SWR variation with feedline length
 

Along several decades of radio hamming on the HF bands, I noted that the
measured SWR of all the antennas I have mounted (Yagis, dipoles) slightly varies
when the feedline length is changed by several meters. For 100W of forward
power, the reflected power could vary somewhat, e.g. from 2W to 5W or so,
measured on a Bird wattmeter. This behavior would seeem to deny the theory,
according to which SWR is independent of feedline length (as long as the cable
attenuation remains constant).

Clearly the measured SWR change cannot be due to the change in the feedline
attenuation as, at HF, adding or cutting a few meters of cable would yield a
very small change in attenuation and hence a negligible impact on measured SWR.

Reading here and there, the most common theory explaining such phenomenon is
that, in presence of RF on the coaxial cable braid, the SWR meter reading is
influenced by the feedline length. I am not too convinced of that explanation,
also because I have invariably experienced the measured SWR variation phenomenon
with all antenna I have had, and I never had hot braid problems.

At that regard I got an idea that could explain the phenomenon, at least part of
it.

Reading coaxial cable data sheet, I noted that manufacturers typically give a
small tolerance on cable impedance (2 to 3 ohm). Let us then assume that the
feedline cable has a 53-ohm impedance, whilst the Bird wattmeter is 50 ohm
sharp.

If the 53-ohm cable is terminated on an e.g. 75-ohm (purely resistive) antenna,
the real SWR on the line would be 75/53=1.41 independently of feedline length
(if the attenuation variation with length is neglected). But the impedance seen
by the wattmeter obviously varies with the feedline length, and it can be easily
calculated that the seen impedance range results in an apparent SWR, on the
50-ohm wattneter, reading that varies from a maximum of 1.5 (when feedline
length is an even multiple of half wavelenght) down to a minimum of 1.33 (when
feedline length is an odd multiple of wavelenght quarters). For 100W of forward
power, the reflected power varies from about 4W down to about 2W.

Repeating the exercise with an e.g. 85-ohm load, the apparent SWR measured on
the 50-ohm wattmeter would vary from 1.7 down to 1.51 (reflected power varying
from 7W down to 4W).

You can get easily convinced that such variation is only due to the assumed
3-ohm difference in cable impedance.

With older cables having a nominal 52-ohm impedance, instead of 50, the
situation could get even more evident.

Any comment would be appreciated.

73

Tony I0JX

SWR variation with feedline length
 

"Antonio Vernucci" wrote in message
. ..
Along several decades of radio hamming on the HF bands, I noted that the
measured SWR of all the antennas I have mounted (Yagis, dipoles) slightly
varies
when the feedline length is changed by several meters. For 100W of forward
power, the reflected power could vary somewhat, e.g. from 2W to 5W or so,
measured on a Bird wattmeter. This behavior would seeem to deny the
theory,
according to which SWR is independent of feedline length (as long as the
cable
attenuation remains constant).

Clearly the measured SWR change cannot be due to the change in the
feedline
attenuation as, at HF, adding or cutting a few meters of cable would yield
a
very small change in attenuation and hence a negligible impact on measured
SWR.

Reading here and there, the most common theory explaining such phenomenon
is
that, in presence of RF on the coaxial cable braid, the SWR meter reading
is
influenced by the feedline length. I am not too convinced of that
explanation,
also because I have invariably experienced the measured SWR variation
phenomenon
with all antenna I have had, and I never had hot braid problems.

At that regard I got an idea that could explain the phenomenon, at least
part of
it.

Reading coaxial cable data sheet, I noted that manufacturers typically
give a
small tolerance on cable impedance (2 to 3 ohm). Let us then assume that
the
feedline cable has a 53-ohm impedance, whilst the Bird wattmeter is 50 ohm
sharp.

If the 53-ohm cable is terminated on an e.g. 75-ohm (purely resistive)
antenna,
the real SWR on the line would be 75/53=1.41 independently of feedline
length
(if the attenuation variation with length is neglected). But the impedance
seen
by the wattmeter obviously varies with the feedline length, and it can be
easily
calculated that the seen impedance range results in an apparent SWR, on
the
50-ohm wattneter, reading that varies from a maximum of 1.5 (when feedline
length is an even multiple of half wavelenght) down to a minimum of 1.33
(when
feedline length is an odd multiple of wavelenght quarters). For 100W of
forward
power, the reflected power varies from about 4W down to about 2W.

Repeating the exercise with an e.g. 85-ohm load, the apparent SWR measured
on
the 50-ohm wattmeter would vary from 1.7 down to 1.51 (reflected power
varying
from 7W down to 4W).

