"Reg Edwards" wrote
Provided the mechanical connection is sound, you can use any coaxial
connectors you like, regardless of nominal impedance, at frequencies less
than about 300 MHz without any observed ill effects.
_________________

??? Using connectors that don't maintain the characteristic impedance of the
transmission lines they connect _will_ produce undesired effects. The
effects may be negligible to amateur radio operators used to operating
transmitters into rather high mismatches, but they would never be tolerated
in most professional operations, including high-power broadcast systems.

Years ago a common impedance for rigid transmission line used in broadcast
systems was 51.5 ohms. Later the more common value was/is 50 ohms. While a
mechanical adapter was available to allow connecting a 51.5 ohm inner
conductor to 50 ohm inner conductor (SWR= 1.03), better installations
installed an RF transformer section at these interfaces.

A 1.1 SWR at the input of a TV transmit antenna using ~500 or more feet of
transmission line will produce a visible "ghost" in the transmitted picture,
as seen by a careful observer. At 1.25 SWR it can be seen, and will
objectionable to almost everyone.

RF

Visit http://rfry.org for FM broadcast RF system papers.

To save the trouble of calculating it I'll take a guess. A connector less
than 1" long of impedance 51.5 ohms in a 50 ohm system will NOT produce an
SWR of 1.03:1 or anything anywhere near to it at frequencies less than 1000
MHz.

A 1" long connector WILL produce an SWR of 1.03:1 around 3 GHz but no worse.
If you can reliably measure it.

What matters is the ratio of connector length to wavelength along the line.

For the same reason, at HF, bringing the two wires of an open wire line
close together for the purpose of drawing them through a single small hole
in the wall, will not produce any noticeable effect on line performance.
----
Reg.

=================================

"Richard Fry" wrote in message
...
"Reg Edwards" wrote
Provided the mechanical connection is sound, you can use any coaxial
connectors you like, regardless of nominal impedance, at frequencies
less
than about 300 MHz without any observed ill effects.
_________________

??? Using connectors that don't maintain the characteristic impedance of
the
transmission lines they connect _will_ produce undesired effects. The
effects may be negligible to amateur radio operators used to operating
transmitters into rather high mismatches, but they would never be
tolerated
in most professional operations, including high-power broadcast systems.

Years ago a common impedance for rigid transmission line used in broadcast
systems was 51.5 ohms. Later the more common value was/is 50 ohms. While
a
mechanical adapter was available to allow connecting a 51.5 ohm inner
conductor to 50 ohm inner conductor (SWR= 1.03), better installations
installed an RF transformer section at these interfaces.

A 1.1 SWR at the input of a TV transmit antenna using ~500 or more feet of
transmission line will produce a visible "ghost" in the transmitted
picture,
as seen by a careful observer. At 1.25 SWR it can be seen, and will
objectionable to almost everyone.

RF

Visit http://rfry.org for FM broadcast RF system papers.

Connector salesmen (and no doubt ladies) have the habit of exaggerating the
importance and magitude of SWR ON THE LINE associated with the precision of
connector manufacture. The habit transfers itself into magazine articles
without any supporting practical experiments.

"Reg Edwards" wrote

A 1" long connector WILL produce an SWR of 1.03:1 around 3 GHz
but no worse. If you can reliably measure it.

AND

Connector salesmen (and no doubt ladies) have the habit of exaggerating
the
importance and magitude of SWR ON THE LINE associated with the precision
of
connector manufacture. The habit transfers itself into magazine articles
without any supporting practical experiments.
______________________

REG: Please consider this.

Most TV transmit antenna systems have an adjustable RF transformer installed
at the antenna input connector. This transformer consists of 4 or 5 brass
"pins" of about 5/8" diameter spread evenly across a 90 degree section of
rigid transmission line. These pins can be inserted radially into the space
between the outer and inner conductors of that line section. When the pins
are withdrawn fully, they have no affect on the natural impedance of that
line section, but can produce a spatially discrete SWR as function of their
insertion distance into the line.

I, personally, and many other broadcast engineers have been involved in the
adjustment of such RF transformers to optimize the match between the main
transmission line and the antenna input [including its elbow complex(es)],
at the frequencies used in commercial VHF/UHF television -- which start at
54 MHz.

This requires (1) purchase and installation of the transformer, (2)
deployment of a tower crew to adjust it, and (3) use of a qualified field
engineer in the tx bldg with the appropriate test equipment and field
experience to direct the adjustment of that variable transformer.

Obviously, these processes are not inexpensive, and would not be undertaken
if there was no reason. The reason: to optimize the match between the main
line and the antenna, and thus to transmit the "cleanest" video.

This PRACTICAL experience illustrates that an impedance change even in a
54-60MHz TV channel occurring within a physical space of less than one inch
can produce important and commercially supportable system benefits, despite
your statements quoted above.

