Antenna Design Guide

DIY Antenna Design Guide for RTL-SDR Radio Reception

A practical guide to designing and building antennas for FM and DAB radio reception.

See also Antennas for choosing and placing an antenna, and the Antenna Positioning Tools for testing one.

Table of Contents

1. Basic Theory
2. Simple Dipole (Omnidirectional)
3. Dual Dipole (Crossed/Bi-Dipole)
4. Periodic Antennas
5. Construction Tips
6. Optimization and Testing

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Basic Theory

Wavelength Formula

All antenna calculations start with wavelength:

Wavelength (λ) = Speed of Light / Frequency
λ (meters) = 300 / Frequency (MHz)

Examples:

• FM 100 MHz: λ = 300 / 100 = 3.0 meters
• DAB 200 MHz: λ = 300 / 200 = 1.5 meters

Velocity Factor

Real-world antennas are affected by wire diameter and nearby materials:

Effective Length = Calculated Length × Velocity Factor

Common Velocity Factors:

• Thin wire (1-2mm): 0.95
• Thick wire (6-10mm): 0.97
• Aluminum tubing (12-25mm): 0.98
• In free space (ideal): 1.00

Rule of Thumb: Start with 0.95 for DIY wire antennas.

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Simple Dipole (Omnidirectional)

What is a Dipole?

A half-wave dipole is the fundamental antenna design:

• Two equal-length elements extending from center
• Total length = half wavelength (λ/2)
• Omnidirectional in horizontal plane
• Vertically polarized when mounted vertically

Calculation Formula

Total Length = (300 / Frequency_MHz) × 0.5 × Velocity_Factor

Each Element = Total Length / 2

Example 1: FM Band Center (98 MHz)

Step 1: Calculate wavelength
λ = 300 / 98 = 3.061 meters

Step 2: Half wavelength
λ/2 = 3.061 / 2 = 1.531 meters

Step 3: Apply velocity factor (0.95)
Total Length = 1.531 × 0.95 = 1.454 meters = 145 cm

Step 4: Each element
Each Element = 145 / 2 = 72.5 cm

Result: FM dipole = 145 cm total (72.5 cm per element)

Example 2: DAB Band III Center (207 MHz)

Step 1: Calculate wavelength
λ = 300 / 207 = 1.449 meters

Step 2: Half wavelength
λ/2 = 1.449 / 2 = 0.725 meters

Step 3: Apply velocity factor (0.95)
Total Length = 0.725 × 0.95 = 0.689 meters = 69 cm

Step 4: Each element
Each Element = 69 / 2 = 34.5 cm

Result: DAB dipole = 69 cm total (34.5 cm per element)

Quick Reference Table - Simple Dipole

Band	Center Freq	Total Length	Per Element
FM Low (88 MHz)	88 MHz	161 cm	81 cm
FM Mid (98 MHz)	98 MHz	145 cm	73 cm
FM High (108 MHz)	108 MHz	132 cm	66 cm
DAB Low (174 MHz)	174 MHz	82 cm	41 cm
DAB Mid (207 MHz)	207 MHz	69 cm	35 cm
DAB High (240 MHz)	240 MHz	59 cm	30 cm

All values include velocity factor 0.95

Bandwidth Considerations

A simple dipole has limited bandwidth:

• Thin wire (2mm): ±5 MHz around center frequency
• Thick wire (10mm): ±10 MHz around center frequency
• Tubing (25mm): ±20 MHz around center frequency

For wideband reception: Use thicker elements or dual-band design.

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Dual Dipole (Crossed/Bi-Dipole)

What is a Dual Dipole?

