On Tue, 28 Jun 2005 15:39:57 -0700, Frank Gilliland
wrote:

The energy in a coax travels on the conductors -and- in the dielectric
-and- within the magnetic fields. The propogation delay of a line is
the combined phase delays of distributed capacitance -and- distributed
inductance in the line. The dielectric constant only -seems- to be the
determining factor of coax propogation delay because the conductors
are straight. IOW, if you replace the center conductor with a coil you
will introduce an additional propogation delay into the coax which is
-independent- of the dielectric constant (and will have constructed a
device known to us old farts as a 'helical resonantor'). Regardless,
it has no relevance to this discussion.
*****

Well the dielectric constant does have a direct effect on the
capacitance as well as the spacing between the two conductors. Still
the TEM wave propogates through the dielectric and induces currents in
the center and outer conductor. Propogation of a TEM wave can be
mathematically describe by the Pyonting Vector. The TEM wave is an
alternating E and H field.

The currents induced into the conductors have depth only to that of
sigma or the skin depth. I am not sure a coiled center conductor would
introduce anymore delays than a solid or even stranded center
conductor. On face evidence it would seem that it might but only if
the coil's turns per inch were suffieciently low enough as to not
appear to the traveling wave as a solid conductor.

james

if you have over a 2:1 standing wave you can do damage to your finals
or linear

Oh brother. Here we go again with more nonsense and myths about SWR!

73 Tom

On Tue, 28 Jun 2005 15:39:57 -0700, Frank Gilliland
wrote:

One of the most misunderstood terms in radio is "common-mode current".
It simply means that current is moving in the same direction, and in
phase, on two or more conductors. It occurs in a coax when current on
the -inside- of the shield is in phase with the current on the center
conductor. Any RF current on the -outside- of a coax has -nothing- to
do with common-mode currents -- it's simply the result of RF spilling
out of the coax or being induced onto it from an external field.
*******

Yes misunderstood.

I have yet to really see any coax of decent quality that has
suffiecient gaps in the shield to allow a 27 Mhz wave to have
appreciable leakage. Even with 80% coverage the holes in the shield
are so little of a wavelength that I would dare say less than 1/10,000
of the energy of the TEM wave propogating down the coax can "leak"
out.

As for common mode currents the coax itself can have induced currents
in the chield from fields radiated from the antenna. Depending on
where the coax is located to a conducting surface, you can develope
various intensity of currents. Yes you need two conductors minimum to
have comnmon mode. Earth can be one of those conductors.

On Tue, 28 Jun 2005 15:39:57 -0700, Frank Gilliland
wrote:

Often common mode currents are also rich in
harmonic energy and that is what reradiates and cause TVI and
interference.


Hogwash. Harmonics don't just appear because of common-mode currents.
They must come from a source -- i.e, the transmitter. And conductors
of common-mode currents don't have any magical properties that let
them conduct or radiate harmonics any better than the fundamental
frequency. That's RF voodoo.
*****

Harmonics are not there due to just common mode. My mistake there.

What I was thinking and what I wrote were not well alligned.

To generate harmonics off a the shield of the coax from an external
induced current, one needs a means of rectification. That can come
from two dissimilar metals that are not properly electrically
connected. Then the shield can become a radiator of externally induced
currents. It is the diode effect of two dissimilar metals that is the
source of harmonics.

james

Reg Edwards wrote:

Roy, to cut things short, why don't you just say SWR meters don't
measure SWR on anything.

He did, he just felt compelled to add a page or so of explaination.

All they do is indicate whether or not the
transmitter is terminated with its correct load resistance. So they
are quite useful.

Well, to be picky all they do is indicate the divergence from the Zo of
the meter, in a certain way. If the TX is designed for that Zo then
they do what you say, and they're quite useful.

They won't even tell you what the load resistance actually is unless
the load is exactly correct.

Yes

Stop fooling and confusing yourselves. The solution to everybody's
problems is simple - just change the name of the thing to TLI.
(Transmitter Loading Indicator).

Hmm. I'll stick with SWR meter -- which it is if it's used properly.

--
-------------------------------------------
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Buck wrote:

I believe he is right. Radios drop power when they don't like the SWR
and raise it when it does.

That depends on the radio.

Without SWR protection one with a class C final, like the ones in the
little QRP rigs I have lying around, will deliver more power if it sees
a lower RF impedance at the final transistor than it was designed for.
It will also overheat said final transistor* and possibly damage it.
This will happen for some SWR mismatches but not all.

Again without SWR protection one with a class AB or B final, properly
tuned for a 50 ohm resistive load, will deliver less power for some
mismatches (the same mismatches that would be _higher_ power for the
class C final). With other mismatches it would exhibit higher gain but
more distortion. At some mismatch and level of drive you could probably
expose the finals to too much power dissipation, too high an RF current
or too high an RF voltage, and do damage. This depends _heavily_ on the
design of the final stage.

With SWR protection, of course, the radio will automatically back off,
perhaps even in a way that will do some good.

