[TowerTalk] RG-149: 50 ohm/70 ohm - does it matter?

[Jim Lux]

On 12/14/13 10:23 AM, Dan Schaaf wrote:
> Tom, the tuner does not fool me. I know exactly what it does. It
> presents a 50 ohm impedance to my transmitter.
>
> My antennas are resonant at one frequency per band, and of course
> deviate from resonance either +/- from that point. So, to get max
> usefulness across the band(s) I use the tuner. In my humble opinion, the
> least amount of reflected power getting back into my transmitter, the
> better for longevity of the transistors.
>
>


Interestingly, the output Z of your amplifier is probably not 50 ohms 
resistive in any case, but most autotuners don't "match to 50 ohms", 
they "match to minimum reflected power" or "match to minimum VSWR", so 
they take out the effect of changing amplifier Z too.

As W9AC pointed out, you need to look at the whole system in any case. 
Minimizing reflected power might not be the same as maximizing radiated 
power, which is really what you're interested in. (after all, a 30 dB 
pad will minimize reflected power).


And when you start throwing in aspects of "how much power is dissipated 
in the active device" it gets even hairier.  While we speak as if the 
ampliifer is a Thevenin equivalent of a voltage source and a series 
impedance (or current and parallel impedance), and the maximum power 
transfer occurs when the source and load impedances are conjugate, that 
is NOT necessarily the maximum DC to RF efficiency point or the "minimum 
power dissipation in the active device" point.

People speak of the reflected power from a mismatch being dissipated in 
the source and making it hot, but that's not necessarily the case.  It 
could be dissipated in resistive losses in a resonant network, it could 
be fed back to the power supply (something you can make use of in phased 
arrays if your amplifier has full 4 quadrant capability.. the power in 
those negative impedance elements can be recycled to the positive 
impedance elements).

A lot of ham lore is based on the behavior of fairly simple triode 
amplifiers with some simplifying assumptions as far as linearity goes, 
and with a plate tank that stores energy.

Real amplifiers are more complex.  Some are basically voltage sources 
(like a DC power supply) and their output is basically voltage limited: 
  It will put the same voltage into any impedance.  Some are current 
sources.  And their efficiency varies all over the place depending on 
where they are in their operating curves.