Somehow, and using Nec2, I seem not always able to get expected
impedance values when using the results produced by L.B. Cebik's stub
matching software at http://www.cebik.com/trans/ant-match.html

Does anybody know a source for (similar) stub matching software
(freeware or evaluation), so I can check where things go wrong ?

Thanks in advance,
Arie.

4nec2 wrote:
Does anybody know a source for (similar) stub matching software
(freeware or evaluation), so I can check where things go wrong ?

I still use ARRL MicroSmith for such. EZNEC will
also do some stub stuff for you.
--
73, Cecil http://www.w5dxp.com

On 6 Dec 2006 03:15:54 -0800, "4nec2" wrote:

Somehow, and using Nec2, I seem not always able to get expected
impedance values when using the results produced by L.B. Cebik's stub
matching software at http://www.cebik.com/trans/ant-match.html


Arie,

The versions of NEC that I have used all treat transmission line
elements defined as such as lossless, and so the results will be in
error to some extent, depending on the scenario. Allowing for that
error, models I have built and checked appear correct.

The spreadsheet by L B Cebik linked from the page above depends on an
assumption that Z is real for some of the match types, and again
assumes that the line sections are lossless (implying Zo is real).

The assumption of lossless lines might often be acceptable for open
wire lines, but less so for common commercial coaxial lines.

My experience with stub matching is that significant trimming of the
theoretical design is usually necessary. I suspect tolerances of
transmission line parameters is a large contributor to the error, and
some stub designs become extremely sensitive to small changes in
components. Assuming that stub matching is 100% efficient (ie that
line sections are lossless) is another significant contributor.

Does anybody know a source for (similar) stub matching software
(freeware or evaluation), so I can check where things go wrong ?


The line loss calculator at http://www.vk1od.net/tl/tllc.php will
calculate the transformation in series line sections of about 100
common line types, having regard for line loss. Impedances are dealt
with as complex quantities. This also allows calculation of the input
impedance of an o/c or s/c stub by specifying an appropriate
termination impedance. The line loss model used is described on the
page above.

For example, what is the impedance of a quarter wave stub of RG58C/U
at 14MHz? You know it cannot be infinite, but that is what lossless
estimates will produce. The calculator above suggests that 3.53492m of
Belden 8262 s/c stub has an input impedance of about 2300+j0 ohms. At
7MHz, the same stub would have an input impedance of about 2.3+j49.96
(a quite lossy equivalent inductor) where lossless estimates would
suggest 0+j50.

A useful feature of a "Smith chart program" would be a decent
transmission line loss model for transmission line segments. Most that
I have seen seem to ignore it, Winsmith allows specifications of line
loss, but it is tedious and nevertheless only allows specification of
real values for Zo.

XLZIZL and the derived TLDetails are applications that consider the
line loss in calculations (but whilst estimating Xo, assume that Ro is
equal to nominal Zo). There are other programs that use a similar
approximation but seem to have unrealistic built in line loss data.

Owen
--

On 6 Dec 2006 03:15:54 -0800, "4nec2" wrote:

Somehow, and using Nec2, I seem not always able to get expected
impedance values when using the results produced by L.B. Cebik's stub
matching software at http://www.cebik.com/trans/ant-match.html

Just working the example of Bramhams solution in LB's spreadsheet with
http://www.vk1od.net/tl/tllc.php :


Line 2:
Belden 8262,
length=0.848*0.66=0.560m,
Zload=50+j0,
Zin=58.34+j20.44

Line 1:
Belden 9204,
length=0.848*0.66=0.560m,
Zload=58.34+j20.44,
Zin=75.22-j1.33

Seems close enough!

Which match type did you have problems with?

Owen
--

Hello Owen, thanks for the comprehensive explanation.

Which match type did you have problems with?

The 'problem' however was not directly related to lossy lines, but more
about why Nec2 would not correctly model the results as achieved by
Cebik's spreadsheet. This because both use lossless lines. I can
imagine the outcome has a spread around 50 ohms or so, I did not expect
results around 20 ohms or even worse.

At the moment I do not have the corresponding models available
overhere. I'll post them tomorrow, maybe someone else can figure out
what I am doing wrong...