So I will use an SWR meter to keep the transmitter happy, and a field
strength meter to make sure my antenna is doing it's job.

* assuming I got the heatsink design right, neither overly conservative
nor overly optimistic.

-------------------------------------------
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

james wrote:
Lancer wrote:
So thats all my tuner does, lengthen or shorten the coax?

Are you sure about that?

Essentially yes. Without having to go into detailed mathematics, it is
the simplest form to explain what is happening.

If you include the possibility of changing the Z0 of the
coax as well as the length, you will be closer to the truth.
--
73, Cecil http://www.qsl.net/w5dxp


----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups
---= East/West-Coast Server Farms - Total Privacy via Encryption =---

Buck wrote:

I believe he is right. Radios drop power when they don't like the SWR
and raise it when it does.

This is an illustration of a common problem. It's really improper
*impedances* presented to the radio that disturb it; it doesn't know or
care about the actual SWR on whatever transmission line may or may not
be connected. Transmitters typically specify and show this load
impedance as "SWR". But they can't tell the difference between a half
wavelength 50 ohm line with 100 ohm load, which has a line SWR of 2:1;
any length of 100 ohm line with a 100 ohm load, which has a line SWR of
1:1; a quarter wavelength of 300 ohm line with a 900 ohm load, which has
a line SWR of 3:1; or a 100 ohm resistor. All these and an infinite
number of other combinations will present 100 ohms to the rig, all will
cause the rig's SWR meter to read 2:1, and all will have exactly the
same effect.

Roy Lewallen, W7EL

On Tue, 28 Jun 2005 21:01:52 -0700, "Steveo"
wrote:

if you have over a 2:1 standing wave you can do damage to your finals
or linear

*****

Only if you are exceeding the power dissapation of the devices.
Considering the way most CBers use radios and amplifiers, your
statement maybe more ture than false.

Generaly if the power disapation of the finals is not exceeded and
there is sufficient margin to handle the reflected power, it will just
dissapate as heat in the output circuits and the fianls. Then this is
only true if the reflection coefficient of the radio is zero. If other
than zero then there will be some of the antenna power reflected back
to the transmitter reflected back to the load. Then the rest is
dissapated as heat. Then you get all kinds of funny things happening
inside the coax. But that is another subject.

james


"Frank Gilliland" wrote in message
.. .
On 28 Jun 2005 17:51:10 -0700, "K7ITM" wrote in
. com:

snip
But there's a real problem in communicating this. If you hook a 50
ohm
SWR meter to the input of a 75 ohm, 300 ohm, or line of any
impedance
other than 50 ohms, the meter reading won't be the SWR on the
transmission line. That can mislead people into thinking that the
SWR is
changing with line length when it actually isn't.


In addition, most hams (and other non-professionals -- and even many
professionals) don't bother to check that their SWR meter is
properly
calibrated to the impedance they think it is. Most are nominally 50
ohms, but they can be built for any practical line impedance.
Checking
calibration is not all that difficult, if you take the time to do
it.
In addition, your nominally 50 ohm line (or 75 or whatever) can have
an
actual impedance 10% or more from the nominal value. If you have
properly calibrated your meter to 50 ohms, and your line is 60 ohms,
you would read 1.2:1 SWR when your line is actually 1:1. And if the
SWR on the 60 ohm line is 1.2:1, that 50 ohm SWR meter can read
anything between 1:1 and 1.44:1, depending on the line length and
its
load. Finally, though you may have checked that the meter to reads
1:1
with a 50 ohm load and infinity to 1 with a short or open load, the
construction of inexpensive meters may cause them to have
significant
errors at other load impedances.


Impedance matching of an SWR meter is generally unimportant since
most
SWR meters used for HF have a directional coupler that is much
shorter
than the operating wavelength. Regardless, I'm not a big fan of SWR
meters -- they are good for detecting a major malfunction but that's
about it. Antenna tuning/matching is best done with a field strength
meter.







----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet
News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World!
120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via
Encryption =----

On Wed, 29 Jun 2005 11:07:17 -0400, "Fred W4JLE"
wrote:

What is the reason a 2:1 SWR can cause such havoc?

How can I avoid this catastrophic condition?

I feed my dipoles with 450 Ohm ladder line, but the last 20 feet or so is 50
Ohm coax, I guess that makes it work ok. I haven't blown up my finals yet.

Lions and tigers and bears Oh my...
*****

Actually can happen if you push the finals to where there is
insufficeint margin to the maximum heat dissapation. Tubes are a bit
more forgiving. Transistor inadequately heatsinked and overdriven,
typical CB usage, often have little of no margin for heat dissapation.

If the transmitter has a refelction coefficient of zero and the load
say .3, then that reflected power from the load is dissapated as heat
in the output circuits and any final transistors or tubes. Now if the
radio has a reflection coefficient other than zero that will lessen
the heat dissapation on the transimiiter. Now you get load and source
reflections convoluting within the transmission line.

