portable-antennas - Extras, Reference Data

Extras - Reference data and charts

Reference data for parameters used in this site. Access these via the tab pages below:

Animated VSWR chart
Reference factors
Wire insulation factors

Animated VSWR chart

Use the slider to set the reflection coefficient - this will have the effect of altering the impedance mismatch between the feedline and the load/antenna. This will affect the values of VSWR, mismatch loss, etc. in the upper row, and also update the chart. The chart displays, for a lossless line, the following:

a forward wave propagated in a feed-line from a source (e.g. a transmitter)
a wave, reflected back to source along the feed-line, by an impedance mismatch at the load
a standing wave resulting from the impedance mismatch

Reflection coefficient, Γ :	VSWR:	Mismatch loss:	Percent loss:	Return loss:
	1.000 : 1	0.000 dB	0.000 %	0.000 dB

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Reference charts and diagrams

A collection of charts and diagrams displaying various correction factors used in the designer pages

Chart of end-effect correction factors

An end-effect correction factor is applied to calculations of antenna wire lengths, and is dependent on the ratio of wavelength to wire diameter, the λ/d ratio.

This λ/d ratio can take a wide range of values, as shown by this table:

λ/d ratios by band and wire diameter

Calculate your own end-effect correction factor:
Frequency:	MHz Wavelength λ: meters
Wire diameter d:	mm
λ/d ratio:
Correction factor:

Chart of dipole feed-point impedance vs. apex angle

A flat dipole in free space has a characteristic feed-point impedance of ~73.1 Ω. When situated above real ground, however, the impedance will be somewhat less.

When a dipole is configured as an inverted vee, the feed-point impedance will depend on the included apex angle of the antenna: the smaller the apex angle, the smaller the impedance.

Feed-point impedance by apex angle

The table and chart show the variation by included apex angle for an inverted-vee dipole in ideal conditions over perfectly conducting ground.

It is recommended to keep the included apex angle between 90° and 180° - if it is less than 90°, the feed-point impedance rapidly decreases, thereby lowering the radiation resistance, and will lead to increased coupling between the two legs of the antenna. This will result in poor antenna performance.

Chart of dipole feed-point impedance vs. antenna height

A flat dipole in free space has a characteristic feed-point impedance of ~73.1 Ω. When a dipole is situated above real ground, however, its' feed-point impedance is found to vary with its' height above the ground.

Feed-point impedance by antenna height

The table and chart show the variation by height in wavelengths above ground for a flat dipole in ideal conditions over a perfectly conducting ground.

Situating a dipole antenna too low above the ground can result in a low feed-point impedance, low radiation resistance, and increased capacitive coupling of the RF field with the ground, leading to poor antenna performance.

Chart of inverted-Vee dipole length correction factor vs. antenna apex angle

A flat dipole in free space will resonate at its' design frequency. In the real world, however, in a dipole configured as an inverted-vee, the resonant frequency will be found to increase with the antenna's central apex angle, while the bandwidth at resonance will decrease.

In order to counteract such an increase in resonant frequency, it is necessary to lengthen the dipole "legs" by a small amount, dependent on the apex angle, to bring the antenna to resonance at the desired frequency.

Inverted-vee dipole - length correction
factors by apex angle

The table and chart show the variation by apex angle for a dipole configured as an inverted vee.

An inverted-vee dipole antenna with an apex angle between 180° (a flat-top) and 90° will function well; angles below 90° will result in poor performance.

Reference wire insulation factors

Charts and diagrams displaying wire-insulation length-correction factors used in the designer pages

Charts of wire-insulation length-correction factors

Radio engineers and radio amateurs have long known that, when a wire antenna is made from insulated wire, the insulation will increase the electrical length of the wire to a degree which depends on the wire diameter, the insulation thickness, and the insulation material.

For insulated wires, therefore, it is necessary to include a suitable length-correction factor to compensate for the electrical lengthening due to the insulation. These charts list such factors by:

insulator type, wire diameter and insulation thickness;
wire diameter, insulator type and insulation thickness.

The values presented here are empirically derived from results obtained from multiple runs of the NEC v4.2 engine, for combinations of various insulator material types, wire core diameters, and insulator thicknesses.

1. Wire insulation correction factors by insulator type:

2. Wire insulation correction factors by wire diameter: mm

Interested parties may download the insulation correction-factors data as a CSV file.