|
CONTENTS
1. Overview
2. Entering Impedances
3. Environment Parameters
4. NETCALC Operations
5. Example - a Pi-Tank
6. NETCALC with Windows, Mac
7. Downloads
End, Back |
NETCALC
I wrote NETCALC to meet a practical need for a simple impedance calculator
for working out RF network design ideas. I hope you find it useful.
Here's how it works.
NETCALC uses a "stack" system (like the one used in HP calculators) because
that is the most powerful and flexible way to implement a simple computer-based
impedance calculator.
NETCALC displays two complex impedance registers, labeled X and Y. These
impedances each consist of two terms: resistance and reactance. You can do
things to the X impedance alone, or combine the X and Y impedances in various
ways.
? is NETCALC's prompt for input. At
the prompt you have three options:
- Enter the value of an impedance into the stack; or
- Carry out operations on impedance values already in the stack; or
- Change one of the "environment" parameters (frequency, system impedance
and velocity factor).
The results of the operations are displayed, and then you are prompted again
by ? . |
|
CONTENTS
Top
1. Overview
2. Entering Impedances
3. Environment Parameters
4. NETCALC Operations
5. Example - a Pi-Tank
6. NETCALC with Windows, Mac
7. Downloads
End, Back
|
Entering Impedances
Impedances are input and displayed in either series or parallel form.
- a+b means resistance a in
series with reactance b.
- a&b means resistance a in
parallel with reactance b.
You must ALWAYS specify both the resistive AND the reactive
part, even if one of them is zero.
In NETCALC's impedance notation, the + sign in a+b does NOT indicate
that reactance b is positive; it means only that a and b are series-connected.
An impedance involving a negative reactance is indicated by a+-b or
a&-b.
NETCALC understands the normal "k", "m" and "M" multipliers - in fact the
whole range from "pico" to "Giga". Simply enter the first letter.
Only "m" and "M" are sensitive to Shift or Caps Lock. For the "micro" (mu)
symbol, type "u" or "U" . The largest and smallest displayable numbers are
0.001p and 999.999G - a wide-enough range for most applications.
A reactance can also be input in terms of capacitance or inductance, using
the notation a+bpF, a&buH, etc.
If NETCALC hasn't been told any differently, it assumes that the frequency
is 1Hz - see later for instructions on changing frequency.
NETCALC will then calculate the reactance, and will enter the correct impedance
into the X register.
A "j" before the reactance value is not necessary. Use it if you find it
helpful, although NETCALC will not display it.
When NETCALC starts up, it displays undefined impedances as
0*0. Your first input must be at least one
impedance value for NETCALC to work on.
Examples of correct impedance inputs
|
50+0 |
50 ohms, resistive |
|
50+5 |
50 ohms resistance in series with 5 ohms inductive reactance |
|
50+j5 |
As above; the j is ignored |
|
50++5 |
Same as 50+5. NOTE: the SECOND "+" sign is the optional one! |
|
50+-5 |
50 ohms resistance in series with 5 ohms capacitive reactance |
|
1E6&5 |
1 megohm resistance in parallel with 5 ohms inductive reactance |
|
1M&+5 |
Same as 1E6&5; the "+" sign is optional |
|
1M&j5 |
As above; the j is ignored |
|
0+5E-3H |
5mH inductance with no series resistance |
|
10+5mH |
5 millihenries inductance (note the small "m") with 10 ohms
series resistance |
|
1M&47pF |
47pF capacitor with 1 megohm leakage resistance |
Impedances of the type "a&0" or "0&b" are acceptable, but are physically
meaningless: one half of the impedance is short-circuited by the other!
Error Protection
If you input something that NETCALC can't interpret, the program either returns
you directly to the ? prompt or produces
an error message.
Press the Enter key to return from an error message to the prompt.
Attempts to use an undefined impedance, or to carry out some physically
meaningless operation, will produce the error message
Can't be done.