You can get easily convinced that such variation is only due to the
assumed
3-ohm difference in cable impedance.

With older cables having a nominal 52-ohm impedance, instead of 50, the
situation could get even more evident.

Any comment would be appreciated.

73

Tony I0JX

The Bird actually measures a combination of capacitive coupled voltage and
inductively coupled current. There is a app note on the Bird website.
Find: "Straight Talk About Directivity".

SWR variation with feedline length
 

Antonio Vernucci wrote:
I noted that the
measured SWR of all the antennas I have mounted (Yagis, dipoles)
slightly varies when the feedline length is changed by several meters.

If the characteristic impedance of the feedline differs from
the characteristic impedance of the calibrated SWR meter,
the indicated SWR will vary with length of feedline.

If the feedline is lossy, the transmitted signal SWR will
decrease between the antenna and the transmitter. All
feedlines have a certain amount of loss.

If common-mode current is present on the SWR meter case,
the SWR reading will vary because the meter has no fixed
ground reference.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

SWR variation with feedline length
 

It's important to know and keep in mind that the SWR meter doesn't
actually measure the SWR on the feedline. So its reading doesn't prove
or disprove anything about how the SWR on a feedline changes with length.

What the meter effectively measures is the impedance seen at that point.
It's calibrated in such a way that if it's connected to a transmission
line of exactly 50 ohms impedance, the indicated SWR will be the SWR on
the line. Otherwise, the indicated SWR won't be the actual line SWR.

There are at least three things which can cause an indicated SWR
variation with line length:

1. The feedline Z0 isn't exactly 50 ohms. The Z0 of coax easily varies
+/- 5 ohms from nominal, and sometimes closer to +/- 10 -- it's seldom
exactly 50. If you connect a perfect 50 ohm load to your transmitter via
a 45 ohm line, the impedance seen by the transmitter will change with
line length. Consequently, the SWR meter reading will also change. The
actual SWR on the line will not, except as dictated by loss, described next.

2. The feedline has loss. The SWR will improve as the line becomes
longer due to line loss. If the line is long enough to be very lossy,
the transmitter will see nearly the line's Z0 regardless of what load is
connected to the other end. The actual SWR on the line will be greatest
at the load, decreasing as you get farther away.

3. There is current on the outside of the coax shield (common mode
current). When this happens, the feedline becomes part of the antenna.
Consequently, changing the feedline length actually changes the
effective antenna length, which in turn changes the feedpoint impedance.

Roy Lewallen, W7EL

Antonio Vernucci wrote:
Along several decades of radio hamming on the HF bands, I noted that the
measured SWR of all the antennas I have mounted (Yagis, dipoles)
slightly varies when the feedline length is changed by several meters.
For 100W of forward power, the reflected power could vary somewhat, e.g.
from 2W to 5W or so, measured on a Bird wattmeter. This behavior would
seeem to deny the theory, according to which SWR is independent of
feedline length (as long as the cable attenuation remains constant).

Clearly the measured SWR change cannot be due to the change in the
feedline attenuation as, at HF, adding or cutting a few meters of cable
would yield a very small change in attenuation and hence a negligible
impact on measured SWR.

Reading here and there, the most common theory explaining such
phenomenon is that, in presence of RF on the coaxial cable braid, the
SWR meter reading is influenced by the feedline length. I am not too
convinced of that explanation, also because I have invariably
experienced the measured SWR variation phenomenon with all antenna I
have had, and I never had hot braid problems.

At that regard I got an idea that could explain the phenomenon, at least
part of it.

Reading coaxial cable data sheet, I noted that manufacturers typically
give a small tolerance on cable impedance (2 to 3 ohm). Let us then
assume that the feedline cable has a 53-ohm impedance, whilst the Bird
wattmeter is 50 ohm sharp.

If the 53-ohm cable is terminated on an e.g. 75-ohm (purely resistive)
antenna, the real SWR on the line would be 75/53=1.41 independently of
feedline length (if the attenuation variation with length is neglected).
But the impedance seen by the wattmeter obviously varies with the
feedline length, and it can be easily calculated that the seen impedance
range results in an apparent SWR, on the 50-ohm wattneter, reading that
varies from a maximum of 1.5 (when feedline length is an even multiple
of half wavelenght) down to a minimum of 1.33 (when feedline length is
an odd multiple of wavelenght quarters). For 100W of forward power, the
reflected power varies from about 4W down to about 2W.

Repeating the exercise with an e.g. 85-ohm load, the apparent SWR
measured on the 50-ohm wattmeter would vary from 1.7 down to 1.51
(reflected power varying from 7W down to 4W).

