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

"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

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

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 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.

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
...
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.

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