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module: My Reflection Sucks - RF Cafe Forums

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Kirt Blattenberger
Post subject: module: My Reflection Sucks Posted: Tue Mar 21, 2006 8:33 pm

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Joined: Sun Aug 03, 2003 2:02 pm
Posts: 308
Location: Erie, PA
NOTICE: I replicated the post of user "module" while moving it to the database on the new server. - Kirt Blattenberger

Posted: 21 Mar 2006 06:28 pm Post subject: My Reflection Sucks


Hello Everyone,

I have been a good person and tried to design my PA around my load line. Our establishment has this bad habit of "just dropping transformers in". I would actually like to try to match my input impedance using that stuff called math. I have calculated my loadline and designed my transformer using UT-34-50 semi rigid coax. Can someone do a quick check on my equations? I hooked it up to my network analyzer and I get nasty reflection. I am trying to match the input of a Polyfet SQ721 to a 50Ohm source. I am using Amawave's UT-34-50 50Ohm semi-rigid coax with a Amidaon BN-61-2402 with an AL=150. My math is here done in Mathcad. Any feedback would be greatly appreciated.


- Kirt Blattenberger
RF Cafe Progenitor & Webmaster


Post subject: ReflectionPosted: Wed Mar 22, 2006 8:53 pm


Joined: Wed Feb 22, 2006 3:51 pm
Posts: 104

I guess I'm not clear on what you're doing.

A quick trip to a Smith chart, matching 50 Ohms to 10-j65 Ohms at 88 MHz, yielded a two-inductor matching solution. One inductor of 45 nH from the amplifier input to ground, and a 83 nH inductor from the amplifier input to the power FET gate. I'm assuming the source is grounded.

In 45nH
o--UUUU-----------| gate
Gnd ``` 83 nH

(ignore the ` characters - PHPBB doesn't let me use spaces for ascii art - which I'm not good at anyway!)

Maybe that's not what you meant. Maybe you meant something like the series 45 nH inductor, with a shorted stub of about VF*10 inches of 50 Ohm coax in parallel (in place of the 83 nH inductor)?

Or did you mean a 1:5 transformer, with approximately 517 nH inductor in series with the input winding?

It doesn't look like I'm "getting it". What configuration is your input matching network? Are we even "on the same page"?


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