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


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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.







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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