWR resulted in much higher
dielectric losses.

Then, someone invented the Pi-network [for all of you with fingers
poised over the keyboard to pounce on my description of history, relax
and breath deeply. I'm making some of the non-technical stuff up to
help hold your attention]. The Pi-network would transform the 50 or 75
ohm impedance at the end of the coax to the several thousand ohm plate
circuit impedance of the PA stage and power would flow to the antenna
unimpeded. It was at this point that sales of SWR indicators soared,
SWR became a household abbreviation in the ham community, and an SWR of
1.000:1 became the Nirvana of ham radio. It has been thus ever since.

As electronics progressed [?] from the vacuous to the solid state, SWR
took on an enhanced importance since the solid state was far less
tolerant of overvoltage and energy dissipation than the 807's, 813's,
and 250TH's of the previous era. The resonant tank circuit faded,
amplifiers now feed non-resonant filter networks and expect to see a
50+j0 ohm load ... or else! This made open wire transmission lines much
more difficult to use, requiring some way to adapt a PL259 to connect
them to the radio which is both mechanically and electrically hard, and
sparked the incorporation of "SWR Alarms" into our radios. It also
spawned the "balun," a mysterious device that may be of alien origin
since no one seems to know exactly what it does or how it does it.

I hope this helps. [:-)

73,
Fred ["Skip"] K6DGW
ex KN6DGW 1953
Sparks NV DM09dn
Washoe County

On 4/9/2019 7:21 PM, W2xj wrote:

> I agree except that even at 500KW a 2:1 or greater is the norm with open wire line.
>
> Sent from my iPhone
>
>> On Apr 9, 2019, at 19:10, Fred Jensen <[hidden email]> wrote:
>>
>> Well, the reflected power is created by a "virtual transmitter" at the feedpoint of the antenna and heads down the coax which has a surge impedance of Z0 ohms. It meets the SO-239 at the TX and sees an impedance of Z1, the impedance presented by the PA and output filters. If Z1=Z0, the power is dissipated as heat in the PA and associated RF circuitry. If Z1<>Z0, some is dissipated and some is reflected, where some is radiated, and some is reflected [virtual transmitter again]. Ad infinitum, and when forever is over, it is all gone and everyone lives happily ever after.
>>
>> SWR and all the associated measuring equipment and concern with it only became an issue when: 1) Coax replaced parallel lines and; 2) Resonant output circuits were replaced by solid state amplifiers with non-resonant filters. When I sat for the Extra in early 1956, the only question that involved standing waves was one about how Lecher Lines could be used to measure transmitter frequency.
>>
>> It's important today but calculating it hasn't changed. At 10 W, a 2:1 SWR will probably work ok. At 1500 W, a solid state amplifier may not be happy with the voltages developed at its output connector.
>>
>> 73,
>> Fred ["Skip"] K6DGW
>> Sparks NV DM09dn
>> Washoe County
>>

Bob McGraw - K4TAX

Re: Measuring SWR (Long)

Hummmm.......see: http://www.dj0ip.de/balun-stuff/

I seem to think this fellow has a good grasp about the "alien origin" of
baluns.

73
Bob, K4TAX

On 4/10/2019 12:23 PM, Fred Jensen wrote:
> It also spawned the "balun," a mysterious device that may be of alien
> origin since no one seems to know exactly what it does or how it does it.

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Bob McGraw - K4TAX

Re: Measuring SWR (Long)

In reply to this post by Don Sanders

And for that reason as Roy stated, those components that are rated
"legal limit" are done so under MATCHED conditions. Use a legal limit
component in a condition where SWR is 2:1 or 3:1 or higher and one is
likely to find it becoming a smoke generator at legal limit or even less.

Case and point, the KAT500 ATU is rated at 600 watts, 3 - 30 MHz, 5 oh

On 4/9/2019 7:56 PM, Don Sanders wrote:
> From memory now, at 2 to 1...feedline voltage nodes and current nodes can
> double -- no small thing when power is 1500 watts; can stress antenna
> system
> components.
>
> 73, Roy K6XK

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K8TE

Re: Measuring SWR (Long)

In reply to this post by k6dgw

We have one in NM, however I am not allowed to talk/write about it. No, it
has nothing to do with Roswell.

