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Comparing Tuner Match to EZNEC Impedance Prediction
I'm fairly new here so if this post is not appropriate for this group,
direct me elsewhere. I am using the EZNEC demo to analyze an Off Center Fed sloper I am using on 75m. The antenna is being fed with 85 feet of 450 ohm ladder line which runs all the way to my operating location. At 3.9405 Mhz, EZNEC shows an impedance of 28.11 -j102.2 at the transmission line input (Alt Z0 of 450 ohms). My MFJ auto tuner (L-Network) is actually matching the sloper at a 1.1 SWR with an inductor value of 5.34 uH and a capacitor value of 605 pf. The capacitor is shunted on the transmitter side of the inductor. I am using a formula I found in the L-Network section of Chapter 25 of the ARRL Antenna Book to take the MFJ tuner values above and calculate the impedance the tuner is matching. There are a couple of statements in this section of the Antenna Book which lead me to believe that L- Networks are used for matching purely resistive loads. It also states that loads containing "reactance" can be accounted for in the L- Network inductor or capacitor values. Do I need to eliminate the portion of the tuner inductor value being used to bring the system to resonance before I calculate the impedance transform? If so is there a way to do this or is my whole approach here off base? Thanks for any replies. Interesting stuff. |
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