On 24 mayo, 17:30, Owen Duffy wrote:
lu6etj wrote in news:62702483-6d3c-43e0-aa8b-
:





Hi all. I have a doubt about if this text from "Another look at
reflections" reflect the author ideas.

"Laboratory and experimental work often requires holding incident
voltage constant with variation in loading. A constant-voltage source
is usually obtained for this purpose by inserting a pad having 15 to
20 dB attenuation between the generator and the line to absorb the
reflected power, preventing it from reaching the generator where it
would alter the line coupling and cause the generator output voltage
to vary. Because of the absorption of the pad, the generator sees a
nearly perfect match for all load conditions and all reflected power
is lost - but these are laboratory control conditions required to
obtain valid test data."

For example this part = "by inserting a pad having 15 to 20 dB
attenuation between the generator and the line TO ABSORB the reflected
power"

I would not refer to the cascading of a practical source with a 15 to
20dB attenautor at a 'constant-voltage source' as Walt did above.

A Thevenin source with Zs=50+j0, with varying load impedances, will
always have terminal v/i such that the Vfwd component of a traveling wave
decomposition of v is constant. I give the explanation / proof athttp://vk1od.net/blog/?p=1028.

Even if a given source has Zs not equal to 50+j0, a sufficiently large
cascaded 50 ohm attenuator will result in a combination that is very
close to the ideal Zs=50+j0, albeit at lower level. This is a common
technique for 'standardising' the equivalent source impedance of a
source.

Owen- Ocultar texto de la cita -

- Mostrar texto de la cita -

Thank you for your answer Owen (but I think that it doesn't answer my
specific question, possibly because I am not able to express it well
due to my translation to English).
......
I agree with what you say in your answer and several things you write
in your good article "Is Zs of to HF ham tx typically 50+j0? ".

For example that for an idealized Theveninj generator, the incident
power is independent of the load impedance (net power Pf-Pr not)
(If an impedances adapting device is inserted between the line and
generator and we insert directional wattmeter between the matching
device and line, of course Pf varies and Pnet also do it).

Regarding that output impedance of a rig be quite near to 50+j0 I
think that depends on the concrete design; such a condition is not
necessary and in my opinion neither optimal.

Best regards

Miguel Ghezzi LU6ETJ