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