On Apr 2, 5:21 pm, Roy Lewallen wrote:
dykesc wrote:
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'm afraid that your model isn't adequate for the antenna. In an off
center fed antenna, there will be considerable common mode feedline
current -- in other words, the currents on the two feedline conductors
won't be equal and opposite, so the feedline will radiate and is an
integral part of the antenna. A model has to include the feedline (as
two wires, not as a transmission line model which doesn't radiate) in
the correct orientation relative to the antenna, plus the entire path to
ground from the transmitter. This path is seldom known. A simpler model
which ignores the very significant contribution of the radiating
feedline won't give meaningful results.

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.

This is not at all true. I'm guessing you're misinterpreting what's
written there. If not, and it really says that, it's an error which
should be corrected.

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?

L network values for matching are best done by beginning with the
required impedances at both ports, then directly calculating the
necessary component values. A number of programs are available to do
this calculation for you, or if you prefer, it can be done quite easily
graphically with a Smith chart.

Roy Lewallen, W7EL

Thanks for your reply Roy and, while I'm at it, thanks for your
continued contribution to Amateur Radio.

I didn't mention that I have a homebrew 1:1 current balun installed at
the antenna feedpoint to eliminate common mode currents which would
otherwise result from the off center feed. Would my EZNEC model still
not be adequate? (I also didn't mention the 4:1 balun in my antenna
tuner which I am accounting for in my comparison.) Also there is
probably some coupling of the feedline to the antenna field due to
geometry, but I tried to keep the feedline perpendicular to the
feedpoint as much as possible. I believe I read in the HELP file that
EZNEC does not model this type of coupling. Am I correct on this
point? I am not experiencing any issues with radiation off the
feedline at my operating location, so I believe I've done a pretty
good job with the balun and feedline routing.

Would you still recommend I don't pursue modeling of my OFC with EZNEC
at least as far as transmission line and feedpoint parameters go. How
about radiation patterns given the additional info in this post?

If I model the antenna feed with two additional wires will the limit
on segments in the demo version of EZNEC start having an appreciable
impact on the quality of results? I'm probably going to get the full
version anyway but I am curious about the affect that segment limits
has.

One more question. Is there a way to set antenna orientation in EZNEC
so that it coincides with my actual antenna end to end compass
bearings? This would allow me to use the "bearings" option in EZNEC
directly. It appears I have to measure X - Y offsets to do this and
can't use a simple circular (degrees) input in EZNEC to do this.

All of this is just an outfall of my interest in RF antenna
technology. The antenna is operating fairly well for stateside
operation.

Thanks again.

Dykes Cupstid AD5VS