AntennaCal

Wire antenna designer

Wire

Length

Wire specification

Height above ground

Station

Operating frequency

MHz

TX power

Feed-line impedance

Feed position

Recommended wire lengths

Antenna diagram

Matching network

V, I & Z along wire

Position	V pk	I pk	\|Z\|

Enter your wire details on the Setup tab and tap Calculate antenna — the multiband comparison will appear here, calculated for your actual wire and frequency.

Quick-select guide

Balun — balanced ↔ unbalanced.
UnUn — unbalanced ↔ unbalanced.
Choke — suppresses common-mode only, no Z transform.

1:1

Current choke balun

8–12 turns of coax on FT240-43 or FT240-31. Suppresses common-mode current. No impedance transform. Essential at every coax-fed balanced antenna.

ChokeBalun

4:1

Guanella current balun

Two bifilar chokes in series/parallel on FT240-43. Transforms 200 Ω → 50 Ω. Best for folded dipoles, delta loops, doublets on ladder line.

Balun

4:1

Ruthroff voltage balun

Trifilar wound on FT240-43. Good only on resistive balanced loads. Degrades at high SWR. Prefer Guanella for field use.

Balun

9:1

UnUn — random wire

Autotransformer 3T:9T on FT240-43. Transforms ~450 Ω → 50 Ω. Pair with an ATU for multi-band random wires.

UnUn

49:1

EFHW UnUn

3 primary / 21 secondary turns on FT240-43 (7:1 turns → 49:1 Z). Transforms ~2450 Ω → 50 Ω. Add 100 pF across primary. Stack two cores for >200 W.

UnUn

16:1

UnUn — high-Z loads

Autotransformer tapped at 1/4 of turns on FT240-61. Transforms ~800 Ω → 50 Ω. Useful for off-resonance end-feds presenting 600–1000 Ω.

UnUn

W2DU

Bead choke (1:1)

50–100 Fair-Rite #43 or #31 beads slid directly over coax — no winding. Very broadband 1–54 MHz.

Choke

About AntennaCal

AntennaCal is a wire antenna designer I built for HF operators who want a sensible starting point before they cut any wire. You tell it what you've got, and it works out how long to make it, what's likely to happen at the feed point, and what you'll need to match it to your radio.

What it does

On the Setup tab you choose your wire (solid and stranded copper, copper-clad steel, aluminium, galvanised steel and a few more), the length, the height above ground, your operating frequency, your transmit power and the feed-line impedance. You also say whether the wire is fed at the end or in the centre. Tap Calculate and you get:

Recommended wire lengths for your frequency
A diagram of the antenna and a suggested matching network
How voltage, current and impedance change along the wire
A band-by-band view, and a multiband comparison for the wire you actually have

How it works

Each wire type in the list has its own velocity factor, resistivity and power rating. The calculator uses those to work out the electrical length of your wire and how much RF resistance it adds, including skin effect, with an allowance for stranded wire. Voltage and current along the wire follow the usual standing-wave pattern, and the feed impedance is estimated from where your feed point sits along that pattern. For an end-fed wire, for example, the feed impedance swings from fairly low to several thousand ohms depending on how close the length is to a half-wave multiple. That's why the length matters so much.

How to use it

Pick the wire you have, or the one you plan to buy.
Enter the length, the height above ground and the frequency you want to work.
Choose your feed-line impedance and whether the antenna is end-fed or centre-fed.
Tap Calculate antenna, then look through the Results, Bands and Multiband tabs.
Check the Ref tab for the balun or unun that suits your feed point.

The balun and unun guide

The Ref tab is a quick guide to the common transformers: 1:1 current chokes, 4:1 baluns, 9:1 and 16:1 ununs for random wires, and the 49:1 unun used for end-fed half-wave antennas. Each entry gives the core, the number of turns and the impedance it's meant for. I put it there because that's the question I get asked most once the wire length is sorted.

A word of caution

This is a calculator, not a field-strength meter. Real antennas are affected by the ground, nearby buildings, trees and the feed line itself, so treat the numbers as a good starting point. Cut the wire a little long, then trim it while you watch an antenna analyser or a NanoVNA. Everything runs in your browser, so nothing you type is sent anywhere.