73, Bill, K8TE

--
Sent from: http://elecraft.365791.n2.nabble.com/
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Vic Rosenthal

Re: Measuring SWR (Long)

In reply to this post by k6dgw

I believe I run the highest SWR of anyone here. It is close to 100:1 in
normal operation according to EZNEC (I have never tried to measure it).

The line is open-wire line made with no. 12 (2mm) wire, only about 10m
long. The highest SWR is on 40 meters, and I calculate the loss as
around 1 dB. I calculated the peak voltage at around 7 kV. You know how
the specs on vacuum relays have to be derated for RF? Believe them.

After using various more or less complicated matching arrangements, I
settled on an old Johnson KW Matchbox. It does the job on all bands from
40 through 10m, with just a little external help on 30m. The antenna is
a simple rotary dipole whose total length is 10m.

73,
Victor, 4X6GP
Rehovot, Israel
Formerly K2VCO
CWops no. 5
http://www.qsl.net/k2vco/
On 10/04/2019 20:23, Fred Jensen wrote:

> Or 10:1 or even 20:1. However ... there's always a "however" or a "but"
> ... it doesn't matter. The dielectric constant of air [the insulator
> between open wire conductors] is very close to 1 whereas it is much
> larger for various forms of coax. Very high SWR creates very high
> voltages along the line. The energy storage with high dielectric
> constants is much higher than with air, and dielectric losses go up
> dramatically. In prehistoric times when we used vacuum tubes and
> resonant tank circuits, we coupled power to the line with a 2-3 turn
> link coil adjacent or in the middle of the resonant tank.
>
> If the line exhibited reactance, it detuned the tank which we just
> retuned to resonance ["Dip the plate, increase the coupling, repeat"].
> The PA tank circuit became a thoroughly mis-named "antenna tuner." It
> didn't matter what the SWR was on the open wire transmission line, and
> in fact no one paid any attention to it. Then flexible coaxial cable
> was invented. It was much more convenient than open wire line, however
> in reasonable physical sizes, it had very low characteristic [surge]
> impedances ... 50 and 75 ohm impedances were the result and now, SWR
> mattered. The dielectric constant inside the cable was very much
> higher, and the higher voltages from high SWR resulted in much higher
> dielectric losses.
>
> Then, someone invented the Pi-network [for all of you with fingers
> poised over the keyboard to pounce on my description of history, relax
> and breath deeply. I'm making some of the non-technical stuff up to
> help hold your attention]. The Pi-network would transform the 50 or 75
> ohm impedance at the end of the coax to the several thousand ohm plate
> circuit impedance of the PA stage and power would flow to the antenna
> unimpeded. It was at this point that sales of SWR indicators soared,
> SWR became a household abbreviation in the ham community, and an SWR of
> 1.000:1 became the Nirvana of ham radio. It has been thus ever since.
>
> As electronics progressed [?] from the vacuous to the solid state, SWR
> took on an enhanced importance since the solid state was far less
> tolerant of overvoltage and energy dissipation than the 807's, 813's,
> and 250TH's of the previous era. The resonant tank circuit faded,
> amplifiers now feed non-resonant filter networks and expect to see a
> 50+j0 ohm load ... or else! This made open wire transmission lines much
> more difficult to use, requiring some way to adapt a PL259 to connect
> them to the radio which is both mechanically and electrically hard, and
> sparked the incorporation of "SWR Alarms" into our radios. It also
> spawned the "balun," a mysterious device that may be of alien origin
> since no one seems to know exactly what it does or how it does it.
>
> I hope this helps. [:-)
>
> 73,
> Fred ["Skip"] K6DGW
> ex KN6DGW 1953
> Sparks NV DM09dn
> Washoe County
>
> On 4/9/2019 7:21 PM, W2xj wrote:
>> I agree except that even at 500KW a 2:1 or greater is the norm with
>> open wire line.
>>
>> Sent from my iPhone
>>
>>> On Apr 9, 2019, at 19:10, Fred Jensen <[hidden email]> wrote:
>>>
>>> Well, the reflected power is created by a "virtual transmitter" at
>>> the feedpoint of the antenna and heads down the coax which has a
>>> surge impedance of Z0 ohms. It meets the SO-239 at the TX and sees
>>> an impedance of Z1, the impedance presented by the PA and output
>>> filters. If Z1=Z0, the power is dissipated as heat in the PA and
>>> associated RF circuitry. If Z1<>Z0, some is dissipated and some is
>>> reflected, where some is radiated, and some is reflected [virtual
>>> transmitter again]. Ad infinitum, and when forever is over, it is
>>> all gone and everyone lives happily ever after.
>>>
>>> SWR and all the associated measuring equipment and concern with it
>>> only became an issue when: 1) Coax replaced parallel lines and; 2)
>>> Resonant output circuits were replaced by solid state amplifiers with
>>> non-resonant filters. When I sat for the Extra in early 1956, the
>>> only question that involved standing waves was one about how Lecher
>>> Lines could be used to measure transmitter frequency.
>>>
>>> It's important today but calculating it hasn't changed. At 10 W, a
>>> 2:1 SWR will probably work ok. At 1500 W, a solid state amplifier
>>> may not be happy with the voltages developed at its output connector.
>>>
>>> 73,
>>> Fred ["Skip"] K6DGW
>>> Sparks NV DM09dn
>>> Washoe County
>>>
>
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k6dgw

