Antenna Design

Part of Radio

The antenna is where electrical signals become radio waves and vice versa. A well-designed antenna built from wire and poles can outperform an expensive radio connected to a poor antenna.

Why Antenna Design Matters

In radio communication, the antenna is the most important component in the entire system. A mediocre transmitter connected to an excellent antenna will outperform an excellent transmitter connected to a poor antenna every time. The antenna converts electrical energy into electromagnetic waves (for transmitting) and captures electromagnetic waves and converts them back to electrical signals (for receiving).

In a rebuilding scenario, you will not have access to commercial antennas. But the good news is that antennas are fundamentally simple — wire, cut to the right length and hung in the right configuration, is all you need. The physics is straightforward, and the materials are among the most basic available.

Fundamental Concepts

Wavelength and Frequency

Radio waves travel at the speed of light. The relationship between frequency and wavelength is:

Wavelength (meters) = 300 / Frequency (MHz)

Frequency	Wavelength	Band Name	Typical Use
0.5 MHz	600 meters	Medium Wave (AM broadcast)	Regional broadcast
3.5 MHz	86 meters	80 meters (HF)	Regional communication
7 MHz	43 meters	40 meters (HF)	Continental communication
14 MHz	21 meters	20 meters (HF)	Worldwide communication
27 MHz	11 meters	CB radio	Local communication
144 MHz	2 meters	VHF	Line-of-sight, local

Resonance

An antenna works best when its length is related to the wavelength of the signal. The most common relationship is the half-wave dipole — a wire whose total length equals half the wavelength. At resonance, the antenna efficiently converts between electrical signals and radio waves. Off-resonance, most of the energy reflects back instead of radiating.

The 95% Rule

Real wire antennas are slightly shorter than the calculated wavelength due to end effects and wire diameter. Use 95% of the theoretical length: Half-wave dipole length (meters) = 142.5 / Frequency (MHz)

Dipole Antenna

The half-wave dipole is the fundamental antenna design. Every other antenna is either a variation of or compared to the dipole.

Construction

1. Calculate the total length: 142.5 / frequency (MHz)
2. Cut two pieces of wire, each half the total length
3. Connect the feed line (transmission line) to the center, one wire to each conductor
4. Insulate the center junction and the two ends
5. Hang horizontally between two supports (trees, poles, buildings) at least a quarter-wavelength above ground

Example: 7 MHz Dipole

• Total length: 142.5 / 7 = 20.36 meters
• Each leg: 10.18 meters
• Height above ground: at least 10 meters
• Feed line: connected at center
• Supports: two poles or trees 21+ meters apart

Performance

Parameter	Value
Gain	2.15 dBi (reference standard)
Pattern	Figure-8 (bidirectional, broadside to the wire)
Bandwidth	Moderate (usable over ~5% of frequency)
Impedance	~73 ohms at center
Difficulty	Very easy

The Inverted-V Dipole

If you only have one tall support, hang the center of the dipole from the top and let the ends slope down at 30-45 degree angles. This “inverted V” configuration performs nearly as well as a flat dipole and requires only one tall pole instead of two.

Vertical Antenna (Ground Plane)

A quarter-wave vertical antenna is omnidirectional — it radiates equally in all directions around the horizontal plane. This makes it ideal for communication when you do not know the direction of the other station.

Construction

1. Calculate the radiator length: 71.25 / frequency (MHz)
2. Cut a vertical wire or rod to this length
3. Mount it vertically with the base at least 2-3 meters above ground
4. Add 3-4 radial wires at the base, each the same length as the radiator, extending horizontally or sloping slightly downward
5. Connect the feed line: center conductor to the vertical element, shield to the radials

Ground Plane Radials

The radials simulate a ground plane (mirror image of the vertical element). More radials improve performance:

Number of Radials	Impedance	Efficiency
0 (earth ground only)	Variable	Poor (30-50%)
2	~40 ohms	Fair (60%)
4	~35 ohms	Good (75%)
8	~32 ohms	Very good (85%)
16+	~28 ohms	Excellent (90%+)

Vertical Height

A vertical antenna’s performance depends critically on height. Every meter higher improves range significantly. Mount it on a rooftop, hilltop, or tall mast whenever possible. A quarter-wave vertical at 20 meters height can communicate 10 times farther than the same antenna at 3 meters.