You ought to model a 400 Mhz square wave with source and load
refelctions coefficients other than zero. It can get ugly


james

On Tue, 28 Jun 2005 22:29:55 GMT, Lancer wrote:

Thanks James;
I don't think my post was a smart ass post. Nor do I think I have
ever made a smart ass post directed at you. I posted what I learned
from the guys in the antenna group. It seems that you know all the
answers, no point in me continuing this conversation with you...

Lance
*****

No I don't know all the answers. Never claimed to.

Nor did I intend the post to be such or a smart ass post. If you took
it that way I apologize for it was not intended to be that.

Just that current in a inner conductor and shield of the coax is
induced by the TEM (Transverse ElectroMagnetic) wave that travels from
the source to the load in the space between the two conductors. That
is a hard concept for many to understand. But once that is understood
then many of the mysteries of transmission lines becomes rather
simple. All to often many think that the signal travels down the
inside and outside conductors. It does not.

james

On Tue, 28 Jun 2005 15:40:40 -0700, Frank Gilliland
wrote:

I think you have that a little misconstrued..... reflection of the
load to the transmitter by a half-wavelength coax is equal to the
-load- regardless of the characteristic impedance of the -coax-.

****

You may be right there. Don't have all my library restored here at my
new place and having to remember.

And Lancer was right, RF on the shield at the feedpoint -will- change
the input impedance of the coax because the shield is no longer
grounded, which is a necessary condition for proper operation of the
coax.
********

1) The impeadance of the coax never changes unless its dielectrtic, or
ratio of outer diameter to inner diamter changes.

2) the reflection coefficient of the load does not change unless there
is a physical change in the load.

3) currents on the shield from other sources will not change the
impedance of the load reflected towards the source.

4) currents on the shield may alter what the source sees as a load
impedance. It may well be possible that the source could see the
antenna and some other load in paralell and then vectorally add the
two impedances.

james

On Wed, 29 Jun 2005 16:17:33 GMT, james wrote
in :

On Tue, 28 Jun 2005 15:39:57 -0700, Frank Gilliland
wrote:

The energy in a coax travels on the conductors -and- in the dielectric
-and- within the magnetic fields. The propogation delay of a line is
the combined phase delays of distributed capacitance -and- distributed
inductance in the line. The dielectric constant only -seems- to be the
determining factor of coax propogation delay because the conductors
are straight. IOW, if you replace the center conductor with a coil you
will introduce an additional propogation delay into the coax which is
-independent- of the dielectric constant (and will have constructed a
device known to us old farts as a 'helical resonantor'). Regardless,
it has no relevance to this discussion.
*****

Well the dielectric constant does have a direct effect on the
capacitance as well as the spacing between the two conductors. Still
the TEM wave propogates through the dielectric and induces currents in
the center and outer conductor. Propogation of a TEM wave can be
mathematically describe by the Pyonting Vector. The TEM wave is an
alternating E and H field.


Well, let's put it this way: the radio and antenna don't connect to
the dielectric of a coax.


The currents induced into the conductors have depth only to that of
sigma or the skin depth. I am not sure a coiled center conductor would
introduce anymore delays than a solid or even stranded center
conductor. On face evidence it would seem that it might but only if
the coil's turns per inch were suffieciently low enough as to not
appear to the traveling wave as a solid conductor.


I'm sure you have studied the lumped-constant equivalent of a
transmission line.....






----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----

On Tue, 28 Jun 2005 23:17:15 -0500, Cecil Moore
wrote in :

Frank Gilliland wrote:
Impedance matching of an SWR meter is generally unimportant since most
SWR meters used for HF have a directional coupler that is much shorter
than the operating wavelength.

Point is that they are usually calibrated for Z0=50 ohms
and are in error when used in Z0 environments differing
from Z0=50 ohms, e.g. Z0=75 ohms.


The point is that the error is insignificant when the directional
coupler is much shorter than the wavelength. The error is even more
insignificant when there are a host of variables and confounds between
the SWR meter and the transmitted field that can (and frequently do)
affect the objective -- field strength. It's much simpler (and just
plain logical) to measure the field strength directly instead of
measuring an abstract value halfway towards the objective and relying
on nothing more than speculation that the rest is working according as
expected.





----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----

On Wed, 29 Jun 2005 20:53:42 GMT, james wrote
in :

On Tue, 28 Jun 2005 15:40:40 -0700, Frank Gilliland
wrote:

I think you have that a little misconstrued..... reflection of the
load to the transmitter by a half-wavelength coax is equal to the
-load- regardless of the characteristic impedance of the -coax-.

****

You may be right there. Don't have all my library restored here at my
new place and having to remember.

And Lancer was right, RF on the shield at the feedpoint -will- change
the input impedance of the coax because the shield is no longer
grounded, which is a necessary condition for proper operation of the
coax.
********

1) The impeadance of the coax never changes unless its dielectrtic, or
ratio of outer diameter to inner diamter changes.