Arie.

4nec2 wrote:
Somehow, and using Nec2, I seem not always able to get expected
impedance values when using the results produced by L.B. Cebik's stub
matching software at http://www.cebik.com/trans/ant-match.html

Does anybody know a source for (similar) stub matching software
(freeware or evaluation), so I can check where things go wrong ?

Thanks in advance,
Arie.

Arie,

If you are using the 1/4 wavelength series section worksheet, there is
a factor of four error. Instead of calculating a 1/4 wavelength line,
it calculates a full wavelength. If you will divide each of the values
in cells G10 through G12 by 4 you will get the correct line length.

The Beta matching worksheet seems to be correct. I use stub matching
on my 20-meter Yagi and the modeled values and my measured values are
remarkably close. The stub length given in the worksheet correlates
fine with the model.

Regards,

Wes N7WS

I could not find my original data, so I create a new set to try to show
where I have a 'problem'.

Below, a random short inverted-V for 80, delivering a Z-in of around
16.3 - j 826 ohms

CM Short Inverted V for 80 Mtr.
CE
GW 1 25 -9.829825 0 14.117083 -0.3 0 21 5e-4
GW 2 3 -0.3 0 21 0.3 0 21 5e-4
GW 3 25 0.3 0 21 9.8298245 0 14.117083 5e-4
GE
LD 5 0 0 0 58000000
GN 2 0 0 0 14 .006
EX 0 2 2 0 1 0
FR 0 1 0 0 3.6
XQ

************************************************** ******

Next, the data that was entered in the Cebik spreadsheet, Tab E
http://www.cebik.com/download/ant-match.xls

Using 50 ohm 'coax' (0.66) as feed/match line and 300 open-line (0.8)
as stub.

Yes, a strange combination, but I choose these to get exceptional
results.

Antenna Load Resistance RL 16.3 Ohms
Antenna Load Reactance XL -826 Ohms
Frequency in MHz FQ 3.6 MHz
Zo of Matchline ZL 50 Ohms
Velocity Factor of Matchline VFL 0.66
Zo of Main Feedline ZF 50 Ohms
Velocity Factor of Feedline VFF 0.66
Zo of Stub ZS 300 Ohms
Velocity Factor of Stub VFS 0.8

Match-Line Length A 84.56 degrees 12.910 meters ----
Shorted stub length 0.33 degrees 0.061 meters
Open stub length 90.33 degrees 16.716 meters ----

Match-Line Length B 88.51 degrees 13.513 meters
Shorted stub length 179.67 degrees 33.249 meters
Open stub length 89.67 degrees 16.594 meters


Electical lengths (ant. to stub) 12.910/0.66 = 19.5606 mtr
(stub itself) 16.716/0.8 = 20.895 mtr

************************************************

Below the Nec-file with Tr-Line lengths as calculated above and a third
50 ohm line from
stub junction to the house.

Delivering a Z-in of 0.93 + j 24.2 ohms ?, If you ask me this should
have been around 50 ohms.

CM Short Inverted V for 80 Mtr.
CE
GW 1 25 -9.829825 0 14.117083 -0.3 0 21 5e-4
GW 2 3 -0.3 0 21 0.3 0 21 5e-4
GW 3 25 0.3 0 21 9.8298245 0 14.117083 5e-4
GW 989 1 -0.1 0 0.5 0.1 0 0.5 5.e-4
GW 998 1 -0.1 16.72 8.09 0.1 16.72 8.09 5.e-4
GW 999 1 -0.1 0 8.09 0.1 0 8.09 5.e-4
GE
LD 5 0 0 0 58000000
GN 2 0 0 0 14 .006
EX 0 989 1 0 1 0
FR 0 1 0 0 3.6
TL 2 2 999 1 50 19.5606
TL 999 1 998 1 300 20.9
TL 999 1 989 1 50 7.59
XQ

Arie.

On 8 Dec 2006 05:05:33 -0800, "4nec2" wrote:

....
Below the Nec-file with Tr-Line lengths as calculated above and a third
50 ohm line from
stub junction to the house.

Delivering a Z-in of 0.93 + j 24.2 ohms ?, If you ask me this should
have been around 50 ohms.