NETCALC smoothly handles calculations that produce infinite results. These
are automatically approximated to extremely large values, and the calculation
continues. Further operations are then likely to produce extremely small values,
which NETCALC will correct to zero. |
|
CONTENTS
Top
1. Overview
2. Entering Impedances
3. Environment Parameters
4. NETCALC Operations
5. Example - a Pi-Tank
6. NETCALC with Windows, Mac
7. Downloads
End, Back
|
Environment Parameters
NETCALC uses three "environment" parameters -
- F is the operating frequency (initially 1Hz)
- Z is the system impedance (initially 50 ohms)
- V is the velocity factor for transmission lines (initially 1)
To alter any of these values, press the appropriate key (F, Z or V) and enter
a new value at the bottom of the screen. To leave the present value unchanged,
just press Enter.
F can be entered using the normal "k", "M" and "G" multipliers
(as an exception to the normal rule, lower-case m
is
also assumed to mean megahertz, e.g. 14.345M = 14.345m = 14.345MHz).
Z and V are entered as plain numbers. |
|
CONTENTS
Top
1. Overview
2. Entering Impedances
3. Environment Parameters
4. NETCALC Operations
5. Example - a Pi-Tank
6. NETCALC with Windows, Mac
7. Downloads
End, Back
|
NETCALC Operations
NETCALC keeps an operating stack of two impedances. The "current" value is
called X, the other Y.
Previous values of X and Y scroll up the screen. You can look at them for
reference, but you can't do anything with them.
Operations involving Only X
| P |
Converts the impedance in X into parallel form (from series - ignored
if X is already in parallel form). |
| S |
Converts the impedance in X into series form (from parallel - ignored
if X is already in series form). |
| J |
Converts the impedance in X to its conjugate impedance (i.e. it
flips the sign of the reactance). |
| * |
Multiplies both parts of the impedance in X by a scaling factor
of your choice. |
| / |
Divides both parts of the impedance in X by a scaling factor of
your choice. |
| Mn |
Copies the impedance in X into memory n, where n =
1 to 9. X is unaffected, but the previous contents of memory n (if any)
are lost. |
| ?n |
Displays the contents of memory n on the bottom line. This
does not affect either the impedance in X or the memory contents. |
|
For the following X-only operations, you need to have
specified the frequency (F command), and also possibly the system impedance
(Z) and velocity factor (V) - |
| Q |
Displays the Q of the impedance in X, on the bottom line. X itself
is unaffected. |
| X |
Calculates the capacitance or inductance corresponding to the reactive
part of the impedance in X, and displays the result on the bottom line.
X is unaffected. |
| L |
Calculates the length of transmission line equivalent to the reactive
part of the impedance in X, and displays the result on the bottom line.
X is unaffected. |
| T |
Adds a transmission line of characteristic impedance Z in series
with the impedance in X, transforming its impedance in most cases. You
are asked to input the length of the line. Inputs < 1.0 are assumed
to be in electrical wavelengths. Inputs > 1.0 are assumed to be physical
millimetres, so V and F are used to calculate the electrical length.
The impedance in X changes to its new impedance-transformed value. |
| W |
Calculates the VSWR and return loss of X, according to the prevailing
system impedance Z, and displays the result on the bottom line. X itself
is unaffected. |
Operations on Both X and Y
Keyboard
entry |
Entering an impedance from the keyboard will affect both X and Y
- the new impedance value goes into X, the old X is "pushed" up into
Y, and the old Y is lost (scrolls up the screen). |
| |
|
| E |
Exchanges X and Y (useful in connection with memory operations,
which only operate on the X register). To swap back, press E again. |
| Rn |
Recalls the impedance stored in memory n and enters it into
X, just as if it had been entered from the keyboard. The old X is pushed
up into Y, and the old Y value is lost (scrolls up the screen). Rn
does not affect the memory contents. |
| + |
Adds impedance X and impedance Y in series (whatever their original
forms). The result appears in X in series form, and Y becomes empty
(displayed as 0*0).