2) the reflection coefficient of the load does not change unless there
is a physical change in the load.

3) currents on the shield from other sources will not change the
impedance of the load reflected towards the source.

4) currents on the shield may alter what the source sees as a load
impedance. It may well be possible that the source could see the
antenna and some other load in paralell and then vectorally add the
two impedances.


And #4 is exactly why #1 is incorrect: the 'characteristic' impedance
of a coax is constant, but it's 'input' impedance varies according to
load mismatch at the other end. If it wasn't for this fact, a tuner at
the radio end would be useless. But the point here is that if the SWR
meter is left floating with the coax shield (both of which should be
RF grounded) then the measurement can be darn near anything.






----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----

"Frank Gilliland" wrote in message
...
On Tue, 28 Jun 2005 23:17:15 -0500, Cecil Moore
wrote in :

Frank Gilliland wrote:
Impedance matching of an SWR meter is generally unimportant since most
SWR meters used for HF have a directional coupler that is much shorter
than the operating wavelength.

Point is that they are usually calibrated for Z0=50 ohms
and are in error when used in Z0 environments differing
from Z0=50 ohms, e.g. Z0=75 ohms.


The point is that the error is insignificant when the directional
coupler is much shorter than the wavelength.

It is the directional coupler that is balanced for a particular value of Z0.

Tam/WB2TT


The error is even more
insignificant when there are a host of variables and confounds between
the SWR meter and the transmitted field that can (and frequently do)
affect the objective -- field strength. It's much simpler (and just
plain logical) to measure the field strength directly instead of
measuring an abstract value halfway towards the objective and relying
on nothing more than speculation that the rest is working according as
expected.





----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet
News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+
Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption
=----

Frank Gilliland wrote:
Point is that they are usually calibrated for Z0=50 ohms
and are in error when used in Z0 environments differing
from Z0=50 ohms, e.g. Z0=75 ohms.

The point is that the error is insignificant when the directional
coupler is much shorter than the wavelength.

Nope, that's not the point at all. It is true that a 50 ohm
SWR meter designed for HF may not work on 70 cm but the error
I'm talking about is the calibration error in a 50 ohm SWR meter
designed for HF and used on HF in, for instance, a Z0 = 450 ohm
environment instead of its calibrated-for 50 ohm environment. It
works perfectly in a 50 ohm environment at the HF frequency of
operation. Here's the proof using a 50 ohm SWR meter:

XMTR--1/2WL 450 ohm line--SWR meter--1/2WL 450 ohm line--50 ohm load

The 50 ohm SWR meter will read 1:1, nowhere near the actual SWR

XMTR--1/4WL 450 ohm line--SWR meter--1/4WL 450 ohm line--50 ohm load

The 50 ohm SWR meter will read 81:1, nowhere near the actual SWR

An SWR meter calibrated for 450 ohms will correctly read 9:1
in both cases.
--
73, Cecil http://www.qsl.net/w5dxp


----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups
---= East/West-Coast Server Farms - Total Privacy via Encryption =---

james wrote:
If the transmitter has a refelction coefficient of zero and the load
say .3, then that reflected power from the load is dissapated as heat
in the output circuits and any final transistors or tubes.

Sometimes yes, sometimes no. If the reflected current arrives out
of phase with the forward current, then the final dissipation can
actually be *reduced* by the mismatch.
--
73, Cecil http://www.qsl.net/w5dxp


----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups
---= East/West-Coast Server Farms - Total Privacy via Encryption =---

On Tue, 28 Jun 2005 16:18:56 -0700, Frank Gilliland
wrote:

Yep. And I should add that 18' of coax is recommended not because of
it's propogation characteristics -inside- the coax, but because of
it's velocity factor on the -outside- of the shield which is nearly 1.
IOW, when the shield of an 18' length of coax is grounded only at one
end, that ground will be reflected at the other end of the coax. At
least that's the theory. In practical use it's not perfect, but it's
still better than a fully ungrounded radio or antenna
*****

your reasoning does not pass the common sense rule.

if 18 feet is special because the velocity factor outside the shield
is nearly 1, then no other length has a velocity factor of nearly 1?
What would cause the outside velocity factor to change?

Beisdes I really don't care to rehash this topic to much more. Been
there, done that and don't care to review it right now.

james

On Wed, 29 Jun 2005 16:42:49 -0500, Cecil Moore
wrote:

Sometimes yes, sometimes no. If the reflected current arrives out
of phase with the forward current, then the final dissipation can
actually be *reduced* by the mismatch.
*****

Power is power. Phase is not a problem. Take the mafnitude of the
transmitted power and teh magnitude of the reflected power. The
results are phaseless. The magnitudes add linearly.

QED

james

On Wed, 29 Jun 2005 17:28:01 -0400, "Tam/WB2TT"
wrote:

It is the directional coupler that is balanced for a particular value of Z0.

