On Tue, 14 Jun 2005 13:13:01 -0400, Buck wrote:

[snip]

I am not sure what I just did or saw. I had several colored graphs
charted and one gray. The gray didn't seem to move around, but the
colored ones sure did. I am afraid I didn't see what you were trying
to show me. I believe the colors were the different feed lines,
capacitors, etc used in the feedline shown. I tried removing them and
only leaving the feedline I chose to use. I don't know if I did
something wrong or not, but the gray line appeared to be the antenna
and it never appeared to move. The feedline markers moved radically.

I guess you're looking at the Smith chart in XLZIZL. What you are
seeing is correct. The gray trace is the load impedance and absent
changes in the antenna it will be fixed. If you look right to the
legend you will see what each of the colors represents.

So if you've modeled something in EZNEC and read the resulting data
file into ZLZIZL.xls and did a "Refresh and show Smith chart" you will
see the data plotted as the "Load"

Let's do a little (bit long) experiment that will clarify this (I
hope) and also demonstrate some of what happens in a matching network.

1. Open EZNEC and open the file "BYdipole.ez".

2. Go to "Setups" (at the top of the window) and select "Frequency
Sweep."

3. When the pane opens, select "On" and type in 14.35 for the "Stop
Frequency" and .05 for the "Frequency Step."

4. Under output. check the box "Microsmith Files" and for file name
type "g5rv" and hit Ok.

5. Go to the "Wires" menu and change the Y value for End 1 to -51.
Change the Y value for End 2 to 51.

6. Do a "Save As" and type "g5rv".

7. Perform a Freq Swp. (Bottom left button).

8. Open XLZIZL, clear any old data or networks and read the g5rv.gam
file.

9. Press "Refresh and show Smith chart."

10. The gray trace is the plotted impedance of the feedpoint of the
102' dipole over the 20-meter band.

11. Now at this point Varney (G5RV) recommended a "matching section"
of 34' of open wire line, followed by a run of 72 ohm line to the
shack. (If you want to see how bad this idea was, you can experiment
with adding the lines in the Network dialog)

12. We are going to do it the "modern" way, by using ladderline and a
tuner.

13. In the Network definition dialog, select element position 5 and
set the type to "Transmission Line", select "Wireman 554" and set the
length to 50'. Set the element and close the pane.

14. Press the "Refresh and show Smith chart." The red trace shows the
impedance at the input end of the 50' transmission line. Since the
desired target is the center of the chart, we can seen that this was a
negative "improvement."

Note: I didn't contrive this feeder length, it was just a round
number that seemed like an average situational value that follows the
"conventional wisdom" that says, "Use a length long enough to reach
from the antenna to the rig."

15. This is what we're stuck with if we decide to use ladderline and
a tuner, so let's muddle on.

16. In the ZIZL worksheet at row 38 there are some boxes for
Component Q. Enter 500 for Qu(C), 250 for Qu(L) and 14 for Q ref
freq.

17. Now let's explore a slick feature of this program and design a
matching network. Highlight the cells F106 and F107.

18. Look at cell I43 and press "Copy Selected R,X to T-Pi-L".

19. Focus will transfer to the T-Pi-L worksheet and a query window
may open asking whether you want to copy the Q values from the ZIZL
sheet. Answer Yes. You will no doubt get a second advisory pane that
says that frequencies differ. Answer "Ok". Cell H10 will be
selected. Type 14.2 into it and press Enter or Tab to input the data.

20. We are now presented with four built-in matching network
solutions. The most used topography for commercial tuners these days
is the "High Pass T." (Option 1) So we will use it. You can press
"Schematic" and see the configuration. Press the "Copy to FT/Solver"
button. (Cell B24)

21. The last action took the precalculated values and moved them to
the Fine Tune / Solver Area. For now, we won't explore the attributes
of this feature. Press the "Copy to ZIZL 1-3" button. (Cell K26)

22. Focus will return to the ZIZL sheet and a query pane will open.
Answer "No." If you look at the network definition table area you can
note that there are now three components in addition to the
transmission line in evidence.

23. Press "Refresh and show Smith chart."

24. Focus returns to the Smith chart where a wealth of information is
presented. In the upper right corner there is a box that shows
exactly what the feed system consists of. The "Plot Segments" box
shows the legend for each trace and what the effect of each network
component is. The informational box to the left will show the
parameters of any data point on any trace by just double clicking the
point.

Of course, the trace for Plot Segment 1, the input series C, is what
we are after. All but the 14.0 MHz point fall inside a 2:1 SWR
circle. Let's see if we can fix that.

25. Press the "Tune / Set" button (lower right corner). Long years
of experience tell me to tune the input capacitor first, so that's
what we'll do, but this may not always be the case. In the pane that
opens, select, "1:Series C", which if all of the calculations are the
same, should equal 51.2 pF. Mouse down a bit and find the +-1% option
and select it. We are going to "tune" the input capacitor in 1%
steps. Click the "Increase" up-arrow three times. Voila! The SWR is
less than 2:1 over the whole 20-meter band with one set of
adjustments. With each click you could see the action of "tuning"
this component. Note that none of the other traces moved at all. All
we changed was the match between the input and the junction of the
input C and the shunt L. The match from the junction to the antenna
was unaffected.

Because this is getting a bit long-winded and off-topic, I'm going to
stop here and pick this up in a new thread.



I haven't thought a lot about the losses in ladder-type feedline, but
I do understand how the losses in coax can reduce High SWR by reducing
both the outgoing and incoming reflected signals in the coax. This
may make the antenna desirable to the rig, but it doesn't do any good
for the operator who is trying to reach or copy that weak signal.

Correct.

It
may well be that the Twin-lead has the same effect.

It does.

I often read that
one of the advantages of the twin-lead is that it can handle higher
SWR without the higher losses of coax.

I seem to have been seeing a lot about the technology of the twin-lead
tuning technology, but then I can't be sure it wasn't from the same
people here on this forum. I'll continue to look into it to see what
I might learn.

Some of the people here know what they are talking about... others I'm
not so sure. I'm unlocking the tool box, so you can figure it out
yourself.


I was able to get past whatever block I had on learning to use EZNEC,
so at least some good came from this discussion. Hopefully, I'll
learn more and can model this antenna and others I read about.

I'm sure you will.


Thanks very much for taking the time to address this with me.

No problem. The guys that really deserve the thanks are the geniuses
1: who developed the NEC engine, 2: Roy, W7EL, (among others) who put
a pretty face on it, and 3: Dan Maguire, AC6LA, who did the Excel
stuff.

And the absolutely amazing thing is, they give away their work! Is
this a great country or what?

Regards,

Wes N7WS