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Measuring a Transmitter's Output Impedance Components - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

Amateur Radio | Antennas | Circuits & Components | Systems | Test & Measurement

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
Posts: 1
Hi everyone.
I was asked to develop a linear amplifier for an RC transmitter (35MHz).
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 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 inductive reactance.
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
Posts: 21
Location: Burnaby
Hi Laser,

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
Posts: 5
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.

Posted  11/12/2012

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