Calculators · HAMnet.gr

Antenna length

Common antenna lengths for a frequency. Cut a little long and trim for best SWR.

Frequency (MHz) Shortening factor

λ = 300 / f · dipole = λ/2 × k · ground plane = λ/4 × k

SWR from your meter, or from forward and reflected power.

SWR (e.g. 1.5) Transmitter power (W)

Γ = (SWR − 1) / (SWR + 1) · reflected = Γ² · SWR = (1 + Γ) / (1 − Γ), Γ = √(Pr / Pf)

Power or voltage ratio to dB and back, and dBm to watts.

dB dBm

power: dB = 10·log(P2/P1) · voltage: dB = 20·log(V2/V1) · dBm = 10·log(P / 1 mW)

Power, feed line loss and antenna gain. Loss per 100 m is on the cable datasheet for your frequency.

Transmitter power (W) Cable length (m) Loss (dB/100 m) Antenna gain Gain unit

dBi = dBd + 2.15 · EIRP = P × 10^((gain dBi − loss)/10) · ERP = EIRP / 1.64

Fill in any two and leave the rest blank.

Voltage V (V) Current I (A) Resistance R (Ω) Power P (W)

V = I × R · P = V × I = I² × R = V² / R

Resonant frequency, and XL and XC at that frequency.

L (µH) C (pF)

f = 1 / (2π√(LC)) · XL = 2πfL · XC = 1 / (2πfC)

A λ/4 transformer between two impedances, and how much cable to cut for its velocity factor.

Z1 (Ω) Z2 (Ω) Frequency (MHz) Velocity factor

Z = √(Z1 × Z2) · physical length λ/4 = (300 / f) / 4 × Vf

Your locator (or lat/long) and the other station’s. Get the distance and beam heading.

Me (locator or latitude, longitude) Other station (locator or latitude, longitude)

Maidenhead (field, square, subsquare) · great circle with an Earth radius of 6371 km

Roughly, the distance at which power density falls below the limit (far field). The exam bank gives 2 W/m² at 28 MHz (Q 20).

EIRP (W) Limit (W/m²) Duty cycle (%)

S = EIRP × duty / (4π r²) → r = √(EIRP × duty / (4π × limit))

Note: a free-space estimate, not an official assessment.