You can get easily convinced that such variation is only due to the
assumed 3-ohm difference in cable impedance.

With older cables having a nominal 52-ohm impedance, instead of 50, the
situation could get even more evident.

Any comment would be appreciated.

73

Tony I0JX

SWR variation with feedline length
 

On Mon, 1 Jun 2009 22:50:52 +0200, "Antonio Vernucci"
wrote:

Along several decades of radio hamming on the HF bands, I noted that the
measured SWR of all the antennas I have mounted (Yagis, dipoles) slightly varies
when the feedline length is changed by several meters. For 100W of forward
power, the reflected power could vary somewhat, e.g. from 2W to 5W or so,
measured on a Bird wattmeter. This behavior would seeem to deny the theory,
according to which SWR is independent of feedline length (as long as the cable
attenuation remains constant).

Hi Antonio,

Yes, this is the accepted wisdom.

Clearly the measured SWR change cannot be due to the change in the feedline
attenuation as, at HF, adding or cutting a few meters of cable would yield a
very small change in attenuation and hence a negligible impact on measured SWR.

A good point.

Reading here and there, the most common theory explaining such phenomenon is
that, in presence of RF on the coaxial cable braid, the SWR meter reading is
influenced by the feedline length.

This, too, is accepted wisdom.

I am not too convinced of that explanation,
also because I have invariably experienced the measured SWR variation phenomenon
with all antenna I have had, and I never had hot braid problems.

You ARE describing a hot braid problem. At the slight shift of 3W out
of 100W, it is a small problem by the same degree (nothing you would
notice by other indications).

At that regard I got an idea that could explain the phenomenon, at least part of
it.

Well, I am going to skip that quote to cut to the chase. What you
describe is called mismatch uncertainty. It exists in a cable that
has a nominal Z that matches neither the load nor the source.
Depending upon the amount of mismatch at each end, you have a zone of
confusion between those ends that will result in as many different
readings as you have insertion points to measure at. As most modern
transmitters have a source Z of 30 to 70 Ohms, you might note a very,
very small perturbation when the other end of ANY line is mismatched -
but I doubt it. To provoke this condition into revealing readings
that are significantly beyond the range of error requires mismatches
at both ends on the order of 3:1. You don't describe that.

More likely your problem is Common Mode currents - what you call hot
braid. One test is to use a snap-on choke and slide it along the line
and note if the SWR meter reading moves in concert with the hand
motion (or the reading simply shifts by the addition of the choke).

73's
Richard Clark, KB7QHC

SWR variation with feedline length
 

On 1 jun, 22:50, "Antonio Vernucci" wrote:
Along several decades of radio hamming on the HF bands, I noted that the
measured SWR of all the antennas I have mounted (Yagis, dipoles) slightly varies
when the feedline length is changed by several meters. For 100W of forward
power, the reflected power could vary somewhat, e.g. from 2W to 5W or so,
measured on a Bird wattmeter. This behavior would seeem to deny the theory,
according to which *SWR is independent of feedline length (as long as the cable
attenuation remains constant).

Clearly the measured SWR change cannot be due to the change in the feedline
attenuation as, at HF, adding or cutting a few meters of cable would yield a
very small change in attenuation and hence a negligible impact on measured SWR.

Reading here and there, the most common theory explaining such phenomenon is
that, in presence of RF on the coaxial cable braid, the SWR meter reading is
influenced by the feedline length. I am not too convinced of that explanation,
also because I have invariably experienced the measured SWR variation phenomenon
with all antenna I have had, and I never had hot braid problems.

At that regard I got an idea that could explain the phenomenon, at least part of
it.

Reading coaxial cable data sheet, I noted that manufacturers typically give a
small tolerance on cable impedance (2 to 3 ohm). Let us then assume that the
feedline cable has a 53-ohm impedance, whilst the Bird wattmeter is 50 ohm
sharp.

If the 53-ohm cable is terminated on an e.g. 75-ohm (purely resistive) antenna,
the real SWR on the line would be 75/53=1.41 independently of feedline length
(if the attenuation variation with length is neglected). But the impedance seen
by the wattmeter obviously varies with the feedline length, and it can be easily
calculated that the seen impedance range results in an apparent SWR, on the
50-ohm wattneter, reading that varies from a maximum of 1.5 (when feedline
length is an even multiple of half wavelenght) down to a minimum of 1.33 (when
feedline length is an odd multiple of wavelenght quarters). For 100W of forward
power, the reflected power varies from about 4W down to about 2W.

