Are all RG8s created equal? (as promised)
 

Owen- see my comments below


The mark of a beginner is blind faith in meters.
Most hams get over believing an "S" meter after a
few years, but very few get to the point of
doubting that SWR or power meter. You ought to try
hooking half a dozen SWR bridges and power meters
in series with your transmission line--all at the
same time. If the line is not pro*perly terminated
you'll probably get as many different readings as
you have meters.

The author is not stating what has happened, what he has observed, but
an opinion on what he thinks may happen.

What happens is IN a HIGH SWR Mis-match, is that
the SWR (true) can only be measured at the load
end. The reflected wave (Say infinity:1 is NOT a
constant, over the entire length of your coax, but
will take on the sinusoidial of the wavelength
for voltage (or current). Obviously, the reflected
wave repeats the voltage, every 1/2 wavelength
(- coax loss's), going from a 1:1 match, to a
infinity to 1 mis-match, every 1/4 wave length
and, then back to a 1:1 match,the next 1/4 wave.
If you were to graph this out for 1 cycle, you
would see a start at 0 volts, raiseing to maximum
volts, then dropping to 0 volts, and then to
Maximum Negative volts.This repeats all the way
from the source to the load! A perfectly
matched coax, on the other hand will have the same
voltage measured , no matter the length of the
line (again ignoreing the line loss's). (straight
line graph, not a sin wave).

Assuming that he is talking about the common reflectometer /
directional wattmeter instruments that sample current and voltage in a
very small region to determine SWR, that they are suited to the
frequency and line, and they are of quality...

The fact remains that on practical lines at HF and above with
practical loads, the SWR decreases smoothly from the load to the
source because of line loss, the lower the loss, the lower the rate at
which SWR falls as you approach the source.

This is academic, as the True VSWR still exists at
the load. If it is 3:1, at the load, but you
measure 1.5/1 at your rig, the difference is
because of line loss, NOT that the 3:1 swr has
been corrected! Worse, (usually on short coax
runs with a bum load, is the effect, that you
have MORE POWER reflected, than was applied to
the coax (an impossible situation)!

Any difference between instruments for direct or indirect measurement
of VSWR placed in a uniform practical line is accounted for by line
loss and instrument error (eg its indication or disruption of the
line). The line loss between adjacent meters in practical coaxial
lines (which is where you are likely to be using an SWR meter) on
practical loads is very low, and so quality instruments should read
almost identically.

To take an extreme example, using RG213 at 1296MHz, and two SWR meters
spaced 0.3m (~1') apart, if the SWR on the line at the one nearest the
source is 3:1, the SWR on the line 0.3m closer to the load is 3.1...
you would probably not see the difference. If on actual test, two
meters showed significant difference, there is instrument error in at
least one of the instruments.

Keep in mind, that WHEN a swr exists, the sin wave
effect also exists irregardless of the freq you
are on. and also, IF you measure at a null point
where the forward power wave voltage cancells the
reflected voltage (for 0 volts), the introduction
of a 1/4 wave length (electrical), will force your
meter to the voltage MAXIMUM ! 'corse, the best
place to measure a mismatch is at the load,but
try it sometime! Try about 13-1/2 inch long
piece of coax, on 2 meters added to your
mismatched load, and then remeasure the swr-
you will see what Im talking about!



SWR bridges and power meters are very valuable
instruments, properly used. End.

Prophetic!

Also, found a article by the same, useing a 400
foot run of RG-58 U for a dummy load, terminated
with couple 2 Watt 100 ohm resistors (truth be
known, at 6 meters, and up you dont even need the

Yes, the input SWR of a 400' o/c stub of RG58 is about 1.1 at 50MHz,
and gets lower with increasing frequency.



Only to say this why your swr APPEARS lower (coax
loss)
but that doesnt effect the REAL swr of the load!
With the 400 foot of coax, the losses are so high,
you wont even see any reflected at the source end-
but at the load end, you will have infinity :1 SWR!
Jim NN7K

Are all RG8s created equal? More
 

Jim - NN7K wrote:
Owen-

Also check out :
http://beradio.com/departments/radio...aves_antennas/

http://www.physics.montana.edu/demon...cherwires.html

http://www.iop.org/EJ/abstract/0031-9120/8/1/002

Tho, both refer to the equivilant of ladder line,
and one is on Lecher Lines, the effect on swr is
the same. The lecher line was the way frequency
was measured before tuned circuit/freq counters
but is based on the phenomina that is described
in measureing swr at various places in a feedline.
It is an observable thing! Jim NN7K

Are all RG8s created equal? (as promised)
 

On Tue, 05 Sep 2006 19:12:38 GMT, Jim - NN7K wrote:

Owen- see my comments below


The mark of a beginner is blind faith in meters.
Most hams get over believing an "S" meter after a
few years, but very few get to the point of
doubting that SWR or power meter. You ought to try
hooking half a dozen SWR bridges and power meters
in series with your transmission line--all at the
same time. If the line is not pro*perly terminated
you'll probably get as many different readings as
you have meters.

