Shortened Dipole Calculator

Calculator Inputs

Frequency

MHz

Velocity factor

Shortening

Percent shorter than adjusted full dipole.

End effect correction

Percent length reduction.

Loading placement

Loading units

Coil Q

Extra loss resistance

Ohms

Target impedance

Ohms

Length unit

Formula Used

The calculator first estimates an adjusted half wave dipole length.

L = 142.6464 × VF × (1 - E / 100) / f

Here, L is total dipole length in meters. VF is velocity factor. E is end effect correction. f is frequency in MHz.

The shortened total length is:

Ls = L × (1 - S / 100)

Each side is one half of the shortened total length.

Estimated radiation resistance uses:

Rr = 73.1 × ratio^2.1

Estimated inductance per loading unit uses:

uH = X / (2 × pi × fMHz)

These coil values are planning estimates. Nearby objects, height, wire shape, and feed routing can change the final tuning.

How to Use This Calculator

Enter the operating frequency in MHz.
Add the wire velocity factor.
Enter the desired shortening percentage.
Add an end effect correction if needed.
Select the loading placement method.
Enter coil count, coil Q, and loss values.
Select the length unit for your result.
Press calculate to show the result above the form.
Use CSV or PDF download for records.

Example Data Table

Frequency	Velocity Factor	Shortening	Full Total Length	Shortened Total Length	Each Side
7.150 MHz	0.95	30%	18.95 m	13.27 m	6.64 m
14.200 MHz	0.95	30%	9.54 m	6.68 m	3.34 m
28.400 MHz	0.95	30%	4.77 m	3.34 m	1.67 m

Shortened Dipole Planning Guide

A shortened dipole helps when a full half wave antenna will not fit. It keeps the familiar two leg shape. It also reduces the required span. The tradeoff is important. A shorter wire normally needs loading. Loading adds reactance that makes the antenna appear electrically longer.

Why the Estimate Matters

This calculator starts with the common half wave dipole length. It then applies velocity factor, end effect, and the selected shortening percentage. The result gives the total wire length and the length for each side. These values are practical starting points, not final construction guarantees.

Loading and Loss

Loading coils are estimated from the missing electrical length. The tool shows an approximate reactance and inductance. Coil quality factor is included because every real coil has loss. Lower Q coils waste more power as heat. Higher Q coils usually improve efficiency. Placement also matters. Center loading is convenient. Mid leg loading is often balanced. End loading can need different values and may behave differently near nearby objects.

Resistance and Tuning

The feed resistance estimate is useful for planning. A very short dipole can have low radiation resistance. This makes coil loss and connection loss more important. The calculator compares the estimated feed resistance with the target line impedance. It also gives a simple SWR clue after the loading reactance is assumed to be tuned out.

Building Notes

Use the results as a design notebook. Cut the wire slightly long when possible. Trim in small steps after measuring resonance. Keep both legs equal unless your installation needs compensation. Use strong insulators, stable supports, and weather protected coil connections.

Real World Effects

Nearby walls, gutters, trees, masts, and feed lines can change resonance. Height above ground also changes impedance. A compact antenna may still work well when it is tuned carefully. It may have narrower bandwidth than a full size dipole. Recheck tuning after rain, season changes, or hardware changes.

Record Keeping

Good records save time. Export the result after each trial. Compare frequency, length, coil Q, and measured SWR. Over several tests, the table becomes a clear build history. That history helps you improve the next version faster.

Final Check

For best results, pair the estimate with an antenna analyzer. Measure outdoors, away from metal clutter. Change one variable at a time. Small careful changes usually beat large guesses during tuning.

FAQs

What is a shortened dipole?

A shortened dipole is a dipole antenna made physically shorter than a normal half wave dipole. It often uses coils, hats, bends, or other loading methods to restore resonance near the target frequency.

Is the calculated length final?

No. It is a starting point. Real tuning changes with height, wire diameter, ground, nearby metal, supports, and feed line routing. Cut slightly long when possible, then trim after measurement.

Why does a shortened dipole need loading?

Short wire has less electrical length. Loading adds reactance that helps the antenna resonate at the desired frequency. Coils are common, but they can add loss and narrow usable bandwidth.

What does coil Q mean?

Coil Q describes coil quality. A higher Q value usually means lower loss. A low Q coil may waste more power as heat and reduce antenna efficiency, especially in very short designs.

Which loading placement is best?

There is no single best placement. Center loading is easy to build. Mid-leg loading can be efficient. End loading can work well in some layouts. Actual performance depends on construction and surroundings.

Why is efficiency lower for short antennas?

As a dipole gets shorter, radiation resistance often drops. Loss resistance then becomes more important. Coil loss, wire loss, and connection loss can take a larger share of input power.

Can this calculator design a multiband antenna?

It can help with one frequency at a time. Multiband shortened antennas need extra traps, links, tuners, or switching. Use separate calculations for each band as an early planning step.

Why is the SWR only a clue?

The SWR value assumes the loading reactance has been tuned out. Real feed impedance may include reactance, ground effects, coupling, and feed line transformation. Always confirm with a meter or analyzer.