Repeating the exercise with an e.g. 85-ohm load, the apparent SWR measured on
the 50-ohm wattmeter would vary from 1.7 down to 1.51 (reflected power varying
from 7W down to 4W).

You can get easily convinced that such variation is only due to the assumed
3-ohm difference in cable impedance.

With older cables having a nominal 52-ohm impedance, instead of 50, the
situation could get even more evident.

Any comment would be appreciated.

73

Tony I0JX

Hello Antonio,

In my opinion (when dealing with actual antennas) it can be:

1. your coaxial cable is part of the antenna (common mode current).
Changing the length, changes the common mode impedance. You can rule
this out by sliding some large ferrites along the cable close to the
VSWR meter, or change the grounding a bit and watch the difference (if
present).

2. your bridge inside the VSWR meter is not perfect. You can check
this by connecting known impedances (for example 56 Ohms resistor and
a 44.6 Ohms resistor and 100 ohms versus 25 Ohms).

3. The cables you are using are not exactly 50 Ohms. I think your
analysis is right. When you have cable with slightly different Z0,
readings depend on length. Of course when you extend with a good 50
ohms cable (directly connected to the meter), the reading should not
change. I did the math also and found also VSWR=1.7 and VSWR=1.51 for
85 ohms load connected to cable with Z0=53 Ohms. I didn't expect such
difference for just 3 ohms deviation from 50 Ohm.

4. Harmonics in the final amplifier (I hope that is not the reason).

Best regards,

Wim
PA3DJS
www.tetech.nl
remove abc in case of pm

SWR variation with feedline length
 

"Antonio Vernucci" wrote in
:

Along several decades of radio hamming on the HF bands, I noted that
the measured SWR of all the antennas I have mounted (Yagis, dipoles)
slightly varies when the feedline length is changed by several meters.
For 100W of forward power, the reflected power could vary somewhat,
e.g. from 2W to 5W or so, measured on a Bird wattmeter. This behavior
would seeem to deny the theory, according to which SWR is independent
of feedline length (as long as the cable attenuation remains
constant).

Tony,

You may find my thoughts at
http://www.vk1od.net/transmissionlin...splacement.htm of interest.

Owen

SWR variation with feedline length
 

On Jun 1, 1:50*pm, "Antonio Vernucci" wrote:
Along several decades of radio hamming on the HF bands, I noted that the
measured SWR of all the antennas I have mounted (Yagis, dipoles) slightly varies
when the feedline length is changed by several meters. For 100W of forward
power, the reflected power could vary somewhat, e.g. from 2W to 5W or so,
measured on a Bird wattmeter. This behavior would seeem to deny the theory,
according to which *SWR is independent of feedline length (as long as the cable
attenuation remains constant).

Clearly the measured SWR change cannot be due to the change in the feedline
attenuation as, at HF, adding or cutting a few meters of cable would yield a
very small change in attenuation and hence a negligible impact on measured SWR.

Reading here and there, the most common theory explaining such phenomenon is
that, in presence of RF on the coaxial cable braid, the SWR meter reading is
influenced by the feedline length. I am not too convinced of that explanation,
also because I have invariably experienced the measured SWR variation phenomenon
with all antenna I have had, and I never had hot braid problems.

At that regard I got an idea that could explain the phenomenon, at least part of
it.

Reading coaxial cable data sheet, I noted that manufacturers typically give a
small tolerance on cable impedance (2 to 3 ohm). Let us then assume that the
feedline cable has a 53-ohm impedance, whilst the Bird wattmeter is 50 ohm
sharp.

If the 53-ohm cable is terminated on an e.g. 75-ohm (purely resistive) antenna,
the real SWR on the line would be 75/53=1.41 independently of feedline length
(if the attenuation variation with length is neglected). But the impedance seen
by the wattmeter obviously varies with the feedline length, and it can be easily
calculated that the seen impedance range results in an apparent SWR, on the
50-ohm wattneter, reading that varies from a maximum of 1.5 (when feedline
length is an even multiple of half wavelenght) down to a minimum of 1.33 (when
feedline length is an odd multiple of wavelenght quarters). For 100W of forward
power, the reflected power varies from about 4W down to about 2W.

Repeating the exercise with an e.g. 85-ohm load, the apparent SWR measured on
the 50-ohm wattmeter would vary from 1.7 down to 1.51 (reflected power varying
from 7W down to 4W).

You can get easily convinced that such variation is only due to the assumed
3-ohm difference in cable impedance.

With older cables having a nominal 52-ohm impedance, instead of 50, the
situation could get even more evident.

Any comment would be appreciated.