The author is not stating what has happened, what he has observed, but
an opinion on what he thinks may happen.

What happens is IN a HIGH SWR Mis-match, is that
the SWR (true) can only be measured at the load

The definition of VSWR is the ratio of a voltage maximum to the
adjacent voltage minimum on the transmission line.

That definition might imply that VSWR cannot be directly measured at a
point, but it is possible to use a directional coupler to measure the
forward and reflected wave components in an very small region (wrt
wavelength) and to calculate the VSWR at that point (for all intents
and purposes).

Using a directional coupler / sampler, VSWR can be measured by this
indirect method anywhere along a transmission line by insertion of the
coupler / sampler.

It is reasonable to talk of VSWR at a point.

end. The reflected wave (Say infinity:1 is NOT a
constant, over the entire length of your coax, but
will take on the sinusoidial of the wavelength
for voltage (or current). Obviously, the reflected

No, you appear to have a misconception.

It is true that the reflected wave component is not constant with
displacement for a lossy line. The amplitude of the reflected wave
component decreases exponentially from the load end towards the source
end, and the rate at which it decreases is given by the line's
"matched line loss". The phase of the reflected wave component also
changes with displacement (distance along the line).

The amplitude of the forward wave component decreases exponentially
from the source end towards the load end, and the rate at which it
decreases is given by the line's "matched line loss". The phase of the
forward wave component also changes with displacement.

The resultant of these two travelling waves is the standing wave
pattern that can be observed with a voltage probe.

You can find the derivation of SWR from those travelling waves in any
reputable text book, I won't repeat it here.

On practical lines at HF and above with practical loads, the SWR
decreases smoothly from the load to the source because of line loss,
the lower the loss, the lower the rate at which SWR falls as you
approach the source.

If you know the matched line loss and VSWR at one point, you can
calculate the VSWR at any other point.

.... clipped

Owen
--

Are all RG8s created equal? (as promised) try again!
 

So: therefore, Lecher Lines can't work?? They
DON'T show the TRUE wavelength of a signal??
So, then, why were these devices used? to
get light to see to read the manual?? Obviously
V/F vs. V/R is EQUAL everywhere on a given line!
which makes the lamp glow at the SAME intensity
everywhere on the line?? Don't think so!
Jim

Owen Duffy wrote:
On Tue, 05 Sep 2006 19:12:38 GMT, Jim - NN7K wrote:

Owen- see my comments below


The definition of VSWR is the ratio of a voltage maximum to the
adjacent voltage minimum on the transmission line.

NO! the defination is V(MAX) forward/ V(MAX)
reverse (at a given point)! That why they use 2
directional couplers (or one that rotates to
change directions , like BIRD)! Like 2 signals,
going in opposite directions, at the same freq,
at the same time!



That definition might imply that VSWR cannot be directly measured at a
point, but it is possible to use a directional coupler to measure the
forward and reflected wave components in an very small region (wrt
wavelength) and to calculate the VSWR at that point (for all intents
and purposes).

This is HOW the Isotron "Antenna" is made to work-
literally loading your coax to radiate!


No, you appear to have a misconception.

It is true that the reflected wave component is not constant with
displacement for a lossy line.

Point being : it ain't even, in a LOSSLESS line!

The amplitude of the reflected wave
component decreases exponentially from the load end towards the source
end, and the rate at which it decreases is given by the line's
"matched line loss". The phase of the reflected wave component also
changes with displacement (distance along the line).

AS does the forward wave, combined with it!

The amplitude of the forward wave component decreases exponentially
from the source end towards the load end, and the rate at which it
decreases is given by the line's "matched line loss". The phase of the
forward wave component also changes with displacement.

The resultant of these two travelling waves is the standing wave
pattern that can be observed with a voltage probe.

Which is why a lecher wire works

But, you will find points that will measure a
great swr, and yet it will be higher than a kite
at the load end! Jim.
Jim

Are all RG8s created equal? (as promised) try again!
 

On Tue, 05 Sep 2006 23:48:41 GMT, Jim - NN7K wrote:

The definition of VSWR is the ratio of a voltage maximum to the
adjacent voltage minimum on the transmission line.

NO! the defination is V(MAX) forward/ V(MAX)
reverse (at a given point)!

Hi Jim,

Are you so quick to impeach your Lecher line testimony from only one
paragraph earlier? Owen's definition is perfectly correct, classic in
fact. It may not be the only one, but historically (as in Lecher
lines) it certainly preceeds yours by decades. However, it does
suffer from accuracy (but that hardly vindicates the common Bruene
circuit). There are at least half a dozen ways to measure SWR, each
one being more appropriate for certain regions of the magnitude.

