About all that anyone needs to know about an amateur radio amplifier
(transmitter) in order to use it properly is the output power level and the
required load resistance. The latter is usually 50 ohms for a variety of
reasons, most of which relate to convenience, availability of coax cables,
test equipment impedance environment, etc. Beyond those values, there is
nothing about the amplifier design which is used in designing and adjusting
the remainder of the tuner, transmission line and antenna system. The power
level is of importance only in telling us how much voltage and current is
involved in various parts of the system.

The result is the ultimate in convenience. We need have no intimate
knowledge of "what is in the black box" in order to use it properly. In
fact, even if we had full knowledge of all the particulars of the design, we
would still use only its required load resistance and power levels
associated with it modulation waveforms, etc.

Our modern amateur transmitters and amplifiers even have a convenient meter
on the front panel that tells us when we have met our obligation to provide
a 50+j0 ohm load. It may be labeled "SWR" and calibrated in an unusual
scale, but the important thing is that when it reads 0 or "1:1 SWR" that
tells us that we have met the load resistance obligation - nothing more or
less.

I think that a great deal of confusion over this whole issue comes from two
sources:

1. vague efforts to apply the infamous "Maximum Power Transfer Theorem" from
the early days in undergrad EE school; and

2. confusing an r-f transmitter output stage with the classical "signal
generator" with a dissipative 50-ohm internal resistance.

Forget both of those irritants and concentrate on the required load for the
transmitter, which the designer will provide and insist upon, and then
adjust the antenna system to provide that load and all will be well.

At no point will anyone, including the r-f amp designer in all likelihood,
know or even care what the so-called "internal resistance" of the amplifier
happens to be. He demands only one thing: the specified load resistance.
Given that, his design will deliver the required power, efficiency, heat
load, harmonic content, distortion levels, etc. etc.

I know of no instance in the design of everything connected to the output
port of the transmitter where there is need to know anything other than the
required load resistance for the amplifier and the power levels (average,
peak, etc.).

Why do folks make this so complicated, Ian?

73/72, George
Amateur Radio W5YR - the Yellow Rose of Texas
Fairview, TX 30 mi NE of Dallas in Collin county EM13QE
"In the 57th year and it just keeps getting better!"






----- Original Message -----
From: "Ian White, G3SEK"
Newsgroups: rec.radio.amateur.antenna
Sent: Thursday, August 14, 2003 1:42 AM
Subject: Length of Coax Affecting Incident Power to Meter?


Dr. Slick wrote:

As Roy says, the equations relating any one of these parameters to any
other are all well known. NONE of them ever involves source impedance.

Assuming the source impedance is 50 ohms, which it usually isn't
with most PAs.

NO - and this is the central point.

When Roy and I are saying:
NONE of them ever involves source impedance.
- that is exactly what we mean.

We didn't mean there is a hidden assumption about what the source
impedance is - we meant what we said: it isn't there at all, in any of
the equations we're talking about.

Look them up; and then go deeper and look at how they are derived. They
involve only the load impedance and Z0. That's "only", as in "no hidden
additives."


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek