microwave software®
MStrip+® User Manual
Transmission Line Analysis and Synthesis Utility
SUSPENDED SUBSTRATE MODULE
A suspended substrate transmission line is formed by etching a line
onto a thin sheet of dielectric
material, with the backside plating removed. The thin dielectric is only
there to function as a support
for the line etched onto its surface. Ideally, we would eliminate the
dielectric altogether if it weren't needed for mechanical reasons. The
"line" is then suspended, i.e.,
it lies upon a small ledge, within a housing, with air both above and
below it.
Introduction
Ideally, the etched line has zero thickness. However, in practice, excellent results are achieved with 1/4 or 1/2 oz. metallization. Note: 1 oz. = 0.0014 in. = 1.4 mils.
This type line structure is excellent for filters, multiplexers, etc. in the frequency range between (roughly) 5 thru 40 GHz. It offers low loss, and, because the effective relative dielectric constant is very close to that of air, the physical line dimensions are quite reasonable, even at 20 GHz, making manufacture easier.
Propagation of a signal down these lines is slightly dispersive, though not nearly as much as in microstrip. But, because of this mild "quasi-TEM" mode, you must still use an effective relative dielectric constant, ER', to compute line lengths. At the time of this writing, 1986, most suspended substrate design work is done using Rogers RT/duroid (Er=2.2), and Er' is approximately 1.1
The characteristic impedance, Z0, and the effective relative dielectric constant, ER', are a function of the following physical and electrical parameters.
- Line width
- Dielectric thickness
- Dielectric constant
- Metallization thickness
- Air gap top/bottom
- Side wall spacing
The Suspended Substrate Menu
| Suspended Substrate Menu
|
| (1) Synthesize
|
| (2) Analyze
|
| (3) Tabular Data
|
| (4) Main Menu
|
| Choose (1-4) ? _
|
Line Synthesis Mode
When use choose "(1) Synthesize" from the Suspended
Substrate Menu, in the same manner as before, the screen
display will look as follows:
Synthesis Mode
Enter Z0,A,H1,H2,H3,Er [<Enter>=Quit] ? _
Enter each of the six values requested, using a comma between each entry. The units are ohms and inches. You cannot enter any zeroes or spaces.
Let's synthesize a 50 ohm line on Rogers RT/duroid (Er=2.2) with a dielectric thickness of 0.005 in. (5 mils). The metallization is 1/2 oz. copper. The housing is 0.4 in. wide, and the top/bottom air gap is 0.035 in. (35 mils).
Enter the following data and then press <Enter>: The units are ohms and inches.
50,.4,.035,.005,.035,2.2
MStrip+ will then ask:
# of terms [Max=180/<Enter>=30] ? _
We've found that computing 30 terms of the infinite series expression for Z0 gives good results. However, if you don't mind a slightly longer computation time, MStrip+ will let you use up to 180 terms in many cases. This will improve accuracy slightly. For now, press <Enter> and then experiment later if you desire.
Note: At this point, you'll be asked "Print Logfile (Y/N) [<Enter>=No] ? _" Please press <Enter>.
If you choose the option to 'Print Logfile,' the data will be directed to 'Logfile.txt' within the \mwdata3 sub-directory, and to the screen. Right-click on 'Logfile.txt' to print it, and then DELETE the file; it will re-create when next needed.
MStrip+ will display the input data on-screen, IF you pressed <Enter>, as follows:
| MStrip+ SS Synthesis
|
| Input Data
|
| Z0
= 50 ohms (# Terms = 30)
|
| A
= .4 in.
|
| H1
= .035 in.
|
| H2
= .005 in.
|
| H3
= .035 in.
|
| Er
= 2.2
|
| Iterating
|
| Z0 = 54.04757308959961
|
| Z0 = 50.22720336914062 |
| Z0 = 50.00232696533203 |
| Z0 = 50.00032424926758 |
| Z0 = 49.99989318847656 |
| Z0 = 49.99990081787109 |
| Output Data
|
| W = 0.1041 in.
|
| Er' = 1.0779
|
| - Press any Key - |
Line Analysis Mode
When use choose "(2) Analyze" from the Suspended
Substrate Menu, the screen display will
look as follows:
Analysis Mode
Enter W,A,H1,H2,H3,Er [<Enter>=Quit] ? _
The format is the same as before; the only difference is that we substitute W for Z0. Try this mode working backwards with the data from the last example.
The on-screen output should be as follows :
| MStrip+ SS Analysis
|
| Input Data
|
| W =
.1041 in. (# Terms = 30)
|
| A
= .4 in.
|
| H1 = .035 in.
|
| H2 = .005 in. |
| H3 = .035 in. |
| ER = 2.2
|
| Output Data
|
| Z0 = 50.0023
ohms
|
| Er' = 1.0779
|
| - Press any Key - |
Follow the on-screen instructions to "- Press any Key -" and return to the Suspended Substrate Menu.
Tabular Data Mode
When use choose "(3) Tabular Data" from the Suspended
Substrate Menu, the screen display will
be as follows:
| Tabular Data
|
| (1) Z0 vs A/W
|
| (2) SS Menu
|
| Choose (1-2) ? _ |
A/W is the ratio of side wall/line width. Values for A/W must be >1
The following is a tabulation of Z0 vs A/W on 5 mil Rogers RT/duroid, (Er=2.2), with 35 mil air gaps. Notice the influence of wall spacing on Z0.
Choose "(1) Z0 vs A/W" from the Tabular Data menu.
Z0 vs A/W Mode
Enter A/W Start, Stop, Step [<Enter>=Quit] ? _
Type the following and press <Enter>.
1.2,3,.2
Next, enter the line data as requested by the prompt and press <Enter> once again.
.035,.005,.035,2.2
Your on-screen display will be as follows:
| MStrip+ SS Z0
vs A/W Analysis
|
| Input Data
|
| H1 = .035 in.
9# Terms = 30)
|
| H2 = .005 in.
|
| H3 = .035 in.
|
| Er
= 2.2
|
| Output Data |
| A/W
Z0
Er'
|
| 1.2 46.5954 1.1891 |
| 1.4 54.4387 1.1449 |
| 1.6
58.4857
1.1231
|
| 1.8
60.6543 1.1108
|
| 2.0
61.8297
1.1036
|
| 2.2
62.4637 1.0993
|
| 2.4
62.7806 1.0966
|
| 2.6
62.9717 1.0950
|
| 2.8
63.0322 1.0940
|
| 3.0
63.0682 1.0933
|
| - Press any Key - |
Follow the on-screen instruction to " - Press any Key - " to return to the Tabular Data menu.