If you wanted the answer in parallel form, press P. |
| & |
Adds impedance X and impedance Y in parallel (whatever their original
forms). The result appears in X in parallel form, and Y becomes empty
(displayed as 0*0).
If you wanted the answer in series form, press S. |
Other Commands
| Esc |
Exits from NETCALC (press Y to confirm). |
| H |
Displays a Help screen containing a summary of this information.
When not in use for input/output, the bottom line of the screen also
displays the available command keys. |
|
|
CONTENTS
Top
1. Overview
2. Entering Impedances
3. Environment Parameters
4. NETCALC Operations
5. Example - a Pi-Tank
6. NETCALC with Windows, Mac
7. Downloads
End, Back
 |
Example - a Pi-Tank
You can tackle almost any network problem by entering the appropriate resistances
and reactances, and adding them in series or in parallel.
For example, here is a pi-network design from the ARRL Handbook. It is intended
to transform a 50-ohm load impedance into a load of 1.5kohms resistive, as
seen by the anode of the tube.
To see how this happens, start up NETCALC and follow these instructions.
|
Action |
In NETCALC |
|
1. Set frequency to 3.75MHz. |
Enter F 3.75M. |
|
2. Enter the 50 ohm load impedance in
parallel with C2, 1375pF. |
Enter 50&1375pF.
NETCALC converts 1375pF into reactance, so the result is 50 & -j30.866. |
|
3. Convert the result from Step 2 into
series-equivalent form. |
Press S.
The result is 13.797 + -j22.349. |
|
4. Enter L1, 7.02 microhenries. |
Enter 0 + 7.02uH.
NETCALC converts this into 0 + j165.405, and the result from
Step 3 moves up into the Y register. |
|
5. Add the impedance in X and the impedance
in Y. Get the result in series-equivalent form. |
Press +.
The result is 13.797 + j143.056.
Note: you could have skipped Step 3, because when you press
+, NETCALC automatically converts all impedances into series-equivalent
form before adding them together. |
|
6. Convert the result into parallel-equivalent
form. |
Press P. The result is 1.497k & j144.386 - so here
is the 1.5k resistive impedance that the pi-tank presents to the tube. |
|
7. The 1.5k impedance has a reactive compnent,
which is tuned out by C1. Enter C1 = 294pF. |
Enter 0+294pf.
The result from Step 6 moves up into the Y register. |
|
8. Almost there!
Combine the X and Y registers in parallel, and then convert into series-equivalent
form. |
Press &, then press S.
The result is 1.497k + -j3.011, in other words almost exactly
1.5k resistive, as intended.
(The small remaining j term is due to the rounded-off values that we
entered for C2, L1 and C1.) |
|
|
CONTENTS
Top
1. Overview
2. Entering Impedances
3. Environment Parameters
4. NETCALC Operations
5. Example - a Pi-Tank
6. NETCALC with Windows, Mac
7. Downloads
End, Back
|
NETCALC with Windows, Mac
NETCALC was originally a DOS program. It runs in a DOS box under Windows 95,
98 and ME, but these operating systems will not display special characters
such as "ohms" correctly (if this worries you, "rem-out" the CODE PAGE commands
in your \AUTOEXEC.BAT file). Windows XP restores support for special characters.
NETCALC is also available as a very neat click-button
calculator for the Macintosh, written by Harry Whitfield, G6AUC. |
|
CONTENTS
Top
1. Overview
2. Entering Impedances
3. Environment Parameters
4. NETCALC Operations
5. Example - a Pi-Tank
6. NETCALC with Windows, Mac
7. Downloads
|
Downloads
Download NETCALC.EXE
(51KB)
NETCALC for
Macintosh
Copyright
NETCALC is copyright 1986 - 1998 Ian White, G3SEK (the early version called
NETAN published in "Amateur Radio Software" is the copyright of the publisher,
RSGB). NETCALC is placed in the public domain for non-commercial use, entirely
at the user's own risk.
Back to GM3SEK's Amateur Radio
Technical Notebook |