Two dipoles mounted at 90 degrees to each other:

• Better polarization diversity
• Reduces multipath fading
• Maintains omnidirectional pattern
• Each dipole same length as simple dipole

Design Configurations

Configuration 1: Diversity Reception (In-Phase)

• Both dipoles connected in phase (0 degrees)
• Better signal consistency
• Reduces nulls in pattern
• Simple construction

Configuration 2: Circular Polarization (Quadrature)

• Dipoles fed 90 degrees out of phase
• True circular polarization
• Requires phasing network
• More complex construction

Calculation (Same as Simple Dipole)

Each dipole calculated independently:

Dipole A Length = (300 / Frequency_MHz) × 0.5 × 0.95
Dipole B Length = (300 / Frequency_MHz) × 0.5 × 0.95

Result: Both dipoles identical length, crossed at 90 degrees.

Example: DAB Dual Dipole (207 MHz)

Each Dipole:
- Total: 69 cm (from earlier calculation)
- Per element: 34.5 cm

Configuration:
- Dipole A: Horizontal (East-West)
- Dipole B: Horizontal (North-South)
- Crossing point: Center
- Vertical separation: 0 cm (same plane) or 5-10 cm (stacked)

Mounting Options

Option A: Crossed in Same Plane

         Element A
             |
             |
Element B ---+--- Element B
             |
             |
         Element A

• Simple construction
• Good for diversity
• Minimal interaction

Option B: Stacked with Spacing

Dipole A: ----+----
              |
          5-10cm spacing
              |
Dipole B: ----+----

• Better isolation
• Easier feeding
• Requires vertical support

Feeding the Dual Dipole

Simple Parallel Feed (Diversity):

RTL-SDR Coax → Power Splitter → Dipole A
                              → Dipole B

Advantages:

• Both dipoles active simultaneously
• Signal combining improves SNR
• Simple implementation

Component: 2-way power splitter (75 ohm or 50 ohm)

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Periodic Antennas

What are Periodic Antennas?

Arrays of elements with systematic spacing:

• Yagi-Uda: Directional, high gain
• Log-Periodic: Wideband, directional
• Multiple elements work together
• More complex but higher performance

Yagi-Uda Antenna

Basic Structure:

• Reflector: Longest element (behind driven element)
• Driven Element: Connected to feedline (dipole length)
• Director(s): Shorter elements (in front)

Element Length Relationships:

Driven Element = λ/2 × 0.95 (standard dipole calculation)
Reflector = Driven Element × 1.05 (5% longer)
Director 1 = Driven Element × 0.90 (10% shorter)
Director 2 = Driven Element × 0.85 (15% shorter)
Director 3 = Driven Element × 0.82 (18% shorter)

Element Spacing:

Reflector to Driven: 0.15λ to 0.25λ
Driven to Director 1: 0.10λ to 0.20λ
Director spacing: 0.15λ to 0.30λ

Example: 3-Element Yagi for DAB (207 MHz)

Step 1: Calculate wavelength
λ = 300 / 207 = 1.449 meters

Step 2: Driven element (dipole)
Driven = (1.449 / 2) × 0.95 = 69 cm

Step 3: Reflector (5% longer)
Reflector = 69 × 1.05 = 72 cm

Step 4: Director (10% shorter)
Director = 69 × 0.90 = 62 cm

Step 5: Spacing
Reflector-Driven: 0.20λ = 0.20 × 1.449 = 29 cm
Driven-Director: 0.15λ = 0.15 × 1.449 = 22 cm

Result:

• Boom length: 51 cm (29 + 22)
• Reflector: 72 cm at position 0
• Driven: 69 cm at position 29 cm
• Director: 62 cm at position 51 cm
• Expected gain: 6-8 dBi
• Beamwidth: ~60 degrees

Quick Yagi Design Table (DAB Band)

Configuration	Elements	Gain	Beamwidth	Boom Length
3-element	Ref + DE + Dir	6-8 dBi	60°	50 cm
5-element	Ref + DE + 3 Dir	9-11 dBi	45°	100 cm
7-element	Ref + DE + 5 Dir	11-13 dBi	35°	150 cm

DE = Driven Element, Ref = Reflector, Dir = Director

Log-Periodic Dipole Array (LPDA)

Characteristics:

• Wideband (3:1 frequency ratio typical)
• Constant gain across band
• Directional (like Yagi)
• More complex design

