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

"Reg Edwards" wrote
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.
__________________

How convenient. Why _would_ you want to make measurements, and prove
yourself wrong?

On Mon, 17 May 2004 15:02:19 -0500, "Richard Fry"
wrote:

"Reg Edwards" wrote
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.
__________________

How convenient. Why _would_ you want to make measurements, and prove
yourself wrong?

Ah Richard!

Confrontation? Feeble at best when I doubt you could even get his
recipe for RF Mud. ;-)

Poor Reggie can quote Dead White Scientists like Lord Kelvinator with
high dudgeon and still ignore the tenet of the plagiarized message.

73's
Richard Clark, KB7QHC

Reg,

You sent the 2nd post below within ten minutes of the 1st one below.

Exactly WHAT is your position on this subject?

- RF
_______________________

"Reg Edwards" wrote first:

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.

AND THEN, quoting Lord Kelvin:

"When you can measure what you are speaking about and express
it in numbers you know something about it. But when you cannot
measure it, when you cannot express it in numbers, your knowledge
is of a meagre and unsatisfactory kind. It may be the beginning of
knowledge but you have scarcely in your thoughts advanced to the
state of science."

Reg Edwards wrote:
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?


The practical answer for amateurs is somewhere between the two extreme
positions that Richard and Reg are taking.

Richard quotes a case where even very small impedance bumps do matter;
but it's in full-quality TV broadcasting, not amateur radio.

Reg, on the other hand, wants to dumb it down too far. There *are* cases
in amateur radio where small impedance bumps are at least noticeable.

At 2MHz or 30MHz, the effect is so small that no amateur would notice
it. Even using professional test equipment, you'd be hard-pressed to
measure the effect of a single connector of the wrong impedance.

At 150(144)MHz, even a single connector is noticeable... but that's not
the problem.

The real problem is that if people believe a simple slogan like
"connector impedances don't matter", they will probably go ahead and use
*several* mismatched connectors, at various places along the line.

Then they start to find bewildering problems at 144MHz and above, such
as indicated SWR and power output values that vary according to the
length of the coax jumpers that they use. It still may not matter in
terms of the contacts they can make, but they are completely unable to
understand what is happening - and that *does* matter!

(What is happening, by the way, is that the lengths of the line sections
between the mismatched connectors will determine how the small
reflections from each one combine together. If you're lucky with the
line lengths, they may tend to cancel; if you're not, they may tend to
add... and usually it's somewhere in between.)


No need to make measurements.

In this particular case, that's true. When the impedance bump is small,
it is easier and more accurate to calculate the effect than to measure
it.


--
73 from Ian G3SEK
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Let's do what nobody has ever done before, not even in the ARRL handbooks or
in Terman, and get an idea of the magnitudes involved.

Examine two cases over a range of frequencies.

Case (1). In a 50-ohm system, use of a poor connector having an impedance
deviating 10 percent from its nominal value of 50 ohms.

Case (2). Making the mistake of using a 75-ohm connector in a 50-ohm
system.

In both cases the connector, plug and socket, is 1" (25.4mm) long.

We first calculate the input impedance of a 75-ohm transmission line, 1"
long, terminated with 50 ohms. Zin will not be very much different from 50
ohms.

We then calculate the SWR on a 50-ohm line which is terminated by the
afore-mentioned input impedance.

RESULTS of calculation

MHz SWR Case 1 SWR Case 2
------ ----------------- ----------------
2 1.0002 1.0009
30 1.0028 1.0146
150 1.014 1.073
300 1.029 1.145
1000 1.105 1.524

It is seen that results do not become significant to a radio amateur, and
almost everybody else, until he has made the serious mistake of using the
wrong impedance connector, and the frequency has risen to 1000 MHz for which
he hasn't an SWR meter anyway.

Below 300 MHz the results are submerged well beneath the uncertainty of an
SWR meter. Now we can take a balanced view of the situation.
----
Reg, G4FGQ

"Reg Edwards" wrote in message
...
Let's do what nobody has ever done before, not even in the ARRL handbooks
or
in Terman, and get an idea of the magnitudes involved.

Examine two cases over a range of frequencies.

Case (1). In a 50-ohm system, use of a poor connector having an impedance
deviating 10 percent from its nominal value of 50 ohms.

Case (2). Making the mistake of using a 75-ohm connector in a 50-ohm
system.

In both cases the connector, plug and socket, is 1" (25.4mm) long.

We first calculate the input impedance of a 75-ohm transmission line, 1"
long, terminated with 50 ohms. Zin will not be very much different from
50
ohms.

We then calculate the SWR on a 50-ohm line which is terminated by the
afore-mentioned input impedance.

RESULTS of calculation

MHz SWR Case 1 SWR Case 2
------ ----------------- ----------------
2 1.0002 1.0009
30 1.0028 1.0146
150 1.014 1.073
300 1.029 1.145
1000 1.105 1.524

It is seen that results do not become significant to a radio amateur, and
almost everybody else, until he has made the serious mistake of using the
wrong impedance connector, and the frequency has risen to 1000 MHz for
which
he hasn't an SWR meter anyway.

Below 300 MHz the results are submerged well beneath the uncertainty of an
SWR meter. Now we can take a balanced view of the situation.
----
Reg, G4FGQ


Reg,

I suspect your 1 inch length is overly pessimistic. Clearly, for an F
connector, it is more like 1/2 inch. Lastly, if the load at the end is not
50.0, then any small deviation in the feedline could just as well improve
things, as make it worse; this is probably not true for pulses or video.
Think of an antenna tuner. It does nothing to the SWR on the main piece of
line.

Tam/WB2TT

Reg,

Your math example does not illustrate the reality that the impedance
change produced by a ~5/8" OD brass pin inserted radially into the
dielectric space of coaxial transmission line can match that line into an
adjacent termination of fairly high SWR (1.3:1 or so), at frequencies as
low as 54MHz.

As I wrote earlier, this technique is widely and successfully used to match
the main transmission line of broadcast TV and FM stations to the net input
impedance of their antenna (including its input elbows).

- RF

Your math example does not illustrate the reality that the impedance
change produced by a ~5/8" OD brass pin inserted radially into the
dielectric space of coaxial transmission line can match that line into an
adjacent termination of fairly high SWR (1.3:1 or so), at frequencies as
low as 54MHz.

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

It's not intended to.

But I'm sure you are right. Try not to worry about it.

I won't "worry about it," as long as you refrain from posting absolute
statements about this that are demonstrably untrue.

- RF


"Reg Edwards" wrote in message
...

Your math example does not illustrate the reality that the
impedance
change produced by a ~5/8" OD brass pin inserted radially into the
dielectric space of coaxial transmission line can match that line into
an
adjacent termination of fairly high SWR (1.3:1 or so), at frequencies
as
low as 54MHz.

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

It's not intended to.

But I'm sure you are right. Try not to worry about it.