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

"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".

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

"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

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.

"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.

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

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

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

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