"dykesc" wrote in news:1175497378.760848.33570
@l77g2000hsb.googlegroups.com:

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.

Your values, if they are correct, suggest that Z is more like 18-j211
(depending on the Q of the coil).

Your transmission line may not be the 450 ohms that you think, and is a
likely contribution to error.

EZNEC probably models lossless transmission lines, and probably assumes
Zo is resistive. Combined with probable error in Zo, the input impedance
of the line may be different to model. Further, the feedpoint impedance
may be different to model, modelling an off centre fed antenna is
problematic.


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

No, they are not restricted to resistive loads.

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.



Owen