Tam/WB2TT
*****

Correct

james

Frank Gilliland wrote, among other things, "The point is that the error
is insignificant when the directional coupler is much shorter than the
wavelength."

Certainly "directional couplers" for HF may be built at essentially
zero length, and ideally would have exactly zero length, monitoring the
current and voltage at a single point on a line. Then SWR or
reflection coefficient magnitude or even complex reflection coefficient
may be calculated under the assumption we know the desired reference
impedance. But if the equipment combines the voltage and current
samples in the wrong ratio, you will get the WRONG answer. Even if the
coupler looks like a perfect 50 ohms impedance section of transmission
line (with some attenuation), the error _in_measurement_output_ can be
significant indeed. Just because the coupler looks like a 50 ohm line
to the line it's hooked to doesn't mean it will read zero reflection
when IT's presented with a 50 ohm load.

And by the way, not everyone who measures and cares very much about SWR
(or reflection coefficient) cares a whit about field strength. Not all
loads are antennas.

Indeed, as Reg says, we might do better in amateur applications to
consider the SWR meter as an indicator of the degree to which we're
presenting a transmitter with the desired load. That's really what
we're using it for, most of the time. It may ALSO be interesting to
know the field strength, but please be aware that a transmitter's
distortion products may be significantly higher if it's presented the
wrong load impedance, even though the power output may be increased.
Field strength alone is not acceptable to me as a means to adjust an
antenna load to a transmitter, or as a way to adjust the operating
point of the transmitter.

Cheers,
Tom

james wrote:
On Wed, 29 Jun 2005 11:07:17 -0400, "Fred W4JLE"
wrote:


What is the reason a 2:1 SWR can cause such havoc?

How can I avoid this catastrophic condition?

I feed my dipoles with 450 Ohm ladder line, but the last 20 feet or so is 50
Ohm coax, I guess that makes it work ok. I haven't blown up my finals yet.

Lions and tigers and bears Oh my...

*****

Actually can happen if you push the finals to where there is
insufficeint margin to the maximum heat dissapation. Tubes are a bit
more forgiving. Transistor inadequately heatsinked and overdriven,
typical CB usage, often have little of no margin for heat dissapation.

If the transmitter has a refelction coefficient of zero and the load
say .3, then that reflected power from the load is dissapated as heat
in the output circuits and any final transistors or tubes. Now if the
radio has a reflection coefficient other than zero that will lessen
the heat dissapation on the transimiiter. Now you get load and source
reflections convoluting within the transmission line.

You ought to model a 400 Mhz square wave with source and load
refelctions coefficients other than zero. It can get ugly


james



Consider the MRF 140, a 150 Watt 2.0 - 150.0 Mhz N-Channel
linear RF power fet. From the technical data sheet: "100% Tested
For Load Mismatch At All Phase Angles With 30:1 VSWR." You'd have
a tough time damaging this device with a mere 2:1 VSWR.
How do load and source reflections convolute within the
transmission line? That's a new one on me. My old dictionary
defines 'convolute' as "Rolled or folded together with one part
over another; twisted; coiled." The rest of the post is pretty
fanciful, too. A trip to the library would do wonders.
73,
Tom Donaly, KA6RUH

james wrote:

On Wed, 29 Jun 2005 16:42:49 -0500, Cecil Moore
wrote:


Sometimes yes, sometimes no. If the reflected current arrives out
of phase with the forward current, then the final dissipation can
actually be *reduced* by the mismatch.

*****

Power is power. Phase is not a problem. Take the mafnitude of the
transmitted power and teh magnitude of the reflected power. The
results are phaseless. The magnitudes add linearly.

QED

james


Cecil was talking about current, not power. You can't add
power the way you can voltage and current. If you could, you
could build a very nice perpetual motion machine just by using the
reflections in a transmission line to add power so that the output
was greater than the input.
73,
Tom Donaly, KA6RUH

The rig has no way of detecting any alleged "reflected power". It can't
tell the difference between a feedline with a lot of "reflected power",
a feedline with no "reflected power", and a plain resistor. It behaves
exactly the same in all cases, provided only that the impedance that
each provides to it is the same.

Anyone not convinced of this should put a couple or more dummy loads in
series or parallel, make up a few lengths of transmission line of
various impedances, and see for himself.

Roy Lewallen, W7EL

james wrote:
On Wed, 29 Jun 2005 16:42:49 -0500, Cecil Moore
wrote:


Sometimes yes, sometimes no. If the reflected current arrives out
of phase with the forward current, then the final dissipation can
actually be *reduced* by the mismatch.

*****

Power is power. Phase is not a problem. Take the mafnitude of the
transmitted power and teh magnitude of the reflected power. The
results are phaseless. The magnitudes add linearly.

