In article ,
Ian Jackson wrote:

In message
,
Telamon writes
In article ,
Ian Jackson wrote:

In message
,
Telamon writes
In article
,
RHF wrote:

On Dec 28, 8:36*pm, Telamon
wrote:



You are confusing the characteristic impedance of the coax with its
ability to be an effective transmission line. The coax only behaves as
an effective transmission line when both ends of it are terminated at
its characteristic impedance.

Are you really sure about this sweeping statement?

Yes. It is basic transmission line theory. RF energy entering or leaving
a coax line has to be at the same impedance or energy is reflected. That
is a basic rule.

For starters, please define 'effective'.

The word effective was used in the context of the coax meeting its
specifications within reason.

If the source and load impedances are NOT the same as the characteristic
impedance of the coax, any 'ineffectiveness' as a transmission line will
not as a result of the coax not meeting its specifications. You simply
haven't used coax with the RIGHT specifications.

OK. I don't understand your point though. My original reply is that coax
has a characteristic impedance based on its design not what it has for
terminations on either end.

And are you sure that transmitter output impedances are 50 ohms (or
whatever)?

If it is specified to be 50 ohms and it is not then it should find its
way back to the manufacturer for repair or redesign.

Indeed, the specs for transmitters do sometimes say that the output
impedance is 50 ohms. This is almost certainly wrong. What it really
means is that the transmitter is designed to work into a 50 ohm load.

I have to disagree with you here. I work with a lot of test equipment
and if this was true good luck getting anything to work. Signal
generators and amplifiers have to have output impedances that match the
coax impedance or you would not be able to predict how test setups would
work.

The two are rarely the same. Transmitters are designed for best
efficiency and/or linearity. The actual output impedance is not really
relevant. [Signal generators are different. They SHOULD be 50 ohms. This
subject has been discussed ad nauseam in several NGs.]

I don't want to get nauseated then. The arguments to the contrary make
no sense. Signal generators, amplifiers, or transmitters all have to
have the same system impedance or the RF just gets reflected back at the
source, which has to dissipate that additional energy as heat usually.
So you would have to beef up the output devices in the source and you
would be wasting a portion of the RF generated.

--
Telamon
Ventura, California