CM Short Inverted V for 80 Mtr.
CE
GW 1 25 -9.829825 0 14.117083 -0.3 0 21 5e-4
GW 2 3 -0.3 0 21 0.3 0 21 5e-4
GW 3 25 0.3 0 21 9.8298245 0 14.117083 5e-4
GW 989 1 -0.1 0 0.5 0.1 0 0.5 5.e-4
GW 998 1 -0.1 16.72 8.09 0.1 16.72 8.09 5.e-4
GW 999 1 -0.1 0 8.09 0.1 0 8.09 5.e-4
GE
LD 5 0 0 0 58000000
GN 2 0 0 0 14 .006
EX 0 989 1 0 1 0
FR 0 1 0 0 3.6
TL 2 2 999 1 50 19.5606
TL 999 1 998 1 300 20.9
TL 999 1 989 1 50 7.59
XQ


Arie,

Your design is for an o/c stub. Haven't you modelled a s/c stub by
connecting the transmission line to W998?

I have checked your proposed design on Winsmith, and it gives a 50
ohms input impedance. Nevertheless, it is hypothetical as it is not
practical to connect a 300 ohm stub to 50 ohm line, 50 ohm line will
be coax (in practical cases) and it is not practical to make a 300 ohm
coax. If you were to mix coax and open wire line, you have a serious
balance issue that you have not dealt with in your model.

The other thing about the design is that two of the transmission line
elements are very close to 90 deg in length, which means that
operation is very sensitive to frequency change.

If you really want to make a 40m inverted v work well on 80m, you
might have to match at the feedpoint. It is hard to get an unloaded
dipole of length 25%wl to work well without matching at the feedpoint.

Owen
--

Hello Owen, excuses for the delay, because I was caught by a very bad
cold...

Your design is for an o/c stub. Haven't you modelled a s/c stub by
connecting the transmission line to W998?

No, at least it was not meant to be. When TL cards are specfivied
without
additional real- and imaginary shunt admittance values they are to be
considered as an open end, although a dummy wire/segment is need
for connecting the TL end.

I have checked your proposed design on Winsmith, and it gives a 50
ohms input impedance.

Aha, nice to know. It seems the Cebik program is not in error.

Nevertheless, it is hypothetical as it is not
practical to connect a 300 ohm stub to 50 ohm line, 50 ohm line will
be coax (in practical cases) and it is not practical to make a 300 ohm
coax. If you were to mix coax and open wire line, you have a serious
balance issue that you have not dealt with in your model.

Yes, I know.

Who told you that the 50 ohm line was not symmetrical... (hi, hi, joke)

The other thing about the design is that two of the transmission line
elements are very close to 90 deg in length, which means that
operation is very sensitive to frequency change.

That's an interesting aspect of the situation.

If you really want to make a 40m inverted v work well on 80m, you
might have to match at the feedpoint. It is hard to get an unloaded
dipole of length 25%wl to work well without matching at the feedpoint.

The specified problem was only hypotetical, and does not exist in real
life (at least overhere).

Thank you, and the other posters, for the feedback,
Arie.

On 8 Dec 2006 05:05:33 -0800, "4nec2" wrote:

....
Below, a random short inverted-V for 80, delivering a Z-in of around
16.3 - j 826 ohms
....

Just working through a solution using a single o/c stub match using
Cebik's calculator for Wireman 551 ladder line, Zo=400, vfr=0.902.

Length of series section is 13.029m, length of stub is 19.163m. This
is correct for lossless line.

Now lets look at the Y looking into the series section using the line
loss calculator at http://www.vk1od.net/tl/tllc.php , it is
0.030478+j0.067004. Now shunt that with the Y looking into the o/c
stub of 0.017872-j0.076378. Add them for a Y of 4.835E-002+j0.143382,
or Z of 2.11171146485231-j6.26228362468365 or VSWR of 22:1.

What went wrong?

In some cases, assuming lossless lines is not valid, and you don't
really know if you don't calculate the effects of the loss.

Owen

PS: the correct solution will be a slightly shorter series section and
a slightly longer stub. It is still not a good (practical) solution.
--