QED

james

On Wed, 29 Jun 2005 13:56:53 -0700, Frank Gilliland
wrote:

I'm sure you have studied the lumped-constant equivalent of a
transmission line.....
****

And Maxwell's equations

james

On Wed, 29 Jun 2005 14:24:44 -0700, Frank Gilliland
wrote:

And #4 is exactly why #1 is incorrect: the 'characteristic' impedance
of a coax is constant, but it's 'input' impedance varies according to
load mismatch at the other end. If it wasn't for this fact, a tuner at
the radio end would be useless. But the point here is that if the SWR
meter is left floating with the coax shield (both of which should be
RF grounded) then the measurement can be darn near anything.
*****

What I will agree with is that the impedance seen at the input to the
coax is a reflection of the load impeadance as transformed, altered if
some don't like the word transformed, by the length of the coax. I
have no problem with that. Still this impedance is highly dependant on
the load and its reflection coefficient.

Correct the characteristic impedance of the coax never changes and
that was what I intended with #1.


james

On Wed, 29 Jun 2005 16:37:10 -0500, Cecil Moore
wrote in :

Frank Gilliland wrote:
Point is that they are usually calibrated for Z0=50 ohms
and are in error when used in Z0 environments differing
from Z0=50 ohms, e.g. Z0=75 ohms.

The point is that the error is insignificant when the directional
coupler is much shorter than the wavelength.

Nope, that's not the point at all. It is true that a 50 ohm
SWR meter designed for HF may not work on 70 cm but the error
I'm talking about is the calibration error in a 50 ohm SWR meter
designed for HF and used on HF in, for instance, a Z0 = 450 ohm
environment instead of its calibrated-for 50 ohm environment....


There lies our misperceptions; I was not referring to using an HF SWR
meter designed for coax and plugging it into 450 ohm ladder line.






----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----

On Wed, 29 Jun 2005 13:56:53 -0700, Frank Gilliland
wrote:

Well, let's put it this way: the radio and antenna don't connect to
the dielectric of a coax.
*****

No it does not directly.

I know this concept is not easy to see but at the begining of the
coax, one can then consider the energy that travels down the coax as a
TEM wave. It is inside the dielectric where the E and H fields of the
traveling wave can be measured and found.

In transmission lines it is by far easier to think of E and H fields
within the the transmission line. Once that concept is mastered then
the rest is rather easy.

When the wave reaches the end, you have the final induced currents.
You can take a dipole and look at it as if the legs were an extension
of the transmission line. This can better be seen if you consider a
dipole and it is fed with open twin lead. The leads of the dipole then
are an extention of the twin lead except they are now at 90 degrees to
the transmission line.

Current is high when the magnetic field is high. This is so because
the induced current is controlled by the density of the magnetic
field. The E field is high when magnetic field is low. The E field
does not require current but voltage. On a center fed dipole the
impedance is low and the corresponding currents are high. The E field
off teh antenna is also low. As you progress a quarter wave from the
feed point in either direction the H field increases and the E field
decreases. With increasing H field the RF currents induced in the
antenna are high. Thus Ohm's law hald true. Z = I^2*R. Where R is the
radiation resistance of the antenna. The ends of a center fed dipole
are high impedance.

james

On Wed, 29 Jun 2005 22:58:51 GMT, "Tom Donaly"
wrote:

james wrote:

On Wed, 29 Jun 2005 16:42:49 -0500, Cecil Moore
wrote:


Sometimes yes, sometimes no. If the reflected current arrives out
of phase with the forward current, then the final dissipation can
actually be *reduced* by the mismatch.

*****

Power is power. Phase is not a problem. Take the mafnitude of the
transmitted power and teh magnitude of the reflected power. The
results are phaseless. The magnitudes add linearly.

QED

james


Cecil was talking about current, not power. You can't add
power the way you can voltage and current. If you could, you
could build a very nice perpetual motion machine just by using the
reflections in a transmission line to add power so that the output
was greater than the input.
73,
Tom Donaly, KA6RUH
******

Tom

The problem is that current is not reflected back from the load, power
is. Thus the you can add magnitudes of power.


james

On Wed, 29 Jun 2005 16:29:26 -0700, Roy Lewallen
wrote:

The rig has no way of detecting any alleged "reflected power". It can't
tell the difference between a feedline with a lot of "reflected power",
a feedline with no "reflected power", and a plain resistor. It behaves
exactly the same in all cases, provided only that the impedance that
each provides to it is the same.

*****

Agreed that a rig cannot detect the difference between forward and
reflected power. If the reflection coeffiecient of the source is zero
then final stage of a transmiter will look purely resistive to any
power reflected by the load. Thereby that refelcted power is
dissapated as heat. Other reflection coefficients at the source will
yield lesser amounts of reflected power from the load as heat.

james

Anyone not convinced of this should put a couple or more dummy loads in
series or parallel, make up a few lengths of transmission line of
various impedances, and see for himself.

Roy Lewallen, W7EL

james wrote:
On Wed, 29 Jun 2005 16:42:49 -0500, Cecil Moore
wrote:


Sometimes yes, sometimes no. If the reflected current arrives out
of phase with the forward current, then the final dissipation can
actually be *reduced* by the mismatch.

