"Compound" SWR question
 

I am trying to learn more about antennas and transmission lines and am
currently reading through the transmission line chapter of a recent ARRL
Handbook.

There is a calculated example for a load with a certain SWR connected to a
transmission line with certain losses and the author states:

"Because of losses in a transmission line, the measured SWR at the input of
the line is less than the SWR measured at the load end of the line."



My first reaction was Oh my God, I am soo proud of my decent SWR at the
input of my antenna feedline and in reality I could have a really bad
mismatch between the antenna and the transmission line and wouldn't even
know about it.

And my second thought was what is happening to the energy, since an SWR
gives me an indication of how much power reaches the antenna and how much is
either radiated from the feedline or lost as heat and here I have two
different SWR in the system, one at the antenna and one for the
antenna/feedline system.

I think it can't be that I would loose a whole lot at the antenna (high SWR)
but then wouldn't loose all that much for the system since the overall SWR
is lower.


Where do I go wrong??

Uwe

Jaggy Taggy wrote:
My first reaction was Oh my God, I am soo proud of my decent SWR at the
input of my antenna feedline and in reality I could have a really bad
mismatch between the antenna and the transmission line and wouldn't even
know about it.

Yep, 200 ft. of RG-58 used on 440 MHz will result in an SWR close
to 1:1 whether the antenna is attached or not. :-)

And my second thought was what is happening to the energy, since an SWR
gives me an indication of how much power reaches the antenna and how much is
either radiated from the feedline or lost as heat and here I have two
different SWR in the system, one at the antenna and one for the
antenna/feedline system.

The RF can indeed be lost as heat in the transmission line. The
maximum current in a feedline depends upon the SWR. The losses
in the feedline depend upon the current. The maximum voltage
in a feedline depends upon the SWR. The losses in a feedline
depend upon the voltage. At HF, the current losses tend to
dominate.
--
73, Cecil http://www.qsl.net/w5dxp


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Jaggy Taggy wrote:
. . .
And my second thought was what is happening to the energy, since an SWR
gives me an indication of how much power reaches the antenna

It does not.

and how much is
either radiated from the feedline or lost as heat

It does not.

and here I have two
different SWR in the system, one at the antenna and one for the
antenna/feedline system.

I think it can't be that I would loose a whole lot at the antenna (high SWR)

The SWR has nothing to do with how much power is being lost.

but then wouldn't loose all that much for the system since the overall SWR
is lower.

The SWR has nothing to do with how much is lost.

Where do I go wrong??

In all the above. You need to go back and read again just what SWR is
and what it means.

Roy Lewallen, W7EL

I strongly suggest that you invest in "Reflections II" by Walt Maxwell, W2DU
and sleep with it a few nights.

No offense, but the tone of your questions and comments indicates that you
are working with a number of highly erroneous misconceptions. The sooner you
iron out these kinks, the better you will understand and enjoy your
antennas.

Walt is working on "Reflections III" but it may be this Fall before it
publishes, so the current edition is more than adequate.

However, don't feel bad - the notion of SWR (its causes and effects) is
probably one of the most widely misunderstood concepts in amateur radio.
Walt can help a lot . . .

--
73, George W5YR
Fairview, TX

http://www.w5yr.com


"Jaggy Taggy" wrote in message
...
I am trying to learn more about antennas and transmission lines and am
currently reading through the transmission line chapter of a recent ARRL
Handbook.

There is a calculated example for a load with a certain SWR connected to a
transmission line with certain losses and the author states:

"Because of losses in a transmission line, the measured SWR at the input
of
the line is less than the SWR measured at the load end of the line."



My first reaction was Oh my God, I am soo proud of my decent SWR at the
input of my antenna feedline and in reality I could have a really bad
mismatch between the antenna and the transmission line and wouldn't even
know about it.

And my second thought was what is happening to the energy, since an SWR
gives me an indication of how much power reaches the antenna and how much
is
either radiated from the feedline or lost as heat and here I have two
different SWR in the system, one at the antenna and one for the
antenna/feedline system.

I think it can't be that I would loose a whole lot at the antenna (high
SWR)
but then wouldn't loose all that much for the system since the overall SWR
is lower.


Where do I go wrong??

Uwe

"Jaggy Taggy" wrote in message
...
I am trying to learn more about antennas and transmission lines and am
currently reading through the transmission line chapter of a recent ARRL
Handbook.

There is a calculated example for a load with a certain SWR connected to a
transmission line with certain losses and the author states:

"Because of losses in a transmission line, the measured SWR at the input
of
the line is less than the SWR measured at the load end of the line."



My first reaction was Oh my God, I am soo proud of my decent SWR at the
input of my antenna feedline and in reality I could have a really bad
mismatch between the antenna and the transmission line and wouldn't even
know about it.