Design Formula:

Tau (τ) = Length ratio between adjacent elements (typically 0.7-0.9)
Sigma (σ) = Spacing factor (typically 0.05-0.15)

Element N Length = Element N-1 Length × τ
Element N Spacing = (Length N + Length N-1) / 2 × σ

Practical LPDA for DAB (174-240 MHz):

Choose τ = 0.85 (good bandwidth/gain compromise)
Choose σ = 0.10 (moderate spacing)

Longest element (174 MHz):
L1 = (300 / 174 / 2) × 0.95 = 82 cm

Subsequent elements:
L2 = 82 × 0.85 = 70 cm
L3 = 70 × 0.85 = 59 cm
L4 = 59 × 0.85 = 50 cm
L5 = 50 × 0.85 = 43 cm
L6 = 43 × 0.85 = 36 cm

Spacing calculations:
S1-2 = (82 + 70) / 2 × 0.10 = 7.6 cm
S2-3 = (70 + 59) / 2 × 0.10 = 6.5 cm
S3-4 = (59 + 50) / 2 × 0.10 = 5.5 cm
S4-5 = (50 + 43) / 2 × 0.10 = 4.7 cm
S5-6 = (43 + 36) / 2 × 0.10 = 4.0 cm

Result:

• Total boom length: ~28 cm
• 6 elements covering 174-240 MHz
• Gain: 6-8 dBi across band
• VSWR < 2:1 across entire DAB Band III

When to Use Each Type

Simple Dipole:

• Omnidirectional coverage needed
• Urban environment with signals from all directions
• Simplest construction
• Good for scanning/exploration

Dual Dipole:

• Omnidirectional with better reliability
• Multipath environments
• Mobile/portable use
• Still simple construction

Yagi:

• Specific transmitter direction known
• Rural/distant reception
• Maximum gain needed
• Reject interference from other directions

Log-Periodic:

• Wide frequency range (FM + DAB combined)
• Directional gain needed
• Professional installation
• More complex construction

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Construction Tips

Materials

Wire/Rod Options:

1. Copper Wire (1-3mm)

   • Pros: Easy to work with, good conductivity
   • Cons: Narrows bandwidth, can sag
   • Best for: Indoor dipoles, temporary antennas

2. Aluminum Rod (6-10mm)

   • Pros: Lightweight, doesn't corrode, wider bandwidth
   • Cons: Harder to solder
   • Best for: Outdoor dipoles, Yagi elements

3. Brass Rod (4-8mm)

   • Pros: Easy to work, good conductivity
   • Cons: Can tarnish
   • Best for: Indoor/sheltered outdoor use

4. Aluminum Tubing (12-25mm)

   • Pros: Very wide bandwidth, rigid
   • Cons: Requires special connectors
   • Best for: Professional Yagi/LPDA arrays

Center Insulator/Feedpoint

Simple Dipole:

Element A --+-- Element B
           Coax center
           Coax shield

Materials:

• Plastic terminal block
• PVC pipe section
• 3D printed connector
• Purchased antenna center insulator

Waterproofing:

• Self-amalgamating tape
• Heat shrink tubing
• Silicone sealant
• Coax seal putty

Boom Material (Yagi/LPDA)

Options:

• Wood (2x2 or 2x4): Simple, non-conductive, weatherproof treated
• PVC pipe (25-50mm): Lightweight, weatherproof, easy to drill
• Fiberglass rod: Professional grade, non-conductive
• Aluminum tube: Conductive, requires element isolation

Element Mounting (Yagi/LPDA)

Through-Boom Method:

• Drill holes in boom
• Elements pass through boom
• Secure with compression clamps
• Isolate from boom if boom is metal

U-Bolt Method:

• U-bolts attach elements to boom
• Plastic or nylon U-bolts prevent shorting
• Easy adjustment
• Weather-resistant

Impedance Matching

Dipole Impedance:

• Free space: ~73 ohms
• Near ground: 50-60 ohms
• RTL-SDR input: 75 ohms (but accepts 50 ohms)

Matching Techniques:

1. Direct Connection (Simple)

   • 75 ohm coax direct to dipole
   • Good enough for receive-only
   • VSWR 1.5:1 typical

2. Folded Dipole

   • Two parallel elements
   • 300 ohm impedance
   • Use 4:1 balun to 75 ohms
   • Better bandwidth

3. Gamma Match (Yagi)

   • Adjustable capacitor and rod
   • Matches boom-mounted dipole
   • Professional Yagi antennas use this

Balun Requirements

What is a Balun?