I invite you to post the contrary result(s) of your own "practical
experiments," and/or those of others.

RF

Visit http://rfry.org for FM broadcast RF system papers.

Richard, I have no reason to doubt anything you say.
I have no reason to doubt what I have said.
But why imply there's serious disagreement when there is none?

From the tone of your comments I correctly guessed you were a 'magazine'
author.

Are you also a spare-time connector salesman? ;o) ;o)
----
Reg, G4FGQ

Richard, WE just look at PL259's from somewhat different viewpoints and see
different magnitudes of the factors involved, with different objectives,
even different costs.
----
Reg, G4FGQ

"Reg Edwards" wrote
Richard, I have no reason to doubt anything you say.
I have no reason to doubt what I have said.
But why imply there's serious disagreement when there is none?
_______________________

Clips from our previous posts (below) -

I don't know what constitutes a "serious disagreement" to you, Reg, but I
think most readers would say that we have opposite conclusions about this
topic.

If you have no reason to doubt what I wrote about this, how can you continue
to support what you wrote? Our statements are mutually exclusive.

YOU: "Provided the mechanical connection is sound, you can use any coaxial
connectors you like, regardless of nominal impedance, at frequencies less
than about 300 MHz without any observed ill effects. A connector less than
1" long of impedance 51.5 ohms in a 50 ohm system will NOT produce an SWR of
1.03:1 or anything anywhere near to it at frequencies less than 1000 MHz."

ME: "This PRACTICAL experience illustrates that an impedance change even in
a 54-60MHz TV channel occurring within a physical space of less than one
inch can produce important and commercially supportable system benefits,
despite your statements quoted above."

- RF

Richard,

A radio amateur, by an easy mistake, uses a 75-ohm plug and socket in a
50-ohm coaxial transmission system. The total length of the plug plus
socket is 1"

As a result of the mismatch what is the SWR produced on the 50-ohm line at 2
MHz. At 30 MHz? At 150 MHz?

Is the amateur, or anyone else, likely to be aware of any difference in
performance?

No need to make measurements. If you are unable to make a simple calculation
and answer the question then you are not qualified to continue the
discussion.
----
Reg, G4FGQ

On Mon, 17 May 2004 06:36:43 -0500, "Richard Fry"
wrote:
If you have no reason to doubt what I wrote about this, how can you continue
to support what you wrote? Our statements are mutually exclusive.

Ah Richard!!

Tell me, please, this is the FIRST time you have read our master fence
sitter (in the American historical context otherwise known as a
Mugwump: someone who has his mug on one side of the fence and his wump
on the other).

73's
Richard Clark, KB7QHC

Wait a minute here... Seems to me that Reg is talking about two
opposing steps separated by a very short distance, and Richard is
talking about a single step from one impedance to another. Clearly,
Richard's case results in uncancelled echos related to the ratio of
the impedances at the step. In fact, Reg's example results in
significant uncancelled echos if the steps are separated by enough
distance: worst at odd multiples of pi/2 electrical degrees. When the
connector is only perhaps a few electrical degrees long, the steps
nearly cancel. How long is 1", in electrical degrees? Well, at
1000MHz and a v.f. of perhaps 0.7, longer than one might have
anticipated: almost 45 degrees long. So in fact at 1000MHz, a 1"
section of 51.5 ohm connector might introduce a bit more than 1.03:1
SWR. But didn't we start out talking about a much lower frequency,
and a much larger impedance difference? Another example I ran was 4
electrical degrees of connector...perhaps a bit less than an inch at
150MHz...where it's 75 ohms in a 50 ohm system. The swr for that came
out about 1.06:1.

The TV broadcast engineer should worry about things like that. The
typical ham doesn't have equipment calibrated accurately enough that
s/he should be worried about it, and it's unlikely to make any
substantive difference anyway in typical ham work.

Cheers,
Tom

"Richard Fry" wrote in message ...
"Reg Edwards" wrote
Richard, I have no reason to doubt anything you say.
I have no reason to doubt what I have said.
But why imply there's serious disagreement when there is none?
_______________________

Clips from our previous posts (below) -

I don't know what constitutes a "serious disagreement" to you, Reg, but I
think most readers would say that we have opposite conclusions about this
topic.

If you have no reason to doubt what I wrote about this, how can you continue
to support what you wrote? Our statements are mutually exclusive.

YOU: "Provided the mechanical connection is sound, you can use any coaxial
connectors you like, regardless of nominal impedance, at frequencies less
than about 300 MHz without any observed ill effects. A connector less than
1" long of impedance 51.5 ohms in a 50 ohm system will NOT produce an SWR of
1.03:1 or anything anywhere near to it at frequencies less than 1000 MHz."

ME: "This PRACTICAL experience illustrates that an impedance change even in
a 54-60MHz TV channel occurring within a physical space of less than one
inch can produce important and commercially supportable system benefits,
despite your statements quoted above."

- RF