73

Tony I0JX

There is a very good possibility that your analysis is correct. I see
the same effect, and in fact, it's of particular concern to me right
now, because I'm putting what effectively is an SWR meter into
production, and it's important that we have a test setup that
accurately measures the performance. I've been specifically concerned
that the test setup, as currently configured, may have trouble because
the connecting cables may not be close enough to 50 ohms.

As others have said, IF there is a problem with RF on the outside of
the line, any variation in observed SWR is most likely because the
change in line length has changed the load the other end of the coax
is seeing, NOT because the meter is directly responding to the
"outside" RF. The meter measures transmission line current and
transmission line voltage, and the line itself is all the reference it
needs to do that. Direct response to RF on the outside of the line
could result from poor construction of the meter, but I wouldn't
expect that from a Bird.

One other possibility that I haven't seen mentioned, too, is that the
impedance of the line is not constant along its length. With line of
good construction that hasn't been abused, the variation should be
small. You can detect it by running a network analyzer sweep of just
the line, across a broad frequency range. But a line with
polyethylene dielectric (and especially one with foam polyethylene)
that's gotten too hot--perhaps because of high power at high SWR--can
have the center conductor go "off-center" and change the impedance.

If the effect you are seeing is the result of a line that's not quite
the same impedance that the meter is calibrated to (which itself may
be noticably different from 50 ohms), you could plot the change in
indicated SWR as a function of line length and see it vary in a smooth
and predictable manner. Most likely, though, what you're seeing is
the sum of several effects, and the variation in indicated SWR or
reflected power may not be all that smooth. Very often when I expect
to see a nice smooth spiral centered on one point on my network
analyzer's Smith chart display, what I see is a spiral that follows
along some arc, because of various imperfections. (Sometimes it's fun
to try to figure out just what the imperfections are...)

Cheers,
Tom

SWR variation with feedline length
 

It's important to know and keep in mind that the SWR meter doesn't actually
measure the SWR on the feedline. So its reading doesn't prove or disprove
anything about how the SWR on a feedline changes with length.

Hi, Roy

yes, that is the reason why I was talking of "apparent" or "measured" SWR,
whilst the real SWR does not vary with line length.

1. The feedline Z0 isn't exactly 50 ohms. The Z0 of coax easily varies +/- 5
ohms from nominal, and sometimes closer to +/- 10 -- it's seldom exactly 50.
If you connect a perfect 50 ohm load to your transmitter via a 45 ohm line,
the impedance seen by the transmitter will change with line length.
Consequently, the SWR meter reading will also change. The actual SWR on the
line will not, except as dictated by loss, described next.

2. The feedline has loss. The SWR will improve as the line becomes longer due
to line loss. If the line is long enough to be very lossy, the transmitter
will see nearly the line's Z0 regardless of what load is connected to the
other end. The actual SWR on the line will be greatest at the load, decreasing
as you get farther away.

3. There is current on the outside of the coax shield (common mode current).
When this happens, the feedline becomes part of the antenna. Consequently,
changing the feedline length actually changes the effective antenna length,
which in turn changes the feedpoint impedance.

I would say that in case no. 1 the meter measures an apparent SWR, whilst in
case no. 2 it measures the real SWR existing at the measurement point. I am not
sure what it measures in case no. 3

Regards.

Tony I0JX

SWR variation with feedline length
 

If the characteristic impedance of the feedline differs from
the characteristic impedance of the calibrated SWR meter,
the indicated SWR will vary with length of feedline.

If the feedline is lossy, the transmitted signal SWR will
decrease between the antenna and the transmitter. All
feedlines have a certain amount of loss.

If common-mode current is present on the SWR meter case,
the SWR reading will vary because the meter has no fixed
ground reference.


Hi Cecil.

in the first case the meter measures an apparent SWR, whilst in the second case
it measures the real SWR (occuring at the measurement point).

For the third case, I am unable to figure out whether the meter reads an
apparent or the real SWR.

73

Tony I0JX

SWR variation with feedline length
 

The Bird actually measures a combination of capacitive coupled voltage and
inductively coupled current. There is a app note on the Bird website.
Find: "Straight Talk About Directivity".


Thanks for pinpointing that nice document.

However the document dwells on the directivity of the meter, and I could not
find there any mention of the impedance tolerance issue I had raised.


73

Tony I0JX

SWR variation with feedline length
 

"Antonio Vernucci" wrote in
:

The Bird actually measures a combination of capacitive coupled
voltage and inductively coupled current. There is a app note on the
Bird website. Find: "Straight Talk About Directivity".


Thanks for pinpointing that nice document.

