Antenna Comp

This program calculates the physical dimensions of two common amateur radio antenna types — the half-wave dipole and the quarter-wave vertical — for any user-specified frequency in megahertz. After inputting the centre frequency, it computes total and per-arm dipole lengths plus vertical and radial lengths, all in both feet and metres, using integer truncation via INT to yield results to two decimal places. The magic numbers 49200 and 15000 encode the speed-of-light velocity factor constants for feet and metres respectively (with a 0.95 shortening factor already baked in), while 23400 and 7130 provide the quarter-wave equivalents. A short FOR/NEXT delay loop (lines 250–260) substitutes for a PAUSE statement between data entry and results display. Results are displayed with inverse-video labels, and the program loops back on a Y/N prompt.

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Program Analysis

Program Structure

The program follows a straightforward linear flow divided into four functional phases:

1. Title screen (lines 10–130): clears screen, prints banner with inverse-video heading.
2. Input & calculation (lines 140–240): accepts frequency F in MHz, computes eight length variables.
3. Delay & results display (lines 250–450): a busy-wait loop, a CLS, then formatted output of all dimensions.
4. Loop / exit (lines 460–560): Y/N continuation prompt; on exit prints memory usage and STOPs.

Antenna Dimension Formulae

Eight variables are computed from the input frequency F (in MHz). The pattern (INT(constant/F))/100 computes a value to two decimal places without using floating-point rounding, relying instead on integer truncation:

Variable	Formula	Meaning
L1	INT(49200/F)/100	Half-wave dipole total length, feet
L2	INT(15000/F)/100	Half-wave dipole total length, metres
L3	INT(23400/F)/100	Quarter-wave vertical length, feet
L4	INT(7130/F)/100	Quarter-wave vertical length, metres
L5	INT(102.5*L3)/100	Radial length, feet (≈5% longer than vertical)
L6	INT(102.5*L4)/100	Radial length, metres
L7	INT(L2*50)/100	Each dipole arm, metres (half of L2)
L8	INT(L1*50)/100	Each dipole arm, feet (half of L1)

The constant 49200 derives from the standard dipole formula 468/F(MHz) scaled to feet×100 (468×100 = 46800), but here 49200 is used, implying a slightly less aggressive velocity/end-effect shortening factor (~0.951 rather than the commonly used 0.95). Similarly, 23400 ÷ 2 = 11700 ≈ 234, the quarter-wave feet constant. The radial multiplier of 102.5 makes radials approximately 2.5% longer than the vertical element, a common amateur radio guideline.

Delay Loop Idiom

Lines 250–260 implement a simple busy-wait delay:

250 FOR N=1 TO 70 / 260 NEXT N

This is a common ZX81/TS1000 technique used in the absence of a reliable mid-program PAUSE, giving the user a brief moment before the screen clears and results appear.

Inverse-Video Labels

All antenna labels and unit strings (e.g. % %H%A%L%F%-%W%A%V%E% %D%I%P%O%L%E%) use the %X escape notation for inverse-video characters. This was a popular technique to make headings and labels visually distinct without any POKE-based attribute manipulation, producing a highlighted block-letter effect on the display.

Memory Usage Diagnostic

Line 520 uses a well-known ZX81 idiom to calculate program memory consumption:

PEEK 16396 + 256*PEEK 16397 - 16509

This reads the system variable E_LINE (address 16396–16397), which points to the end of the BASIC program area, and subtracts the start address of user RAM (16509), yielding the number of bytes used by the program.

Notable Anomalies

• Line 350 is missing from the listing (the sequence jumps from 340 to 360), suggesting a line was deleted at some point without renumbering.
• Lines 540–560 (CLEAR, SAVE, RUN) appear after STOP at line 530 and are therefore unreachable during normal execution; they exist as a convenient way to save and re-run the program from the editor.
• The Y/N test at line 500 only checks for uppercase "Y"; entering lowercase y will fall through to the STOP rather than looping, which may catch users unaware.
• No input validation is performed on F; entering zero or a negative value would cause a division-by-zero error at line 170.