The folded dipole antenna is a variation of the conventional half-wave dipole antenna that offers significant advantages in terms of input impedance and bandwidth characteristics. It consists of two parallel dipole elements connected at their ends, forming a narrow loop structure.
Key Concept: The folded dipole was developed to provide a higher input impedance (approximately 300Ω) that matches well with twin-lead transmission lines commonly used in television and FM broadcasting applications.
Why Study Folded Dipoles?
Provides impedance transformation without additional matching networks
Wider bandwidth compared to simple dipoles due to increased conductor thickness
Mechanically more robust than single-wire dipoles
Serves as the driven element in Yagi-Uda arrays
Essential for understanding antenna impedance transformation techniques
2. Physical Structure
Folded Dipole Geometry
L ≈ λ/2d << λ
Figure 1: Basic folded dipole structure showing two parallel conductors connected at ends with center feed point
Dimensional Parameters
Parameter
Symbol
Typical Value
Description
Total Length
L
λ/2
Length of each conductor (resonant length)
Conductor Spacing
d
0.01λ to 0.05λ
Distance between parallel elements (d << λ)
Wire Radius
a
Variable
Radius of conducting elements
Total Loop Length
2L
λ
Circumference of the folded structure
Important Constraint: For proper operation, the spacing d between the two conductors must be much smaller than the wavelength (d << λ) to ensure strong coupling between the elements and maintain the desired impedance characteristics.
3. Theoretical Analysis
3.1 Mode Decomposition Method
The analysis of a folded dipole is based on decomposing the currents into two fundamental modes: the Transmission Line Mode and the Antenna Mode (also called Radiation Mode). This superposition approach allows us to calculate the total input impedance.
Transmission Line Mode
↔ ↔
Currents flow in opposite directions
Fields cancel in far zone No radiation contribution
Antenna Mode
→ →
Currents flow in same direction
Fields add in far zone Responsible for radiation
3.2 Transmission Line Mode Analysis
In this mode, the two conductors act as a short-circuited two-wire transmission line of length L/2. The input impedance for this mode is:
Zt = jZ0 tan(βL/2)
Where Z0 is the characteristic impedance of the two-wire line and β is the phase constant.
3.3 Antenna Mode Analysis
In the antenna mode, the folded structure behaves like a standard dipole of length L with effective radius increased by the folding. The input impedance for this mode is approximately equal to that of a conventional half-wave dipole:
Zd ≈ 73 + j43 Ω (at resonance Zd ≈ 73 Ω)
3.4 Total Input Impedance
Combining both modes using superposition, the total input impedance of the folded dipole is:
Zin = (4ZtZd) / (2Zd + Zt)
At Resonance (L = λ/2):
When the antenna is resonant, Zt → ∞ (open circuit condition for transmission line mode), simplifying the input impedance to:
Zin = 4 × Zd ≈ 4 × 73 Ω = 292 Ω ≈ 300 Ω
4. Impedance Characteristics
4.1 Step-Up Impedance Ratio
The folded dipole provides an impedance transformation ratio determined by the number of parallel conductors. For a standard two-wire folded dipole with equal conductor diameters:
Zin = (1 + c)2 × Zdipole
Where c is the current division factor. For equal diameter conductors, c = 1, giving a step-up ratio of 4:1.
Impedance Comparison
~73Ω
Half-Wave Dipole
→
~292Ω
Folded Dipole
4:1 Impedance Step-Up Ratio
4.2 Generalized Folded Dipole
For folded dipoles with different conductor diameters (a1 and a2), the step-up ratio becomes:
Zin = (1 + a)2 × Zd
Where the factor a is given by:
a = ln(v) / [ln(v) - ln(μ)]
with v = d/a1 and μ = a2/a1 (ratio of conductor radii).