However the document dwells on the directivity of the meter, and I
could not find there any mention of the impedance tolerance issue I
had raised.

I wrote some notes on the operation of the Bruene type VSWR meter at
http://www.vk1od.net/transmissionlin.../VSWRMeter.htm . The Bird 43 is
not a Bruene type meter, but a similar derivation could be done, and for
all practical purposes, the explanation applies.

Keep in mind that the usual practice of calibrating a VSWR meter is to
adjust it for nil reflected indication with a load of 50+j0. Without
arguing the tolerances implications, its indicated VSWR can only be
applied exactly to an adjacent low loss line with Zo=50+j0... which you
do not have, so you must expect some error in the measurements.

If you read the article I gave earlier, you will see the plots of VSWR
along a specification RG58C/U line with a 50+j0 +/-0% load. Those are the
indications you would expect of a *perfectly* calibrated Bird 43 on
RG58C/U exactly meeting the specification from which the RLGC parameters
were derived.

These are very small effects, but they exist.

Layer on top of that cable tolerance and you have more variation.

But, if you are making measurements using an antenna as a load, I
respectfully submit you probably are not in a sound position to assert
that there is zero common mode current effect.

As a brain teaser, think of the situation in which rho, the magnitude of
the voltage reflection coefficient Gamma could be greater than 1. Of
course many think they have proven that cannot happen my citing
measurements made with a Bird 43... but can it capture what is happening?

Owen

SWR variation with feedline length
 

"Antonio Vernucci" wrote in message
. ..
Along several decades of radio hamming on the HF bands, I noted that the
measured SWR of all the antennas I have mounted (Yagis, dipoles) slightly
varies when the feedline length is changed by several meters. For 100W of
forward power, the reflected power could vary somewhat, e.g. from 2W to 5W
or so, measured on a Bird wattmeter. This behavior would seeem to deny the
theory, according to which SWR is independent of feedline length (as long
as the cable attenuation remains constant).


Surely this sounds about right for a Bird 43. Assuming a directivity for the
meter of 30dB and an swr of 1.4:1 this would give a possible indicated
return loss of between about 13 to 16dB depending on the relative phases of
the forward and reflected signals. So moving the meter would give an
indicated reflected power anywhere in the 2 to 5W range.

73
Jeff

SWR variation with feedline length
 

Antonio Vernucci wrote:

1. The feedline Z0 isn't exactly 50 ohms. The Z0 of coax easily varies
+/- 5 ohms from nominal, and sometimes closer to +/- 10 -- it's seldom
exactly 50. If you connect a perfect 50 ohm load to your transmitter
via a 45 ohm line, the impedance seen by the transmitter will change
with line length. Consequently, the SWR meter reading will also
change. The actual SWR on the line will not, except as dictated by
loss, described next.

2. The feedline has loss. The SWR will improve as the line becomes
longer due to line loss. If the line is long enough to be very lossy,
the transmitter will see nearly the line's Z0 regardless of what load
is connected to the other end. The actual SWR on the line will be
greatest at the load, decreasing as you get farther away.

3. There is current on the outside of the coax shield (common mode
current). When this happens, the feedline becomes part of the antenna.
Consequently, changing the feedline length actually changes the
effective antenna length, which in turn changes the feedpoint impedance.

I would say that in case no. 1 the meter measures an apparent SWR,
whilst in case no. 2 it measures the real SWR existing at the
measurement point. I am not sure what it measures in case no. 3

No, the meter is measuring the exact same thing in all cases. it always
(indirectly) measures the SWR within itself, which is directly related
to the impedance connected to the output end of the meter. It never
measures the SWR on any transmission line outside itself. The three
cases only explain reasons the impedance connected to the output of the
meter -- hence the meter reading -- changes as the transmission line
length is changed.

Roy Lewallen, W7EL

SWR variation with feedline length
 

Again, the typical SWR meters we use do *not* measure the SWR on any
transmission line, except (indirectly) the SWR on the short line within
the instrument -- if it even contains such a line, which some don't. I
don't know of any way to directly measure the SWR on an intact coaxial
line, only on a slotted line.

The SWR on a coax line can readily be calculated, however, from
measurements it is possible to make.

Roy Lewallen, W7EL

SWR variation with feedline length
 

On 2 jun, 07:58, "Antonio Vernucci" wrote:
The Bird actually measures a combination of capacitive coupled voltage and
inductively coupled current. *There is a app note on the Bird website..
Find: *"Straight Talk About Directivity".

Thanks for pinpointing that nice document.

However the document dwells on the directivity of the meter, and I could not
find there any mention of the impedance tolerance issue I had raised.

