Measuring a Transmitter's Output Impedance Components - RF Cafe
Post subject: Measuring a Transmitter´s output Impedance
components Posted: Wed Dec 05, 2007 11:24 pm
Joined: Wed Dec 05, 2007 10:09 pm
I was asked to develop a linear amplifier for an RC transmitter
This 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 and antenna.
So far so good, but
then, as I started to analyze the problem, things started to complicate
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
This is called Conjugate Matching and in order
to adapt my amplifier´s input, I also need to know the magnitude
of this inductive reactance.
Now, my question is:
I measure these values without using high priced equipment, like
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
My intuition tells me that this is possible,
I´ll appreciate any help.
Post subject: You may need a directional coupler.Posted:
Thu Dec 06, 2007 3:50 am
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
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 isolation.