On Wed, 21 Dec 2005 21:29:37 GMT, Owen Duffy wrote:

On Wed, 21 Dec 2005 20:18:05 GMT, John Ferrell
wrote:

On Tue, 20 Dec 2005 03:41:41 GMT, "W. Watson"
wrote:

I'm trying to make
some sense out of why the emphasis of standing waves.

Here is the short version:
A matched transmission line behaves like the theory books say it does.
The rated power from the transmitter goes through the transmission
line with the lowest possible loss to the antenna where it is radiated
just like the book says.

It is true that reducing SWR for a given line does reduce the loss if
the line is long enough. (There are some scenarios where a short line
with high VSWR has less loss than matched line of the same length.)

I will have to take your word for it, I cannot think of any examples.
But is matched line the real goal?

If low loss is the goal, there are often cost effective lower loss
solutions possible with lower loss line operated at high VSWR.

A mismatched transmission just MIGHT work OK. If there is any
possibility of generating interference, especially TVI, it will. The

Why? How is TVI "generated" by line mismatch?

Owen
I really don't know why there is more TVI with a high swr. But my
experience has been that there is, especially on 6 meters.
John Ferrell W8CCW

On Thu, 22 Dec 2005 04:17:11 GMT, John Ferrell
wrote:


It is true that reducing SWR for a given line does reduce the loss if
the line is long enough. (There are some scenarios where a short line
with high VSWR has less loss than matched line of the same length.)

I will have to take your word for it, I cannot think of any examples.

John,

It was really a bit of an aside, a lead into the more important point
that followed, however...

In most practical lines at HF, loss is dominated by the series
resistance.

In the regions of a current minimum on a line with high VSWR, the I^2R
losses are less than for the same net power on a flat line, and
conversely, in the region of a current maximum on a line with high
VSWR, the I^2R losses are more than for the same net power on a flat
line. Over a half wave of line, the total losses are higher than a
flat line, but a short line in the regions of a current minimum may
have losses less than the matched line loss.

Whilst that is not often to our benefit as we rarely have relatively
short lines with high Z loads, the converse is true of the more common
situation of a short line with a low Z load. For example, 3m of RG58
with a 5+j0 load (eg mobile antenna) on 3.5MHz has a matched line loss
of 0.08dB, and an actual loss of 0.66dB. Many charts and formulas
would predict the mismatched loss to be only 0.39dB, but it is worse
because the line is short and in the region of a current maximum.

But is matched line the real goal?

If low loss is the goal, there are often cost effective lower loss
solutions possible with lower loss line operated at high VSWR.

A mismatched transmission just MIGHT work OK. If there is any
possibility of generating interference, especially TVI, it will. The

Why? How is TVI "generated" by line mismatch?


I really don't know why there is more TVI with a high swr. But my
experience has been that there is, especially on 6 meters.

If you can't explain the mechanism by which SWR causes TVI, perhaps
they correlate by some other cause. For example, an antenna may
develop loose oxided connections which both change the load impedance
(and hence VSWR), and create intermodulation causing TVI.

If VSWR *does* cause TVI, surely someone will be able to explain how?

Lots of people operate feedlines at high VSWR by design, and they do
not necessarily cause TVI.

When you dismiss the TVI myth, you get closer to understanding how the
transmission lines work and perform, and that for example, operating a
feedline at high VSWR can be part of an efficient and effective
multiband HF antenna. Such a solution should not be dismissed out of
hand because of high VSWR alone.

Owen
--

Owen Duffy wrote:

I really don't know why there is more TVI with a high swr. But my
experience has been that there is, especially on 6 meters.

If you can't explain the mechanism by which SWR causes TVI, perhaps
they correlate by some other cause. For example, an antenna may develop
loose oxided connections which both change the load impedance (and
hence VSWR), and create intermodulation causing TVI.

If VSWR *does* cause TVI, surely someone will be able to explain how?

When people report "high SWR", they are usually talking about a coax-fed
system, and they usually mean "a higher SWR than I expected for this
antenna".

That is a big clue that the antenna is not performing correctly... but
the high SWR is only a symptom. It shouldn't be mistaken for a cause.

One very common cause of RFI is common-mode RF current on the outside of
the coax - in other words, the coax has become an unintended part of the
antenna. The outside of the coax comes back down into the house, and can
be a potent conductor of RFI. The higher than expected SWR is simply
because the addition of the coax makes this a *different* antenna from
the one you thought you were using.

With something like a 6m yagi, the cure is generally to change to a
truly balanced feed system, and to add a feedline choke.

Obviously common-mode current is not the *only* possible connection
between "high SWR" and RFI, but it's more common than many people
suspect.


Just caught Roy's second post about the mistaken belief that high SWR
and feedline radiation. It should be clear from the above that
higher-than-expected SWR and feedline radiation are two separate
*results* of unwanted common-mode currents. Once again, SWR should not
be mistaken for a cause.



--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Sorry, the last paragraph of my previous posting should have included
the word _causes_. It should have read:


Just caught Roy's second post about the mistaken belief that high SWR
_causes_ feedline radiation. It should be clear from the above
that higher-than-expected SWR and feedline radiation are two
separate *results* of unwanted common-mode currents. Once
again, SWR should not be mistaken for a cause.


--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

John Ferrell wrote:
. . .
I really don't know why there is more TVI with a high swr. But my
experience has been that there is, especially on 6 meters.
John Ferrell W8CCW

It might be that the same phenomenon, or related ones, have caused both
the high SWR and the TVI. But high SWR doesn't cause TVI. Or feedline
radiation, which is another mistaken idea.

Roy Lewallen, W7EL

John Ferrell wrote:
I really don't know why there is more TVI with a high swr. But my
experience has been that there is, especially on 6 meters.

SWR can have an indirect effect on common-mode currents by causing
a malfunction of the balun. For an SWR of 1:1, the balun is probably
functioning in the impedance environment for which it was designed.
That balun may cease to choke properly when exposed to the impedances
present in an SWR 1:1 environment and allow more common-mode signals
to develop and radiate.
--
73, Cecil http://www.qsl.net/w5dxp