Maximum power transfer occurs with a zero-ohm source.
Further, the impedance of the vast majority of good 50-ohm coaxial cable is
typically ± 3 %. Here is a quote for Amphenol RG-58U (from the Amphenol
www page for RG58):
[image: image.png]
So the typical impedance of this cable can vary between 51.5 and 48.5
ohms. That's not much and pretty insignificant when considering an "energy
pipe" between an ill-defined impedance of a loop vs. frequency and a
well-defined impedance vs. frequency of an amplifier. So, eliminate the
argument that the coax impedance is variable and impacts ultimate
performance of the loop(s).
Dave - W?LEV
On Tue, Jan 21, 2025 at 5:25?PM Patricio A. Greco via groups.io
<patricio_greco@...> wrote:
This is the typical case if mixing concepts.Its very crear that the
amplifiers don?t have a 50 ohm resistor inside , un less you live in a very
cold place and need some extra heating… My idea were to show that has no
sense to worry about resonance and minimum SWR in this case and also
consider the source impedance that differs of Z2 on VNA. The minimum SWR
that shows the VNA is a calculation using the calibration standards , If
you really would like to know the SWR over the transmission line there are
other methods because the coaxial cables has not exactly 50 Ohms and if we
condider the system as unidirectional the only purpose is to transfer the
maximum power to antenna the minimum SWR must to be measured over the
transmission line. In the other hand the output impedance of an amplifier
don’t only define his efficiency, this is other concept mixing.
Ing. Patricio A. Greco
Taller Aeronáutico de Reparación 1B-349
Laboratorio de Calibración ISO 17025 AREA: RF/MW
Gral. Martín Rodríguez 2159
San Miguel (1663)
Buenos Aires
T: +5411-4455-2557
F: +5411-4032-0072
www.servicios-electronicos.com.ar
On 21 Jan 2025, at 13:24, Fran?ois via groups.io <18471=
[email protected]> wrote:
You are right to ask yourself. There is one fundamental thing that you
forget, that the TX -> line -> Antenna system is only adapted in one
direction.
The antenna adaptation devices (box for example) only serve to bring 50
ohms to the TX terminals. Fortunately, the Tx does not have 50 ohms of
internal impedance but only one or two ohms, otherwise the efficiency would
be catastrophic.
If when looking back at your antenna you see 50 ohms, your antenna
looking towards the TX does not see 50 ohms.
On the other hand, once your agreement is complete with your coupling
box, if you turn the coupling box over, that is to say place 50 ohms in
place of the TX and look, with a NanoVNA, through the antenna base, you
will see the same impedance as that of your antenna. This is a way to
preset a remote adapter.
Keep in mind that if the generator impedance is fixed you will get the
maximum power out of it in a combined load (50% efficiency). But if you are
left free to choose the internal impedance of the generator, whatever the
load, the maximum power will come out of the generator if its internal
impedance is zero (100% efficiency).
--
F1AMM
Fran?ois
--
*Dave - W?LEV*
--
Dave - W?LEV
