"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

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

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

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

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

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

"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

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

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

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