Hairpins and Lecher Lines

A Lecher line, also known as Lecher wires, is a simple but versatile tool used in electronics to work with radio waves in the VHF, UHF, and microwave range (around 30MHz to 3GHz). It's essentially a short section of balanced transmission line, typically made of two parallel wires or rods connected at one end to a signal source

Measuring Wavelength is the classic use of Lecher lines. By sending radio waves through the line and observing the standing wave patterns formed, you can measure the wavelength of the signal. This is similar to how a Kundt's tube measures sound waves.

The distance between two voltage or current maxima on the line corresponds to half the wavelength. By knowing the wavelength of the signal, you can easily calculate the frequency.

Finding nodes on a Lecher line is crucial for measuring wavelength and frequency. Two main methods are employed:

1. Voltage Indicator:

• This method utilizes a high-impedance indicator like an RF voltmeter or a neon bulb connected to sliding contacts on the Lecher wires.

• At a voltage node, the indicator exhibits minimal response (light extinguishes for bulbs).

• Challenges:

  • Low-impedance indicators (regular incandescent bulbs) disturb the standing wave and affect accuracy.

  • Glow discharge bulbs have high striking voltage, making pinpointing the exact voltage minimum difficult.

  • Neon bulbs offer a simpler alternative but may still lack precision.

2. Shorting Bar and Current Measurement:

• This method uses a movable shorting bar on the Lecher line and an RF ammeter in the feeder line.

• As the bar slides, the current drawn from the source varies due to impedance changes caused by the standing wave.

• Current minima occur at every half-wavelength point, coinciding with voltage nodes.

• Advantages:

  • More accurate than voltage indicator method for longer wavelengths.

  • Doesn't depend on indicator characteristics.

Additional notes:

• The distance between two successive nodes of either voltage or current is equal to half the wavelength.

• Knowing the wavelength and the speed of light allows you to calculate the frequency.

Further considerations:

• Calibration: Both methods require careful calibration for accurate measurements.

• Accuracy: The voltage indicator method is generally less accurate than the shorting bar method, especially for longer wavelengths.

Resonance

Lecher lines, called resonant stubs, excel as high-Q parallel resonant circuits at UHF frequencies. Their magic lies in their size: at these frequencies, traditional "lumped component" circuits (separate inductors and capacitors) require miniature components prone to fabrication difficulties and parasitic effects. Lecher stubs, however, leverage standing waves to achieve resonance within their compact structure.

Here's how it works:

• Quarter-wavelength stubs: When shorted at one end, these stubs act like a parallel resonant circuit with high impedance at their fundamental resonant frequency and low impedance at other frequencies.

• Multiple resonances: Unlike lumped LC circuits, which resonate only at their fundamental frequency, Lecher stubs resonate at odd-number multiples of this frequency. This offers additional tuning flexibility.

In essence, Lecher stubs provide a compact, high-Q, and multi-resonant alternative to traditional tuned circuits at UHF frequencies, making them valuable tools for RF engineers.

Hairpin Oscillator Circuits