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.

"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

Owen Duffy wrote in news:Xns9906B0A8B259Enonenowhere@
61.9.191.5:

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

Sorry, I keyed a mistake in frequency in my analysis:

Your values, if they are correct, suggest that Z is more like 32-j110
(depending on the Q of the coil).

On Apr 2, 2:25 am, Owen Duffy wrote:
Owen Duffy wrote in news:Xns9906B0A8B259Enonenowhere@
61.9.191.5:

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

Sorry, I keyed a mistake in frequency in my analysis:

Your values, if they are correct, suggest that Z is more like 32-j110
(depending on the Q of the coil).

Thanks Owen. If not to much trouble, could you tell me how you derived
Z from my values?

"dykesc" wrote in news:1175504328.939428.90870
@n59g2000hsh.googlegroups.com:


Your values, if they are correct, suggest that Z is more like 32-j110
(depending on the Q of the coil).

Thanks Owen. If not to much trouble, could you tell me how you derived
Z from my values?

Nothing too sophisticated, I plugged them into a Smith chart program and
played with the load until it matched to approx 50+j0. It could be done
with a hand calculator nearly as quickly.

BTW, you might find the line loss calculator at
http://www.vk1od.net/tl/tllc.php helpful, you can specify the Z looking
into the line, or at the load end of the line, and it will calculate the
other for you (using a lossy line model).

Owen

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

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

dykesc wrote:

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.

EZNEC does model this type of coupling from wire to wire, but no
coupling to a transmission line model. So the coupling will be accounted
for only if you model the transmission line as wires.

Keeping the feedline perpendicular to the antenna isn't sufficient to
eliminate coupling when the feedpoint is offset, since the line will
still get unequal coupling from the two antenna sides. About the only
way to positively reduce it to a small value is to use multiple current
baluns spaced about a quarter wavelength or closer. The common mode
current might otherwise be high or low depending on the feedline length
and the length of the path to ground. Without modeling the entire path,
the accuracy of the EZNEC model will depend on how successful you've
been in reducing the common mode current to a low value. It's often
difficult with an off-center fed antenna.

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?

I'd always be suspicious of the result unless I verified that the common
mode current on the line is low. You can, of course, measure it with a
simple transformer made from a clamp-on ferrite core clamped over both
feedline wires. Use about 10 turns for the secondary, and terminate the
secondary with 50 ohms. That also allows you to put the detector at a
distance, connected to the transformer with 50 ohm line, with the
termination at the detector end.

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.

You'll probably start getting a noticeable change in results when the
segment lengths exceed about 0.1 wavelength.

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.

You can either define the antenna wires to be in the correct orientation
to begin with, or you can define them in a convenient way then use the
Rotate Wire feature to rotate them to the desired orientation. If you
prefer to work with compass direction rather than degrees CCW from the X
axis, you can chose this in the Options Menu (Angle Convention selection).

Roy Lewallen, W7EL