*****

Power is power. Phase is not a problem. Take the mafnitude of the
transmitted power and teh magnitude of the reflected power. The
results are phaseless. The magnitudes add linearly.

QED

james

On 29 Jun 2005 14:53:29 -0700, "K7ITM" wrote in
.com:

Frank Gilliland wrote, among other things, "The point is that the error
is insignificant when the directional coupler is much shorter than the
wavelength."

Certainly "directional couplers" for HF may be built at essentially
zero length, and ideally would have exactly zero length, monitoring the
current and voltage at a single point on a line. Then SWR or
reflection coefficient magnitude or even complex reflection coefficient
may be calculated under the assumption we know the desired reference
impedance. But if the equipment combines the voltage and current
samples in the wrong ratio, you will get the WRONG answer. Even if the
coupler looks like a perfect 50 ohms impedance section of transmission
line (with some attenuation), the error _in_measurement_output_ can be
significant indeed. Just because the coupler looks like a 50 ohm line
to the line it's hooked to doesn't mean it will read zero reflection
when IT's presented with a 50 ohm load.


SWR is a ratio, not an absolute value. It doesn't matter if the meter
reads a forward power that's off by 1 or 1000 watts just as long as
the reflected power is in error by the same percent, which will be the
case unless you are using two different meters for forward and
reflected power. Calibrated or not, SWR is the same.


And by the way, not everyone who measures and cares very much about SWR
(or reflection coefficient) cares a whit about field strength. Not all
loads are antennas.


That point might be relevant if this thread were cross-posted to
alt.heaters.induction or rec.dummy-loads.


Indeed, as Reg says, we might do better in amateur applications to
consider the SWR meter as an indicator of the degree to which we're
presenting a transmitter with the desired load.


I agree 100%.


That's really what
we're using it for, most of the time.


Unfortunately, it's that "most of the time" part that starts threads
like this. Some radio operators mistakenly think SWR is a measure of
antenna efficiency.


It may ALSO be interesting to
know the field strength, but please be aware that a transmitter's
distortion products may be significantly higher if it's presented the
wrong load impedance, even though the power output may be increased.
Field strength alone is not acceptable to me as a means to adjust an
antenna load to a transmitter, or as a way to adjust the operating
point of the transmitter.


True story. It's certainly better to use a tunable FSM if one is
available. And such meters are readily available -- any receiver with
a good S-meter.






----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----

james wrote:
On Wed, 29 Jun 2005 22:58:51 GMT, "Tom Donaly"
wrote:


james wrote:


On Wed, 29 Jun 2005 16:42:49 -0500, Cecil Moore
wrote:



Sometimes yes, sometimes no. If the reflected current arrives out
of phase with the forward current, then the final dissipation can
actually be *reduced* by the mismatch.

*****

Power is power. Phase is not a problem. Take the mafnitude of the
transmitted power and teh magnitude of the reflected power. The
results are phaseless. The magnitudes add linearly.

QED

james


Cecil was talking about current, not power. You can't add
power the way you can voltage and current. If you could, you
could build a very nice perpetual motion machine just by using the
reflections in a transmission line to add power so that the output
was greater than the input.
73,
Tom Donaly, KA6RUH

******

Tom

The problem is that current is not reflected back from the load, power
is. Thus the you can add magnitudes of power.


james

Nope. You need a course in electromagnetics. Who put all these
ideas into your head, anyway?
73,
Tom

On Wed, 29 Jun 2005 21:44:57 GMT, james wrote
in :

On Tue, 28 Jun 2005 16:18:56 -0700, Frank Gilliland
wrote:

Yep. And I should add that 18' of coax is recommended not because of
it's propogation characteristics -inside- the coax, but because of
it's velocity factor on the -outside- of the shield which is nearly 1.
IOW, when the shield of an 18' length of coax is grounded only at one
end, that ground will be reflected at the other end of the coax. At
least that's the theory. In practical use it's not perfect, but it's
still better than a fully ungrounded radio or antenna
*****

your reasoning does not pass the common sense rule.

if 18 feet is special because the velocity factor outside the shield
is nearly 1, then no other length has a velocity factor of nearly 1?


......huh?


What would cause the outside velocity factor to change?


Change from what?


Beisdes I really don't care to rehash this topic to much more. Been
there, done that and don't care to review it right now.


Good, because I don't think you have a very good grasp of the concepts
involved.







----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----

On Wed, 29 Jun 2005 23:36:26 GMT, james wrote
in :

On Wed, 29 Jun 2005 14:24:44 -0700, Frank Gilliland
wrote:

And #4 is exactly why #1 is incorrect: the 'characteristic' impedance
of a coax is constant, but it's 'input' impedance varies according to
load mismatch at the other end. If it wasn't for this fact, a tuner at
the radio end would be useless. But the point here is that if the SWR
meter is left floating with the coax shield (both of which should be
RF grounded) then the measurement can be darn near anything.
*****

What I will agree with is that the impedance seen at the input to the
coax is a reflection of the load impeadance as transformed, altered if
some don't like the word transformed, by the length of the coax. I
have no problem with that. Still this impedance is highly dependant on
the load and its reflection coefficient.


