Formula Used
DC resistance per meter: R = ρ / A
Temperature adjusted resistivity: ρT = ρ20 × [1 + α × (T − 20)]
Approximate skin depth: δ = √[2ρ / (2πfμ)]
RF resistance per meter: Rac ≈ ρ / (2πaδ)
Loop resistance: Rloop = 2 × Rside × length
Feeder loss: Ploss = I² × Rloss
Efficiency: η = Pload / (Pload + Ploss) × 100
Estimated open wire impedance: Z0 = 120 × acosh(spacing / wire diameter)
How to Use This Calculator
Select the calculation mode first. Use delivered power when you know antenna power. Use source power when you know transmitter output. Use current mode when measured feeder current is available.
Enter one way feeder length, wire diameter, spacing, material, frequency, and temperature. Add estimated SWR if the line is not closely matched.
Press Calculate. The result appears above the form. Use CSV or PDF buttons to save the same calculation.
Example Data Table
| Length |
Wire |
Material |
Load |
Frequency |
SWR |
Expected Use |
| 30 m |
2 mm |
Copper |
450 Ω |
14 MHz |
1.2 |
HF doublet feeder |
| 60 ft |
12 AWG |
Copper |
600 Ω |
7 MHz |
1.5 |
Balanced antenna feed |
| 45 m |
3 mm |
Aluminum |
300 Ω |
28 MHz |
2.0 |
Long open line |
Open Wire Feeder Efficiency Guide
What the Calculator Measures
Open wire feeder is often used for efficient RF power transfer. It has low dielectric loss because air surrounds most of the fields. Still, every conductor has resistance. That resistance converts part of the RF power into heat. This calculator estimates that useful power balance.
Why Resistance Matters
A feeder has two conductors. Current travels out on one wire and returns on the other. Therefore, the calculator uses loop resistance, not only one wire resistance. Longer lines increase total resistance. Thin conductors also raise resistance. Larger wires reduce loss and improve efficiency.
RF Skin Effect
At radio frequency, current crowds near the conductor surface. This is called skin effect. The useful conducting area becomes smaller. The calculator estimates skin depth from frequency, material resistivity, and permeability. It then uses the larger value between direct resistance and RF surface resistance.
Material and Temperature
Copper is usually the best common choice. Aluminum can work well with larger diameter. Steel has higher resistance and magnetic loss concerns. Temperature also changes resistance. A hot conductor usually has more resistance, so the calculator adjusts resistivity from the entered temperature.
Load Power and Source Power
The tool supports several operating cases. If load power is known, it finds the required source power. If source power is known, it estimates delivered load power. If line current is known, it calculates both load power and loss directly from current squared times resistance.
SWR Effect
A perfect line match gives the lowest current stress for a given transfer. Real open wire systems may operate with standing waves. The SWR field adds an approximate mismatch factor. It is useful for planning, but measured data is better for final station design.
Characteristic Impedance
Wire spacing and diameter also estimate characteristic impedance. Wider spacing increases impedance. Thicker wire lowers it. This number helps compare the feeder with antenna and tuner conditions. It does not replace field measurement, but it improves early design choices.
Practical Design Use
Use the results to compare line lengths, wire sizes, materials, and loads. A small decibel loss can still mean many watts at high power. Keep conductors clean, separated, and secure. Good construction preserves balance and keeps RF heating low.
FAQs
What is open wire feeder efficiency?
It is the percentage of source power that reaches the load. The rest is mainly lost as heat in conductor resistance and related RF effects.
Why does the calculator use loop resistance?
Open wire feeder has two conductors. Current flows through both. The total loss path includes the outgoing and return conductors, so loop resistance gives a better estimate.
Does higher frequency increase feeder loss?
Usually yes. Higher frequency reduces skin depth. Current then flows in a thinner surface layer, which increases effective conductor resistance.
Why does wire diameter matter?
Thicker wire has more conducting surface and lower resistance. It usually reduces feeder loss, especially on longer lines or higher frequency bands.
Is copper always required?
No. Copper is common because it has low resistivity. Aluminum can work if sized well. Steel usually has higher loss and needs careful evaluation.
What does SWR do in this calculator?
SWR adds an approximate mismatch factor. It estimates higher average conductor heating when standing waves are present on the feeder.
Is the characteristic impedance exact?
No. It is an estimate using wire spacing and diameter. Nearby objects, insulation, bends, moisture, and construction details can change the real value.
Can I use this for ladder line?
Yes, for an estimate. Enter the wire spacing, diameter, material, length, frequency, and load. Real spacer material may add extra loss.