Loop Antenna

Loop antennas are directional, compact, and excellent for both receiving and direction-finding. They can be made much smaller than dipoles for the same frequency.

Construction

1. Build a frame: square or circular, 0.5-2 meters per side (for HF frequencies)
2. Wind 5-20 turns of insulated wire around the frame
3. Connect a variable capacitor across the loop terminals to tune to resonance
4. Rotate the loop to find the direction of a signal (nulls are perpendicular to the loop plane)

Advantages and Limitations

Advantage	Limitation
Very compact for the frequency	Lower gain than dipoles
Directional (can reject interference)	Narrow bandwidth (needs retuning)
Rejects electrical noise	Requires a tuning capacitor
Indoor use possible	Transmit power limited

Feed Lines

The feed line connects the antenna to the radio. It must carry the signal without excessive loss.

Types

Feed Line Type	Impedance	Loss per 100m at 10 MHz	Best For
Twin-lead (open wire)	300-600 ohms	0.5-1 dB	Long runs, HF dipoles
Coaxial cable	50-75 ohms	2-5 dB	All-purpose, VHF/UHF
Single wire + ground return	Variable	High	Emergency, short runs

Making Open-Wire Feed Line

If you cannot find coaxial cable, make open-wire line:

1. Cut two parallel wires to the needed length
2. Space them 50-100mm apart using wooden or plastic spacers every 0.5-1 meters
3. Keep the line away from metal objects and the ground (at least 2x the wire spacing)
4. This line has very low loss and can carry high power

Impedance Matching

If the antenna impedance does not match the feed line impedance, power reflects back from the antenna. For a dipole (73 ohms) with coaxial cable (50 ohms), the mismatch is small enough to ignore. For more critical applications, build a simple L-network matching circuit from a coil and capacitor.

Antenna Placement

Where you put the antenna matters as much as how you build it:

1. Height: Higher is almost always better. Doubling the height can double your communication range
2. Clear surroundings: Keep antennas away from buildings, trees, and metal objects that absorb or reflect signals
3. Ground conductivity: Wet, mineral-rich soil reflects radio waves better, improving vertical antenna performance
4. Orientation: Point dipole broadside (perpendicular) to the desired communication direction; point its ends toward directions you want to minimize

Common Mistakes

1. Wrong length: An antenna cut 10% too long or short loses significant efficiency. Measure and calculate carefully, then trim to tune.
2. Too close to ground: A dipole less than a quarter-wavelength above ground has poor performance and distorted pattern. Get it as high as possible.
3. Metal near the antenna: Gutters, power lines, fences, and metal roofing absorb and distort antenna patterns. Maintain at least one wavelength clearance from large metal objects.
4. No feed line strain relief: Wind will blow the antenna, putting stress on the connection. Use a strain relief loop or insulated tie-off to prevent the wire from breaking at the feed point.
5. Using corroded wire: Corroded copper wire has higher resistance and can crack. Use clean, bright copper or aluminum wire for best results.

Summary

Antenna Design -- At a Glance

• The half-wave dipole is the fundamental antenna: total length = 142.5 / frequency (MHz), hung horizontally between two supports
• Quarter-wave vertical antennas radiate omnidirectionally and need ground-plane radials for efficiency
• Loop antennas are compact and directional, ideal for receiving and direction-finding
• Height is the single most important placement factor — higher antennas always reach farther
• Open-wire feed line has the lowest loss and can be made from any parallel wires with spacers
• Cut antennas to 95% of theoretical wavelength to account for end effects