Some SWRs that are very low or very high would be impossible to
measure using either method described above. Only academics are
concerned with those methods. I've put my hand to most of them. I
wasn't an academic, but NBS standards did demand their use. Some
techniques are infinitely better than either described above (this
presumes Owen's description consisting of a raw detector which
introduces up to 23% error from square law problems).

73's
Richard Clark, KB7QHC

Are all RG8s created equal? (as promised) try again!
 

On Tue, 05 Sep 2006 23:48:41 GMT, Jim - NN7K wrote:

....

The definition of VSWR is the ratio of a voltage maximum to the
adjacent voltage minimum on the transmission line.

NO! the defination is V(MAX) forward/ V(MAX)
reverse (at a given point)! That why they use 2
directional couplers (or one that rotates to
change directions , like BIRD)! Like 2 signals,
going in opposite directions, at the same freq,
at the same time!

No, you are quite confused / wrong. VSWR is *NOT* V(MAX) forward/
V(MAX) reverse (at a given point).

VSWR=|Vmax|/|Vmin|

VSWR=(|Vf|+|Vr|)/(|Vf|-|Vr|)

You seem to be mixing up the forward and reflected wave components,
and the resultant SWR pattern. You need to go back to the text books
and work it through.

....

Owen
--

Are all RG8s created equal? (as promised)
 

Owen Duffy wrote:
Using a directional coupler / sampler, VSWR can be measured by this
indirect method anywhere along a transmission line by insertion of the
coupler / sampler.

In fact, if one measures the forward power, Pfor, and the
reflected power, Pref, at a point on a transmission line,

SWR = [SQRT(Pfor) + SQRT(Pref)] / [SQRT(Pfor) - SQRT(Pref)]
--
73, Cecil http://www.w5dxp.com

Are all RG8s created equal? (as promised) try again!
 

Jim - NN7K wrote:

Owen Duffy wrote:
The definition of VSWR is the ratio of a voltage maximum to the
adjacent voltage minimum on the transmission line.

NO! the defination is V(MAX) forward/ V(MAX)
reverse (at a given point)!

If V(MAX) forward = 100V and V(MAX) reverse = 0V, then
the SWR is infinite?

If V(MAX) forward = 100V and V(MAX) reverse = 100V, then
the SWR is 1:1?

Doesn't that sound exactly backwards to you?
--
73, Cecil http://www.w5dxp.com

Are all RG8s created equal? (as promised)
 

Quite true- the point I'm tryint to make is, that
when makeing swr measurements, that the APPARENT
measured swr, need have little/no relation to what
the true swr , depending on Where the swr is
measured in a line. at least with cheap meters.
and it really gets disconserting, when you
calibrate a SWR meter, (forward power , to the
set line, and then measure a reflected swr that is
Beyond the set line! Admittedly, much of that
is from too big a sampleing line, but it is
beyond the ability to have MORE power returned
to the source, than the source provided (perpetual
motion machine)! And, that you can prune a feed
line and lower your swr (measured), but that
doesn't change the load's mis-match, does it?
And that lines (1/4 wave, shorted, or 1/2 wave
open) form a very high impedence. Translation:
whatever is on that end of that line will repeat (ZL)
every 1/2 wavelength, will it not ? And , the
opposite would also be true (1/4 wave back, and
in odd multiples thereof, would this not show
a dead short (electrically), in a connected line?
Where am I wrong ? just curious -Jim

Cecil Moore wrote:
Owen Duffy wrote:
Using a directional coupler / sampler, VSWR can be measured by this
indirect method anywhere along a transmission line by insertion of the
coupler / sampler.

In fact, if one measures the forward power, Pfor, and the
reflected power, Pref, at a point on a transmission line,

SWR = [SQRT(Pfor) + SQRT(Pref)] / [SQRT(Pfor) - SQRT(Pref)]

Are all RG8s created equal? (as promised)
 

Jim - NN7K wrote:
... but it is
beyond the ability to have MORE power returned
to the source, than the source provided

Did you know a reflection coefficient can be greater
than 1.0?

that you can prune a feed
line and lower your swr (measured), but that doesn't change the load's
mis-match, does it?

If the Z0 of the feedline is different from the Z0
of the SWR meter, then the meter is not measuring
the SWR on the feedline.

And that lines (1/4 wave, shorted, or 1/2 wave open) form a very high
impedence.

The (virtual) impedance is (Vfor+Vref)/(Ifor-Iref)

The closer that Iref gets to Ifor, the lower the stub
losses and the higher the (virtual) impedance.
--
73, Cecil http://www.w5dxp.com