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Tuning Forks and Radar Speed

Typical Tuning Fork

A police or sports Doppler microwave radar will register a speed from a vibrating tuning fork's narrow side, the higher the tone the higher the speed. A radar will also measure a tone from an audio speaker diaphragm. The radar is measuring the microwave reflection of the fork or speaker diaphragm - a complete end-to-end test.

All radars come with tuning forks tuned for that radar. The supplied forks are the only forks that should be used to test the radar. The radar calibration certificate should include the tuning fork serial numbers that go with that radar. The forks should be labeled with, Speed, Radar Frequency, and Serial Number for tracking and accountability.

- Speed
- Frequency Band
- Radar Frequency

- Serial Number
Tuning Fork Label

TUNING FORK TEST
A radar should be tested with tuning forks on a daily basis. The test should be conducted at start of shift, at a minimum. Some places also require the test when a citation is issued, and/or at end of shift. For dual antenna radars, the test should be conducted on each antenna. Testing the radar stationary mode is straight forward, place a vibrating fork about an inch from the front of the antenna, the radar should read the speed the fork is calibrated to induce. Moving mode is a little more involved and a little confusing.

Radar Moving Mode Tuning Fork Test
Two tuning forks with different tones are required to test moving mode. One fork simulates the patrol car speed, the other fork simulates a moving target reflection. The moving mode opposite direction traffic test is slightly different from the same-lane traffic test.

2 Tuning Forks

Opposite direction mode
1) Strike both forks
2) Put the SLOWER (Larger) fork in front of the antenna.
3) Radar should read slower fork as PATROL SPEED.
4) Put the FASTER (smaller) fork in front of the antenna.
5) Radar should read the DIFFERENCE in fork speeds.
Same-Lane mode
1) Strike both forks
2) Put the FASTER (smaller) fork in front of the antenna.
3) Radar should read faster fork as PATROL SPEED.
4) Put the SLOWER (Larger) fork in front of the antenna.
5) Radar should read the SUM of the fork speeds.


Below illustrations with tuning forks calibrated for 30 and 55 mph.

Fork and patrol speed
opposite direction mode Same-lane mode


Radar Frequency, Tuning Fork Tone, and Speed

The speed reading depends on;
- Radar Frequency
- Tuning Fork Tone
v = c fd / 2 fo
v = speed reading
c = speed of light
fd = Fork tone
fo = radar frequency
Radar Frequencies (U.S.)
Band Frequency Use
X 10.525 GHz Police / Sports
K 24.125 GHz Sports
K 24.150 GHz Police
Ka 33.4 - 36 GHz Police

Tuning fork tone is measured in Hertz (Hz), which equals cycles per second (cps).

Most police radars operate on a single frequency in the Ka band between 33.4 and 36.0 GHz. Police radars that operate in the K band use 24.150 GHz. Few police use X band radar (10.525 GHz) anymore. Most sports radars operate in the K band at 24.125 GHz, some transmit in the X band at 10.525 GHz.

Calculate Speed Reading
v = c fd / 2 fo
Radar Frequency: GHz
Tuning Fork Tone: Hz

mph, kph, knots

Calculate Tuning Fork Tone
fd = 2 v fo / c
Radar Frequency: GHz
Speed:

Tuning Fork Tone

Radar frequency can be estimated from speed and tuning fork resonance. This is an estimate because there is some tolerance in the fork tone, and the equation has inherent rounding errors when calculating transmit frequency.

Estimate Radar Frequency
fo = c fd / (2 v)
Tuning Fork Tone: Hz
Speed:

Radar Frequency



Calibration of Tuning Forks
Each tuning fork should be calibrated (tested) every time the radar is calibrated.

Tuning Fork Calibration Certificate
Fork Serial Number: 82243
Resonance: 5823 Hz
Speed: 55 MPH
Radar Frequency: 35.5 GHz
Temperature: 80 °F
Correction Factor: 0.02 MPH/°F
Tested by: DS
Date: mm/dd/yyyy

Police Radar Basics
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