• Balanced (antenna) to Unbalanced (coax)
• Prevents coax shield radiation
• Improves pattern and reception

When to Use:

• Recommended for all dipoles
• Essential for Yagi fed elements
• Less critical for receive-only

Types:

• 1:1 balun (most common)
• 4:1 balun (for folded dipole)
• Ferrite choke (simplest, works well)

DIY Ferrite Choke Balun:

1. Take 1-2 meters of coax
2. Coil 6-8 turns (15cm diameter)
3. Secure with cable ties
4. Place coil near antenna feedpoint
5. Acts as RF choke on shield

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Optimization and Testing

Using the Plugin's Spectrum Scan

The antenna positioning tools in this plugin are perfect for optimization:

Procedure:

1. Install antenna at test position
2. Open plugin web manager → Antenna Positioning tab
3. Run RF Spectrum Scan
4. Note peak levels on target frequencies
5. Adjust antenna
6. Re-scan and compare
7. Repeat until optimal

Length Adjustment

Tuning a Dipole:

If signal peaks BELOW target frequency → Dipole TOO LONG → Trim 5-10mm
If signal peaks ABOVE target frequency → Dipole TOO SHORT → Add length

Rule: 1% length change = 1% frequency shift

Example:

• Designed for 200 MHz
• Peaks at 195 MHz (2.5% low)
• Dipole 2.5% too long
• 70 cm dipole → trim 1.75 cm (70 × 0.025)

Height Above Ground

Effect on Performance:

Height	Effect
0.1λ (15cm @ DAB)	Poor - ground losses
0.25λ (38cm @ DAB)	Acceptable - minimal ground loss
0.5λ (75cm @ DAB)	Good - low ground loss
1.0λ (150cm @ DAB)	Excellent - negligible ground loss

Recommendation: Mount at least 0.5λ above ground or metal objects.

Polarization

DAB/FM Transmitters:

• Typically vertically polarized
• Some horizontal or circular

Your Antenna:

• Match transmitter polarization when known
• Vertical dipole for vertical transmissions
• Use crossed dipole if polarization unknown

Testing:

• Try vertical orientation
• Try horizontal orientation
• Use spectrum scan to compare
• Choose orientation with strongest signal

Environmental Factors

Avoid:

• Metal roofs (shield signals)
• Dense foliage (absorbs VHF/UHF)
• Proximity to power lines (interference)
• Inside metal buildings (Faraday cage)

Prefer:

• Rooftop mounting
• Clear line of sight
• Away from electronic devices
• Outdoor installation

DAB Channel Validation

Use the plugin's channel validation tool:

1. Build antenna to calculated dimensions
2. Mount in test position
3. Run Channel Validation on target channels
4. Note service counts per channel
5. Adjust antenna orientation
6. Re-validate
7. Find optimal position

Good Result:

• Sync achieved on all target channels
• High service counts (15+ services)
• Low validation time (10-15 seconds per channel)

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Design Workflow Summary

For Omnidirectional Reception (Dipole)

1. Choose center frequency:

   • FM: 98 MHz
   • DAB: 207 MHz

2. Calculate length:

   Length = (300 / Freq_MHz / 2) × 0.95
   

3. Build dipole:

   • Cut two equal elements
   • Mount to center insulator
   • Connect coax

4. Test with spectrum scan

5. Tune if needed:

   • Trim for higher frequency
   • Add for lower frequency

For Directional Reception (Yagi)