73

Tony I0JX

Hello Tony,

If you think cable impedance deviation from 50 Ohms is the problem in
your situation, why not making a resistor bridge type of VSWR meter
(wheatstone bridge)?

By changing the resistor that goes from the source to the output
connector, you can change the reference impedance of the resistor
bridge. For HF and VHF, accuracy is very good when using surface
mount components.

One example is given he
www.w1ghz.org/QEX/A_UHF+_VSWR_Bridge.pdf

Best regards,

Wim
PA3DJS
www.tetech.nl
don't forget to remove first three letters of alphabet in case of PM.

SWR variation with feedline length
 

Roy Lewallen wrote:
It never
measures the SWR on any transmission line outside itself.

Another way of saying it is that the Bird directional
wattmeter Thruline establishes a Z0=~50 ohm environment
within the instrument itself.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

SWR variation with feedline length
 

"Antonio Vernucci" wrote in message
...
The Bird actually measures a combination of capacitive coupled voltage
and
inductively coupled current. There is a app note on the Bird website.
Find: "Straight Talk About Directivity".


Thanks for pinpointing that nice document.

However the document dwells on the directivity of the meter, and I could
not
find there any mention of the impedance tolerance issue I had raised.


73

Tony I0JX

The Voltage indication is non directive while the current is directive.
Voltage varies because of impedance and the reactivity causes phase
differences resulting in complex voltages. You should see variations in
both forward and reflected indications.

Functionally, the Bird is best used to minimize whatever reverse indication
without worrying over the accuracy of the derived measurement.

SWR variation with feedline length
 

On Tue, 02 Jun 2009 01:59:43 -0700, Roy Lewallen
wrote:

Again, the typical SWR meters we use do *not* measure the SWR on any
transmission line, except (indirectly) the SWR on the short line within
the instrument -- if it even contains such a line, which some don't. I
don't know of any way to directly measure the SWR on an intact coaxial
line, only on a slotted line.

The SWR on a coax line can readily be calculated, however, from
measurements it is possible to make.

Roy Lewallen, W7EL

The possibility of common-mode current on the outside of the braid has
been mentioned, but nothing has been mentioned concerning whether a
balun is used if the feedline-antenna connections is unbal to bal. If
there is no balun where should be one, seems to me it's a no-brainer
that the problem is common-mode current causing the different SWR
readings with different lengths of feedline.

Walt, W2DU

SWR variation with feedline length
 

The possibility of common-mode current on the outside of the braid has
been mentioned, but nothing has been mentioned concerning whether a
balun is used if the feedline-antenna connections is unbal to bal. If
there is no balun where should be one, seems to me it's a no-brainer
that the problem is common-mode current causing the different SWR
readings with different lengths of feedline.

Walt, W2DU

Hi Walt,

I have always used a good balun on all my antennas, and therefore I am not too
convinced that, in my case, the SWR change I observe when adding (or removing) a
piece of coax in my station could be due to RF presence on the coax braid.

Anyway, I have not yet read a clear and convincing explanation of why the
presence of RF on the coax braid would cause the SWR meter to give a different
reading when moving it along the line.

I appreciate that, with a hot braid, the coax cable becomes part of the antenna
and then radiates, but I cannot clearly focus why this can cause the SWR meter
to see different impedances at different points of the line. Impedance is the
ratio between RF voltage (between center conductor and braid) and (differential
mode) RF current. So, I do not well visualize how the presence of a common mode
RF current can influence the meter reading.

73

Tony I0JX

SWR variation with feedline length
 

Antonio Vernucci wrote:

Hi Walt,

I have always used a good balun on all my antennas, and therefore I am
not too convinced that, in my case, the SWR change I observe when adding
(or removing) a piece of coax in my station could be due to RF presence
on the coax braid.

Anyway, I have not yet read a clear and convincing explanation of why
the presence of RF on the coax braid would cause the SWR meter to give a
different reading when moving it along the line.

I appreciate that, with a hot braid, the coax cable becomes part of the
antenna and then radiates, but I cannot clearly focus why this can cause
the SWR meter to see different impedances at different points of the
line. Impedance is the ratio between RF voltage (between center
conductor and braid) and (differential mode) RF current. So, I do not
well visualize how the presence of a common mode RF current can
influence the meter reading.

73

Tony I0JX

If you keep the same total line length and move your meter to different
points along the line, current on the outside of the line won't change
the indicated SWR except to the extent that the SWR meter is poorly
shielded and affected by the current. However, I believe the problem was
stated in such a way that the SWR was seen to change as line was added
or removed, that is, as the line length changed. And that could change
the actual line SWR for the reason you described.