4.3 Bandwidth Characteristics
Bandwidth Enhancement Factors:
Increased Effective Diameter: The folded structure effectively increases the conductor diameter, reducing the Q-factor
Stub Effect: The transmission line mode provides reactance compensation over a frequency range
Typical Bandwidth: 15-40% depending on design parameters (conductor spacing and diameter)
VSWR Bandwidth: Usually specified for VSWR < 2:1 or return loss > 10 dB
5. Radiation Characteristics
5.1 Radiation Pattern
The folded dipole exhibits the same radiation pattern as a conventional half-wave dipole because the transmission line mode currents do not contribute to far-field radiation (fields cancel).
Radiation Pattern (Vertical Orientation)
Figure 2: Omnidirectional radiation pattern in the H-plane (doughnut shape)
Parameter
Value
Notes
Radiation Pattern
Figure-8 (E-plane), Omnidirectional (H-plane)
Same as half-wave dipole
Directivity
~2.15 dBi
Theoretical maximum
Gain
~2.15 dBi (lossless)
Practical: 0-2 dBi
Polarization
Linear
Parallel to antenna axis
Beamwidth (E-plane)
78°
Half-power beamwidth
Beamwidth (H-plane)
360°
Omnidirectional
5.2 Current Distribution
The current distribution on a folded dipole is approximately sinusoidal along each conductor, similar to a standard dipole. However, the current divides between the two conductors:
Itotal = I1 + I2 = Ia/2 + It
Where Ia is the antenna mode current and It is the transmission line mode current.
6. Design Guidelines
6.1 Design Procedure
Step-by-Step Design Process:
Step 1: Determine operating frequency f0 and wavelength λ = c/f0
Step 2: Calculate total length L ≈ 0.95 × λ/2 (accounting for end effects)
Step 3: Select conductor spacing d = 0.01λ to 0.05λ (typically λ/100 to λ/50)
Step 4: Choose wire diameter based on mechanical strength and bandwidth requirements
Step 6: Include balun if feeding with coaxial cable (unbalanced line)
6.2 Design Example
Example: Design a folded dipole for FM broadcast reception (100 MHz)
Given: f = 100 MHz, λ = 3 meters
Solution:
• Length L = 0.95 × 1.5m = 1.425m (each arm)
• Spacing d = λ/100 = 30mm
• Expected Zin ≈ 300Ω
• Use 300Ω twin-lead transmission line for direct connection
6.3 Feeding Considerations
Feed Type
Impedance
Matching Requirement
Twin-Lead (Balanced)
300Ω
Direct connection - excellent match
Coaxial Cable
50Ω or 75Ω
Requires 4:1 balun transformer
Ladder Line
450Ω or 600Ω
May require impedance transformer
Important: When feeding a folded dipole with coaxial cable, always use a balun (balanced-to-unbalanced transformer) to prevent current flow on the outside of the coax shield, which can distort the radiation pattern.
7. Applications
📺 Television Reception (VHF/UHF)
The most common application is in TV antennas (rabbit ears) where the 300Ω impedance matches standard twin-lead transmission lines used historically in broadcast television systems.
📻 FM Radio Broadcasting
Used extensively in FM receiver antennas (88-108 MHz) due to the impedance match with 300Ω feed lines and good bandwidth characteristics covering the entire FM band.
🎯 Yagi-Uda Arrays
Serves as the driven element in Yagi-Uda directional antennas. The higher impedance provides better isolation from the parasitic elements and simplifies matching networks.
📡 Amateur Radio
Used in various amateur radio applications, particularly in the VHF and UHF bands (2m, 70cm bands) where simple construction and good performance are desired.
📶 RFID and Wireless Sensors
Modern applications include RFID tag antennas and wireless sensor networks where the folded structure provides compact size and impedance matching to integrated circuits.
🛰️ Cellular and WiFi Systems
Variants of folded dipoles are used in cellular base stations and WiFi access points, often in planar (printed) configurations for compact integration.