Re: Measuring SWR (Long)

In reply to this post by K8TE

What? You mean the guvmint has classified Carlsbad Caverns now?

73,
Fred ["Skip"] K6DGW
Sparks NV DM09dn
Washoe County

On 4/10/2019 11:36 AM, K8TE wrote:
> We have one in NM, however I am not allowed to talk/write about it. No, it
> has nothing to do with Roswell.
>
> 73, Bill, K8TE
>

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k6dgw

Re: Measuring SWR (Long)

In reply to this post by Bob McGraw - K4TAX

It was a feeble attempt at humor, given the nearly countable infinity of
posts here on baluns over the last few years including but not limited
to what they are, what they aren't, how they work, do they even work?,
and do they even exist at all. Perhaps my attempt was more feeble than
I thought. [:-) And, yes I know that "near infinity" is meaningless.

73,
Fred ["Skip"] K6DGW
Sparks NV DM09dn
Washoe County

On 4/10/2019 11:13 AM, Bob McGraw K4TAX wrote:

> Hummmm.......see: http://www.dj0ip.de/balun-stuff/
>
> I seem to think this fellow has a good grasp about the "alien origin"
> of baluns.
>
> 73
> Bob, K4TAX
>
>
> On 4/10/2019 12:23 PM, Fred Jensen wrote:
>> It also spawned the "balun," a mysterious device that may be of alien
>> origin since no one seems to know exactly what it does or how it does
>> it.
>

Richard Corfield

Load Impedance (was Re: Measuring SWR

I'm experimenting with portable antennas at the moment for my KX3 and,
rather than cut the 80m legs of my link dipole to get an antenna that
resonates on 60m, I've experimented with running it off centre. I've found
what looks like resonance at the right frequency range, based on low return
loss.

http://m0rjc.me.uk/screenshot_20190411-132334/

The problem is, that impedance is low. I guess it's being transformed down
by the 10m of feed line, as normal wisdom for an off-centre feed is high
impedance due to the lower currents and higher voltages towards the ends of
a half wave antenna. Considering a velocity factor of 0.66 that feed line
is about 1/4 wavelength long!

Presumably the KXAT-3 will tune this, but do I lose efficiency by having
what would be higher currents in its circuitry?

- Richard

On Wed, 10 Apr 2019 at 22:56, Fred Jensen <[hidden email]> wrote:

> It was a feeble attempt at humor, given the nearly countable infinity of
> posts here on baluns over the last few years including but not limited
> to what they are, what they aren't, how they work, do they even work?,
> and do they even exist at all. Perhaps my attempt was more feeble than
> I thought. [:-) And, yes I know that "near infinity" is meaningless.
>
> 73,
> Fred ["Skip"] K6DGW
> Sparks NV DM09dn
> Washoe County
>
> On 4/10/2019 11:13 AM, Bob McGraw K4TAX wrote:
> > Hummmm.......see: http://www.dj0ip.de/balun-stuff/
> >
> > I seem to think this fellow has a good grasp about the "alien origin"
> > of baluns.
> >
> > 73
> > Bob, K4TAX
> >
> >
> > On 4/10/2019 12:23 PM, Fred Jensen wrote:
> >> It also spawned the "balun," a mysterious device that may be of alien
> >> origin since no one seems to know exactly what it does or how it does
> >> it.
> >
>
> ______________________________________________________________
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