--- title: "VSWR meter and low power (bolometer) measurement" subject: "RF and Microwave Engineering" chapter: 8 key: "semester_7/rf_and_microwave_engineering:vswr-meter-power-measurement" source: https://www.bctengineer.com/q/semester_7/rf_and_microwave_engineering/vswr-meter-power-measurement site: BCT Engineering --- # VSWR meter and low power (bolometer) measurement RF and Microwave Engineering · Chapter 8: RF and Microwave Measurements - Asked in: 2072 Asoj, 2+8 marks; 2070 Bhadra, 4+6 marks (asked 2 times) - Video: [VSWR Measurment | Low & High | Microwave Engineering | Lec-121](https://www.youtube.com/watch?v=EParFhq0Pds) — Education 4u - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/rf_and_microwave_engineering/vswr-meter-power-measurement - Whole chapter: https://www.bctengineer.com/md/semester_7/rf_and_microwave_engineering/chapter-8 ## Question Describe how standing waves and microwave powers are measured with VSWR meter and bolometry (low power measurement) respectively. ## Short answer Standing waves: a slotted-line probe with crystal detector feeds a VSWR meter. For low VSWR, it directly reads E_max/E_min; for high VSWR, the double-minimum width gives S≈λg/[π(d2−d1)]. Low power: a bolometer (barretter/thermistor) in a bridge; RF heating unbalances it, and DC is reduced to rebalance, yielding P_RF=(I1²−I2²)R. ## Answer The **VSWR meter** with a slotted line measures the standing-wave ratio on a line, and the **bolometer bridge** measures low microwave power (below about 10 mW). #### Measuring standing waves with a VSWR meter Set-up: ```text Klystron -> Isolator -> Var. -> Freq. -> Slotted -> Load (1 kHz AM) atten. meter line | probe + crystal | VSWR meter ``` - The source is **square-wave modulated at 1 kHz**. - A probe moves along the **slotted line** and samples the field. A **crystal detector** (square law, V ∝ E²) recovers the 1 kHz envelope. - The **VSWR meter** is a high-gain amplifier tuned to 1 kHz, with a meter calibrated in VSWR and dB for a square-law detector. Steps (low VSWR, S < 10, direct method): 1. Move the probe to a voltage **maximum**; adjust the gain so the meter reads full scale (VSWR = 1.0). 2. Move the probe to the next **minimum**; read the VSWR directly from the scale. 3. S = E_max / E_min. Also |Γ| = (S − 1)/(S + 1). 4. Twice the distance between two successive minima gives λg. For high VSWR (S > 10), the **double minimum method** is used: find the two positions d1, d2 on either side of the minimum where the power is twice the minimum (3 dB up). Then S ≈ λg / [π(d2 − d1)]. #### Low power measurement by bolometry A **bolometer** is a temperature-sensitive resistor: a **barretter** (platinum wire, R rises with power) or a **thermistor** (semiconductor, R falls with power). It is placed in a mount at the end of the line and forms one arm of a Wheatstone bridge. ```text +----R1----+----R2----+ | | | DC --+ G +-- bias | | | +----R3----+--Bolom.--+ ^ RF power ``` Working (DC substitution): 1. With no RF, DC bias current I1 heats the bolometer to the balance resistance R; the galvanometer G reads zero. 2. RF is applied. It also heats the bolometer, so its resistance changes and the bridge becomes unbalanced. 3. The DC current is reduced to I2 until balance returns. The bolometer is again at the same R and temperature, so the RF power has replaced the DC power removed: ```text P_RF = (I1² − I2²) · R ``` Example: R = 200 Ω, I1 = 8 mA, I2 = 5 mA P_RF = (64 − 25)×10⁻⁶ × 200 = 7.8 mW. Limitations: ambient temperature drift (solved by a compensating bridge), low power only, and mismatch errors. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is the source square-wave modulated at 1 kHz in a VSWR meter set-up? A. To increase the microwave power B. To let the crystal's detected 1 kHz envelope be amplified by a tuned high-gain amplifier C. To measure frequency directly D. To heat the bolometer Answer: B. The square-law crystal recovers the 1 kHz envelope, which the VSWR meter (tuned amplifier) amplifies and displays. (Key point: Source modulation and detector) 2. In the direct method for low VSWR, what is done at a voltage maximum? A. The gain is set so the meter reads full scale (VSWR = 1) B. The gain is set to zero C. The probe is removed D. The frequency is changed Answer: A. Set max to full scale, then move to the minimum and read S = Emax/Emin directly. (Key point: Direct VSWR method) 3. Which method is used when S > 10? A. Direct max/min reading B. Double minimum method using the 3 dB points around the minimum C. Counting the minima D. DC substitution Answer: B. S ≈ lambda_g / [pi(d2 - d1)] using positions where power is twice the minimum. (Key point: High VSWR) 4. In bolometer DC substitution, why does the RF power equal the DC power removed? A. The galvanometer absorbs the RF B. The bridge resistors change C. The bolometer returns to the same resistance and temperature at balance D. The RF and DC currents add Answer: C. At rebalance the bolometer is at the same R and temperature, so total heating is the same: RF replaced the DC removed. (Key point: Bolometry balance) ## Related in RF and Microwave Engineering - [Measurement parameters and high VSWR measurement](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Avswr-greater-than-10-measurement) — RF and Microwave Engineering, chapter 8 - [Microwave vs low frequency measurement; static calorimeter](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Ameasurement-differences-static-calorimeter) — RF and Microwave Engineering, chapter 8 - [Measurement parameters and low power device](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Alow-power-measurement-device) — RF and Microwave Engineering, chapter 8 - [Double channel bolometer bridge method](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Adouble-channel-bolometer-bridge) — RF and Microwave Engineering, chapter 8, asked 2× - [Calorimeter wattmeter and single bridge bolometer limits](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Acalorimeter-wattmeter-bolometer-limits) — RF and Microwave Engineering, chapter 8 - [Behavior of systems at microwave vs low frequency](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Amicrowave-vs-low-frequency-behavior) — RF and Microwave Engineering, chapter 1, asked 3× ## Related in other subjects - [Low power design reasons and guidelines](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Alow-power-guidelines) — Instrumentation II, chapter 7, asked 3× - [Power factor and disadvantages of low pf](https://www.bctengineer.com/question.md?key=semester_1%2Fbasic_electrical_engineering%3Alow-power-factor-disadvantages) — Basic Electrical Engineering, chapter 7, asked 3× - [Low power factor causes, disadvantages](https://www.bctengineer.com/question.md?key=semester_1%2Fbasic_electrical_engineering%3Alow-power-factor-causes-disadvantages) — Basic Electrical Engineering, chapter 7 - [Effect of low power factor on equipment ratings](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Alow-pf-effect-ratings) — Industrial Power Distribution and Illumination, chapter 10