8. Comparison: Folded Dipole vs. Standard Dipole
Standard Half-Wave Dipole
✓ Simple construction
✓ Input impedance ~73Ω
✓ Good match to coaxial cable
✓ Well-understood characteristics
✗ Lower impedance limits some applications
✗ Requires balun for twin-lead
✗ Narrower bandwidth (thin wire)
✗ Less mechanically robust
Folded Dipole
✓ Higher impedance ~292-300Ω
✓ Perfect match to twin-lead
✓ Wider bandwidth
✓ Mechanically stronger (loop structure)
✓ Better for Yagi driven elements
✗ Requires matching for coaxial feed
✗ More complex construction
✗ Heavier than simple dipole
Characteristic
Standard Dipole
Folded Dipole
Input Impedance
~73 Ω
~292-300 Ω
Bandwidth (2:1 VSWR)
~8-10%
~15-40%
Gain
2.15 dBi
2.15 dBi (same)
Radiation Pattern
Figure-8 / Omnidirectional
Identical to standard dipole
Typical Feed Line
50/75 Ω Coaxial
300 Ω Twin-lead
Mechanical Strength
Moderate
High (rigid loop)
9. Key Takeaways
Essential Concepts to Remember:
A folded dipole consists of two parallel λ/2 dipoles connected at their ends, forming a narrow loop
The input impedance is approximately 4 times that of a standard dipole (~300Ω vs ~73Ω)
The radiation pattern is identical to a standard half-wave dipole (figure-8 in E-plane, omnidirectional in H-plane)
Wider bandwidth results from increased effective conductor diameter
Commonly used with 300Ω twin-lead transmission lines in TV and FM applications
Serves as the driven element in Yagi-Uda antennas due to its higher impedance
Requires a balun when fed with coaxial cable (unbalanced line)
Exam Tip: When solving folded dipole problems, always remember to apply the 4:1 impedance transformation for equal-diameter conductors. If the conductors have different diameters, calculate the step-up ratio using the logarithmic formula involving the spacing and radius ratio.
📝 Self-Assessment Quiz
Test your understanding of folded dipole antennas. Click "Show Answer" to reveal the solution.
Question 1: What is the approximate input impedance of a half-wave folded dipole with equal diameter conductors?
Answer: Approximately 292-300 Ω. This is roughly 4 times the impedance of a standard half-wave dipole (~73 Ω), achieved through the 4:1 impedance step-up ratio of the folded structure.
Question 2: Why does the transmission line mode in a folded dipole not contribute to radiation?
Answer: In the transmission line mode, currents flow in opposite directions in the two conductors. Since the conductors are closely spaced (d << λ), the fields generated by these opposing currents cancel each other in the far field, resulting in no net radiation.
Question 3: A folded dipole is resonant at 150 MHz. What is its approximate physical length?
Answer: λ = c/f = 300/150 = 2 meters. The physical length L ≈ 0.95 × λ/2 = 0.95 × 1m = 0.95 meters (95 cm). The 0.95 factor accounts for end effects and ensures resonance at the desired frequency.
Question 4: Why is a folded dipole often preferred over a standard dipole as the driven element in a Yagi-Uda antenna?
Answer: The higher input impedance (~300Ω) of the folded dipole provides better isolation from the parasitic elements (directors and reflectors) in the Yagi array. This higher impedance is less affected by the mutual coupling with nearby elements, making the antenna easier to match and providing broader bandwidth.
Question 5: What happens to the input impedance if you use different diameter conductors in a folded dipole?
Answer: The impedance step-up ratio changes from 4:1. The general formula is Zin = (1 + a)2 × Zd, where 'a' depends on the ratio of conductor diameters and spacing. If the unfed conductor is larger, the step-up ratio increases; if smaller, it decreases from 4.
Question 6: Why does a folded dipole typically have wider bandwidth than a standard dipole of the same wire diameter?
Answer: The folded structure effectively increases the conductor diameter, which lowers the Q-factor of the antenna. Additionally, the transmission line mode provides reactance compensation over a frequency range. The combination of increased effective thickness and stub reactance effects results in broader impedance bandwidth.