Roy Lewallen, W7EL

SWR variation with feedline length
 

"Antonio Vernucci" wrote in
:

....
I have always used a good balun on all my antennas, and therefore I am
not too convinced that, in my case, the SWR change I observe when
adding (or removing) a piece of coax in my station could be due to RF
presence on the coax braid.

Perhaps you should set aside your confidence that there is no common mode
current, and do some experiments with shielded loads and coax lines.

For example, you could parallel two 50 ohm loads at a T piece, and
measure the VSWR into the branch line at different lengths and locations.
Try it also with some sections of 75 ohm line if you have some available.

Come back and describe the experiments and results if they do not behave
as expected.

If you get info fine detail about the effect of a VSWR meter calibrated
for 50+j0 on a line that is not exactly 50+j0 (eg practical lines), my
notes at http://www.vk1od.net/transmissionlin...splacement.htm
might help you to understand what may be happening.

When you have convinced yourself about the soundness of the broad
statement that for most practical cases, VSWR decreases smoothly from the
load to the source, and the decrease is accounted for exactly by the line
loss.


Anyway, I have not yet read a clear and convincing explanation of why
the presence of RF on the coax braid would cause the SWR meter to give
a different reading when moving it along the line.

If you maintain the common mode current path exactly, and move the SWR
meter to a different location, the VSWR should be seen to decrease
smoothly from the load to the source, and the decrease is accounted for
exactly by the line loss.

If you change the common mode current path in any way, you may cause a
different load impedance at the far end of the coax, and that will affect
the VSWR.


I appreciate that, with a hot braid, the coax cable becomes part of
the antenna and then radiates, but I cannot clearly focus why this can
cause the SWR meter to see different impedances at different points of
the line. Impedance is the ratio between RF voltage (between center

If the VSWR1, then the impedance varies all along the line.

The VSWR meter doesn't directly read the impedance, but it shows the
VSWR, and although the impedance may vary radically, the VSWR decreases
smoothly from the load to the source, and the decrease is accounted for
exactly by the line loss. (I think I have said that before.)

conductor and braid) and (differential mode) RF current. So, I do not
well visualize how the presence of a common mode RF current can
influence the meter reading.

See above and my notes.

But, while you are fixated on the belief that you have zero common mode
current, what you report cannot be explained by conventional transmission
line theory.

Owen

PS: "most practical cases" means the VSWR as indicated on an instrument
calibrated for 50+j0 and used on low loss nominal 50 ohm line at HF or
above.

SWR variation with feedline length
 

On 9 jun, 22:37, "Antonio Vernucci" wrote:
The possibility of common-mode current on the outside of the braid has
been mentioned, but nothing has been mentioned concerning whether a
balun is used if the feedline-antenna connections is unbal to bal. If
there is no balun where should be one, seems to me it's a no-brainer
that the problem is common-mode current causing the different SWR
readings with different lengths of feedline.

Walt, W2DU

Hi Walt,

I have always used a good balun on all my antennas, and therefore I am not too
convinced that, in my case, the SWR change I observe when adding (or removing) a
piece of coax in my station could be due to RF presence on the coax braid.

Anyway, I have not yet read a clear and convincing explanation of why the
presence of RF on the coax braid would cause the SWR meter to give a different
reading when moving it along the line.

I appreciate that, with a hot braid, the coax cable becomes part of the antenna
and then radiates, but I cannot clearly focus why this can cause the SWR meter
to see different impedances at different points of the line. Impedance is the
ratio between RF voltage (between center conductor and braid) and (differential
mode) RF current. So, I do not well visualize how the presence of a common mode
RF current can influence the meter reading.

73

Tony I0JX

Hello Antonio,

Do you have access to a 2m or 70cm set? If so, take a piece of 50 Ohms
coaxial cable (of about 1m). Plug it onto your set (or a separate VSWR
meter) and add a quarter wave to the other end. Do nothing with the
braid at the other end.

Now select low power setting. Measure the VSWR while you move you hand
along the cable. You will notice change in VSWR for sure.

The reason for this is that the QW antenna requires a good image. The
image impedance is in series with the radiator impedance (simple point
of view). This image is the cable, VSWR meter and set. When you change
this, you will notice change in VSWR. The easiest way to change the
image/ground/counterpoise is capacitively with your hand.

Of course you may assume that you don't have common mode current
(because of a balun), but best is to turn such an assumption into a
fact.

Best regards,

Wim
PA3DJS
www.tetech.nl
please remove the obvious letters in the PM