1. Choose design frequency

2. Calculate driven element (dipole formula)

3. Calculate reflector (5% longer)

4. Calculate directors (10%, 15%, 18% shorter)

5. Calculate spacing (0.15λ to 0.25λ)

6. Build on boom:

   • Reflector at rear
   • Driven element at calculated position
   • Directors at front

7. Point toward transmitter

8. Test with spectrum scan

9. Optimize element lengths if needed

For Wideband Reception (LPDA)

1. Define frequency range (e.g., 174-240 MHz)

2. Choose τ = 0.85 and σ = 0.10

3. Calculate longest element (lowest frequency)

4. Calculate subsequent elements (multiply by τ)

5. Calculate spacing between elements

6. Build on boom (longest at rear)

7. Test across entire band with spectrum scan

8. Verify flat response across frequency range

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Practical Examples

Example 1: Simple DAB Dipole for Urban Reception

Scenario: Urban apartment, signals from multiple directions

Design:

Frequency: 207 MHz (DAB center)
Total length: 69 cm
Per element: 34.5 cm
Material: 3mm copper wire
Orientation: Vertical

Construction:

1. Cut two 35 cm pieces of wire
2. Strip 1 cm from each end
3. Solder to center terminal block
4. Connect coax (center to one element, shield to other)
5. Waterproof connections
6. Mount vertically near window

Expected: Reception of all local DAB ensembles

Example 2: Dual Dipole for Portable DAB

Scenario: Mobile/portable use, unknown signal directions

Design:

Each dipole: 69 cm (34.5 cm per element)
Configuration: Crossed, in-phase
Material: 4mm aluminum rod

Construction:

1. Build two identical 69 cm dipoles
2. Mount crossed at 90 degrees
3. Use 2-way splitter for feeding
4. Connect to single coax
5. Mount on portable mast

Expected: Consistent reception regardless of orientation

Example 3: 5-Element Yagi for Distant DAB

Scenario: Rural location, 50 km from transmitter, known direction

Design:

Center frequency: 207 MHz
Reflector: 72 cm
Driven: 69 cm
Director 1: 62 cm
Director 2: 59 cm
Director 3: 56 cm
Boom length: ~100 cm

Construction:

1. Use 5 cm PVC pipe for boom
2. Drill holes at calculated positions
3. Mount aluminum rod elements
4. Feed driven element
5. Point toward transmitter
6. Mount as high as practical

Expected: Strong reception from distant transmitter, 9-11 dBi gain

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Resources and References

Online Calculators

Many online calculators can verify your calculations:

• Search: "dipole antenna calculator"
• Search: "Yagi antenna calculator"
• Enter frequency, get dimensions

Note: Always apply velocity factor (0.95) to calculator results.

Recommended Reading

• ARRL Antenna Book (amateur radio reference)
• VK5DJ Yagi Calculator (excellent online tool)
• LPDA design resources (search "log periodic antenna design")

Safety Notes

Electrical Safety:

• Antennas are receive-only, low voltage
• Keep away from power lines (lethal)
• Ground mast for lightning protection
• Never install during storms

Mechanical Safety:

• Secure mounting prevents falling
• Wind load considerations for large antennas
• Roof access safety precautions

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Conclusion

Antenna design follows straightforward formulas, but optimization requires testing. The spectrum scan and channel validation tools in this plugin make antenna optimization much easier than traditional trial-and-error methods.

Start with a simple dipole, test with the plugin tools, then progress to more complex designs if needed.

Remember:

• Longer antenna = lower frequency
• Shorter antenna = higher frequency
• Thicker elements = wider bandwidth
• Height above ground matters
• Orientation affects polarization matching

Use the plugin's antenna positioning tools to:

• Verify antenna performance
• Compare different designs
• Optimize placement
• Validate channel reception

Good luck with your antenna projects!

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This guide is provided for educational purposes. Local regulations may apply to antenna installations. The RTL-SDR Radio plugin includes antenna positioning tools to assist with optimization.