And my second thought was what is happening to the energy, since an SWR
gives me an indication of how much power reaches the antenna and how much
is
either radiated from the feedline or lost as heat and here I have two
different SWR in the system, one at the antenna and one for the
antenna/feedline system.

I think it can't be that I would loose a whole lot at the antenna (high
SWR)
but then wouldn't loose all that much for the system since the overall SWR
is lower.


Where do I go wrong??

Uwe
If you post the following, somebody will tell you what the antenna SWR is:

1) Length of coax
2) Type of coax
3) SWR you measure
4) Frequency

The power that gets attenuated is used up to heat the coax. This can be
useful. For instance, I am using 100+ feet of RG58 as a 50 MHz 100W
attenuator.

TYam/WB2TT

Hello George, don't worry, you and the others who pointed out my
misconceptions are not offending me, they are indeed helping.

I started to read the article you recommended, or at least a version of it
available on the ARRL website.

The difficulties I encounter are partially (you mentioned it and Maxwell
mentions this problem in his 'Reflections' as well) because of the
widespread misconceptions about transmission line behavior where everybody
has an opinion, but it seems many of them are wrong. So one has to find
trustworthy sources.

Also, and maybe you know of a source for this, as I gain theoretical
insights I like to strengthen my understanding through measurements and
experiments, a "lab companion book" would be great.

Other than that I will shut up for a while and read.

Best regards from Maine

Uwe
KB1JOW





On 1/31/05 1:56 AM, in article
, "George, W5YR"
wrote:

I strongly suggest that you invest in "Reflections II" by Walt Maxwell, W2DU
and sleep with it a few nights.

No offense, but the tone of your questions and comments indicates that you
are working with a number of highly erroneous misconceptions. The sooner you
iron out these kinks, the better you will understand and enjoy your
antennas.

Walt is working on "Reflections III" but it may be this Fall before it
publishes, so the current edition is more than adequate.

However, don't feel bad - the notion of SWR (its causes and effects) is
probably one of the most widely misunderstood concepts in amateur radio.
Walt can help a lot . . .

On Thu, 03 Feb 2005 02:45:35 GMT, Jaggy Taggy
wrote:



Also, and maybe you know of a source for this, as I gain theoretical
insights I like to strengthen my understanding through measurements and
experiments, a "lab companion book" would be great.

Maxwell has a number of his "Reflections II" chapters up at his web
site. See http://home.iag.net/~w2du/

Bob
k5qwg



Other than that I will shut up for a while and read.

Best regards from Maine

Uwe
KB1JOW





On 1/31/05 1:56 AM, in article
, "George, W5YR"
wrote:

I strongly suggest that you invest in "Reflections II" by Walt Maxwell, W2DU
and sleep with it a few nights.

No offense, but the tone of your questions and comments indicates that you
are working with a number of highly erroneous misconceptions. The sooner you
iron out these kinks, the better you will understand and enjoy your
antennas.

Walt is working on "Reflections III" but it may be this Fall before it
publishes, so the current edition is more than adequate.

However, don't feel bad - the notion of SWR (its causes and effects) is
probably one of the most widely misunderstood concepts in amateur radio.
Walt can help a lot . . .

You can trust the _ARRL Antenna Book_ and Walt Maxwell's
_Reflections_(*). There are many good textbooks on the topic, but most
get too involved for a beginner(**). You're wise to be leery of sources
other than the _Antenna Book_ and _Reflections_, or real textbooks. And
web pages should undergo even more critical scrutiny. I see some pretty
bad misconceptions popping up in the works of some of the most prolific
and, among the amateur community most respected, authors.

(*) No work is perfect. But I know both Walt, and Dean Straw, the
_Antenna Book_ editor, personally, and both know the topic very well. At
least as important is that both are absolutely committed to conveying
accurate information, and do their very best to verify the accuracy of
their work and correct any errors which are found.

(**) One you might consider is _Transmission Lines, Antennas, and Wave
Guides_, by King, Mimno, and Wing. It's well written, and a paperback
edition was printed which you can often find at a reasonably low price.
The paperback edition is by Dover, published in 1965.

Roy Lewallen, W7EL

Jaggy Taggy wrote:
Hello George, don't worry, you and the others who pointed out my
misconceptions are not offending me, they are indeed helping.

I started to read the article you recommended, or at least a version of it
available on the ARRL website.

The difficulties I encounter are partially (you mentioned it and Maxwell
mentions this problem in his 'Reflections' as well) because of the
widespread misconceptions about transmission line behavior where everybody
has an opinion, but it seems many of them are wrong. So one has to find
trustworthy sources.

Also, and maybe you know of a source for this, as I gain theoretical
insights I like to strengthen my understanding through measurements and
experiments, a "lab companion book" would be great.

Other than that I will shut up for a while and read.

Best regards from Maine

Uwe
KB1JOW