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