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Copyright: 1996 - 2024

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RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The World Wide Web (Internet) was largely an unknown entity at the time and bandwidth was a scarce commodity. Dial-up modems blazed along at 14.4 kbps while typing up your telephone line, and a nice lady's voice announced "You've Got Mail" when a new message arrived...

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RF Cafe Forums closed its virtual doors in 2012 mainly due to other social media platforms dominating public commenting venues. RF Cafe Forums began sometime around August of 2003 and was quite well-attended for many years. By 2010, Facebook and Twitter were overwhelmingly dominating online personal interaction, and RF Cafe Forums activity dropped off precipitously. Regardless, there are still lots of great posts in the archive that ware worth looking at. Below are the old forum threads, including responses to the original posts.

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Laser
 Post subject: Measuring a Transmitter´s output Impedance components
Posted: Wed Dec 05, 2007 11:14 pm 
 
Lieutenant
User avatar

Joined: Wed Dec 05, 2007 10:09 pm

Posts: 2

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.

Thanks


 
   
 
Feng
 Post subject: I gave you my suggestion in Circuit board.
Posted: Thu Dec 06, 2007 4:05 am 
 
Captain
User avatar

Joined: Sun Mar 19, 2006 5:25 pm

Posts: 23

Location: Burnaby

Hi Laser,

I gave you my suggestion in Circuit board where you posted your question first. Hope it is useful.


Posted  11/12/2012

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