That's basically what Lance said, just in different words. So what's
the problem?







----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----

james wrote:

On Wed, 29 Jun 2005 16:29:26 -0700, Roy Lewallen
wrote:


The rig has no way of detecting any alleged "reflected power". It can't
tell the difference between a feedline with a lot of "reflected power",
a feedline with no "reflected power", and a plain resistor. It behaves
exactly the same in all cases, provided only that the impedance that
each provides to it is the same.


*****

Agreed that a rig cannot detect the difference between forward and
reflected power. If the reflection coeffiecient of the source is zero
then final stage of a transmiter will look purely resistive to any
power reflected by the load. Thereby that refelcted power is
dissapated as heat. Other reflection coefficients at the source will
yield lesser amounts of reflected power from the load as heat.

james


Anyone not convinced of this should put a couple or more dummy loads in
series or parallel, make up a few lengths of transmission line of
various impedances, and see for himself.

Roy Lewallen, W7EL

james wrote:

On Wed, 29 Jun 2005 16:42:49 -0500, Cecil Moore
wrote:



Sometimes yes, sometimes no. If the reflected current arrives out
of phase with the forward current, then the final dissipation can
actually be *reduced* by the mismatch.

*****

Power is power. Phase is not a problem. Take the mafnitude of the
transmitted power and teh magnitude of the reflected power. The
results are phaseless. The magnitudes add linearly.

QED

james




You need to read _Reflections II, Transmission Lines and Antennas_
by M. Walter Maxwell, W2DU. Even better, get a book on electromagnetics.
You might be able to puzzle some of it out although much of
the math might be too esoteric for you.
73,
Tom Donaly, KA6RUH

On Wed, 29 Jun 2005 22:45:03 GMT, "Tom Donaly"
wrote:

james wrote:
On Wed, 29 Jun 2005 11:07:17 -0400, "Fred W4JLE"
wrote:


What is the reason a 2:1 SWR can cause such havoc?

How can I avoid this catastrophic condition?

I feed my dipoles with 450 Ohm ladder line, but the last 20 feet or so is 50
Ohm coax, I guess that makes it work ok. I haven't blown up my finals yet.

Lions and tigers and bears Oh my...

*****

Actually can happen if you push the finals to where there is
insufficeint margin to the maximum heat dissapation. Tubes are a bit
more forgiving. Transistor inadequately heatsinked and overdriven,
typical CB usage, often have little of no margin for heat dissapation.

If the transmitter has a refelction coefficient of zero and the load
say .3, then that reflected power from the load is dissapated as heat
in the output circuits and any final transistors or tubes. Now if the
radio has a reflection coefficient other than zero that will lessen
the heat dissapation on the transimiiter. Now you get load and source
reflections convoluting within the transmission line.

You ought to model a 400 Mhz square wave with source and load
refelctions coefficients other than zero. It can get ugly


james



Consider the MRF 140, a 150 Watt 2.0 - 150.0 Mhz N-Channel
linear RF power fet. From the technical data sheet: "100% Tested
For Load Mismatch At All Phase Angles With 30:1 VSWR." You'd have
a tough time damaging this device with a mere 2:1 VSWR.
How do load and source reflections convolute within the
transmission line? That's a new one on me. My old dictionary
defines 'convolute' as "Rolled or folded together with one part
over another; twisted; coiled." The rest of the post is pretty
fanciful, too. A trip to the library would do wonders.
73,
Tom Donaly, KA6RUH
******

In electrical engineering it is the instantaneous power density of two
signals passing at the same spot from two directions. That is called
Convolution. It also is a nice mathematical means of modeling SWR at
any point on a transmisison line at a particular time.

Well if you knew CBers, they are not satidied getting 150 watts from
a transistor rated for 150 watts. But in my first paragraph I thought
I made it clear but evidently I did not. I guess I must strive to
better explain myslef.

james

On Thu, 30 Jun 2005 00:12:31 GMT, "Tom Donaly"
wrote:

Nope. You need a course in electromagnetics. Who put all these
ideas into your head, anyway?
73,
Tom
Electromagnetics

james

On Wed, 29 Jun 2005 22:45:03 GMT, "Tom Donaly"
wrote in
:

snip
..... A trip to the library would do wonders.
73,
Tom Donaly, KA6RUH


I keep trying to stress that fact, but some people persist under the
delusion that they can learn everything they need from the internet.






----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----

On Thu, 30 Jun 2005 00:22:48 GMT, james wrote:

In electrical engineering it is the instantaneous power density of two
signals passing at the same spot from two directions. That is called
Convolution.

Hi James,

No, it is called Superposition, and that is done only with voltage or
current. What you are describing may be associated with the Fourier
convolution of power series - an entirely different field (and not
even additive).

73's
Richard Clark, KB7QHC