Measuring a Transmitter´s output Impedance components - RF Cafe
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Post subject: Measuring a Transmitter´s output Impedance components
Posted: Wed Dec 05, 2007 11:24 pm
Wed Dec 05, 2007 10:09 pm
I was asked
to develop a linear amplifier for an RC transmitter (35MHz).
transmitter uses a simple telescopic antenna, and now I will have to
design the amplifier and adapt it´s output to a 50ohm transmission line
So far so good, but then, as I started to analyze the
problem, things started to complicate a little.
In order to adapt
the amplifier´s input to the transmitter´s output, I need to know the
transmitter´s output impedance components (resistive and reactive parts).
The resistive part should be something much lower then 50ohm, which
is the typical impedance of short whip antennas (antennas bellow resonance).
I also know that short whip antennas present a natural capacitive
reactance, and to counteract this, the transmitter has an inductive
reactance part on it´s output to balance with the capacitive reactance
of the antenna.
This is called Conjugate Matching and in order to
adapt my amplifier´s input, I also need to know the magnitude of this
Now, my question is:
How do I measure these
values without using high priced equipment, like network analyzers?
Are there any techniques to do this, with an outside test setup
made of variable capacitors, inductors and resistors and then measuring
phase shifts, voltages and currents in some way on this setup, with
an oscilloscope, that will allow us to calculate the resistive and reactive
parts of the transmitter´s output impedance?
My intuition tells
me that this is possible, but how?
I´ll appreciate any help.
Post subject: You may need a
directional coupler.Posted: Thu Dec 06, 2007 3:50 am
Joined: Sun Mar 19, 2006 5:25 pm
The purpose of impedance matching is to reduce
the reflection wave as less as possible. You may try to use a directional
coupler to obtain the incidence and reflection wave. So if the performance
of your oscilloscope meets the requirement of measurement speed, you
can get their amplitude and phase information. Of course, in order to
achieve a higher precise phase result, the measurement cables should
be identical. Besed on these inforamtion, you can figure out the problem.
Hope it is little useful and good luck.
Post subject: Posted: Mon Jan 14, 2008 6:22 pm
Joined: Fri Feb 02, 2007 5:22 pm
Location: Overland Park, KS
Do you really need to match it? Gain
is cheap when power and noise are not a concern. Maybe just use a 6
dB attenuator and gain block. You'll get decent return loss and good