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

To anybody who may be reading -

(1) There's far too much importance attached to standing waves on
transmission lines. But see (4).

(2) There are colossal standing waves on antennas which are seldom
taken any notice of.

(3) Anyway, of what use does anybody make of standing waves after
taking the trouble to measure them. And the measurements themselves
are the most inacurate in the field of radio engineering.

(4) And to cap it all, the common or garden SWR meter does NOT
measure standing waves on the feedline to the antenna where they might
conceivably be of interest. It's all a gigantic hoax!
----
Season's Greetings from Reg, G4FGQ.

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
--

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

W. Watson wrote:
. . .
Not a bad explanation from Wikipedia:

SWR has a number of implications that are directly applicable to radio use.

1. SWR is an indicator of reflected waves bouncing back and forth
within the transmission line, and as such, an increase in SWR
corresponds to an increase in power in the line beyond the actual
transmitted power. This increased power will increase RF losses, as
increased voltage increases dielectric losses, and increased current
increases resistive losses.

I go along with that.

2. Matched impedances give ideal power transfer; mismatched
impedances give high SWR and reduced power transfer.

That's oversimplified and a misapplication of the rule of maximum power
transfer. Suppose I have a 50 ohm source connected to a 50 ohm load and
adjust the source so it puts 100 watts into the load. Then I put a half
wavelength of 300 ohm line between the source and the load. The
transmission line will have a 6:1 SWR. There will be a 6:1 impedance
mismatch at the transmission line-load junction. Yet
-- The load power will be 100 watts as before.
-- The power produced by the source will be 100 watts as before.
-- The system efficiency will be the same as it was before.
-- 100 watts will be transferred from the source to the line.
-- 100 watts will be transferred from the line to the load.

So in no way did the high SWR result in reduced power transfer.

Now change the load impedance to 300 ohms.

-- There is now a 6:1 mismatch between the source and the line.
-- The line SWR is now 1:1.
-- The load power will be reduced.

The mismatch between source and line didn't cause a high SWR on the
line. In fact, changing the line impedance degraded the match at the
same time it improved the SWR.

3. Higher power in the transmission line also leaks back into the
radio, which causes it to heat up.

That's demonstrably false. For some examples and explanations, see
http://www.eznec.com/misc/food_for_t...se%20Power.txt.
(You might have to splice this URL back together if your browser splits it.)

4. The higher voltages associated with a sufficiently high SWR could
damage the transmitter. Solid state radios which have a lower tolerance
for high voltages may automatically reduce output power to prevent
damage. Tube radios may arc. The high voltages may also cause
transmission line dielectric to break down and/or burn.

That's true. Some transmitters can be damaged from a number of causes
when the load impedance isn't approximately what the transmitter was
designed for. Only one of those possible causes is increased voltage.

Of course, a high SWR can also cause the voltage at the transmitter to
be lower than it otherwise would have been.

Abnormally high
voltages in the antenna system increase the chance of accidental
radiation burn if someone touches the antenna during transmission.

But the antenna doesn't have an SWR, the transmission line does. If you
do have an open wire transmission line, it's best not to touch the line
regardless of the SWR. But if you have a high line SWR, there's just a
good of a chance that the voltage at the point you touch is lower due to
the high SWR than it is than the voltage is higher.

I'll bet if you search the web you can find just about any kind of
possible misinformation about SWR, just as you can about any other topic.

Roy Lewallen, W7EL

John Ferrell wrote:

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.

Mismatched transmission lines also behave like the theory books say. The
rated power from the transmitter goes through the transmission line to
the antenna where it is radiated just like the book says.

I deleted only "with the lowest possible loss" because increased SWR
does increase line loss. But if the line loss is low when matched, the
increased loss due to high SWR is often negligible.

. . .

Roy Lewallen, W7EL

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

I am glad I chose to display my ignorance, I have learned a lot.

I will modify my earlier position that High SWR causes TVI to Roy's
better explanation that they seem to have the same causes.

I have noted that other people seem to get along with High SWR's and
Non resonant antennas. Neither has worked well for me in the past.

I used to operate ATV on 440mhz with a 48 element collinear antenna.
(I did not do the initial construction) It was basically a collection
of dipoles and reflectors tied together with open wire phasing
harnesses. I could never quite figure out what the harness might be
losing. The antenna seemed to be worth about 21db with very primitive
measuring schemes. I speculated that it had a theoretical potential of
48db. I always felt that the phasing harness was the major contributor
to the losses.

With each driven element being resonant, some measure of success was
assured with each one. At 440, feed line losses get critical in a
hurry.

The transmitter final was a military surplus cavity with a 4CX250 that
had been "stuffed" to get it up to 440. As a side effect the output Z
was determined to be pretty low by trying several home brew quarter
wave coaxial matching sections. When a good match was made, a lot of
problems went away. Not only was I managing a better signal, but the
polyethylene cooling ducting was taking a longer time to melt down.

Hence, my position that SWR IS important.

Please note: I am not here to pick an argument, I recently took
W4RNL's antenna course, bought Roy's EZNEC+ program, downloaded (still
studying) Reg's many programs and have taken note that Cecil is a
motorcycle fan like my Dad was.

When I provoke an argument, I am trying to learn something.
BTW, I do have a pretty good library of reference books but you guys
explain it better in terms I can grasp!


On Wed, 21 Dec 2005 22:45:20 -0800, Roy Lewallen
wrote:

John Ferrell wrote:

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.

Mismatched transmission lines also behave like the theory books say. The
rated power from the transmitter goes through the transmission line to
the antenna where it is radiated just like the book says.

I deleted only "with the lowest possible loss" because increased SWR
does increase line loss. But if the line loss is low when matched, the
increased loss due to high SWR is often negligible.

. . .

Roy Lewallen, W7EL
John Ferrell W8CCW

John Ferrell wrote:
I never gave the swr on the radiator any thought. That is a good
point.

Hi John, it was supposed to be humorous. I find it amusing that
someone will say, "I hate high SWRs.", while running a
resonant dipole with an SWR of 20:1 (or whatever) on the
radiating elements.

Superposition and interference between the forward waves and
reflected waves on the radiating elements is what brings the
feedpoint impedance of a 1/2WL dipole down to 50 ohms (or
whatever). If it were not for standing waves on a 1/2WL dipole,
the feedpoint impedance would be in the neighborhood of 1200
ohms, similiar to an infinite dipole.
--
73, Cecil http://www.qsl.net/w5dxp