# Radios at Mach I?

**URL:** <https://boards.straightdope.com/t/radios-at-mach-i/1411>\
**Category:** Cecil's Columns/Staff Reports\
**Created:** [December 29, 1999, 9:32pm UTC](https://boards.straightdope.com/t/radios-at-mach-i/1411 "1999-12-29T21:32:00Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![inertia](https://avatars.discourse-cdn.com/v4/letter/i/d07c76/32.png) [@inertia](https://boards.straightdope.com/u/inertia)\
**Post date:** [December 29, 1999, 9:32pm UTC](https://boards.straightdope.com/t/radios-at-mach-i/1411/1 "1999-12-29T21:32:00Z")

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Would the Doppler effect necessitate moving the dial to keep up with a set frequency as one accelerated to Mach 1?

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**Author:** ![RM\_Mentock](https://avatars.discourse-cdn.com/v4/letter/r/e274bd/32.png) [@RM\_Mentock](https://boards.straightdope.com/u/RM_Mentock)\
**Post date:** [December 29, 1999, 10:08pm UTC](https://boards.straightdope.com/t/radios-at-mach-i/1411/2 "1999-12-29T22:08:00Z")

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Yes, if radios were that sensitive, but they’re not. Mach 1 is only about .3 kilometers per second, and that’s six orders of magnitude smaller than the speed of light which is about 300,000 km/sec. Radio dials would have to have 6 digits on them.

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**Author:** ![The\_LION](https://avatars.discourse-cdn.com/v4/letter/t/8e7dd6/32.png) [@The\_LION](https://boards.straightdope.com/u/The_LION)\
**Post date:** [January 9, 2000, 8:08am UTC](https://boards.straightdope.com/t/radios-at-mach-i/1411/3 "2000-01-09T08:08:00Z")

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The doppler effect is proportional to the frequency of the signal.  
The higher the frequency the lower the speed at which the shift is detectable.  
In the HF band (30khz - 30mhz) the shift at orbital speeds is small.  
In the Microwave band ( 1 ghz and up ) the shift at automotive speeds is very noticeable.  
That is how police radar works, It sends out a beam of microwaves on a set frequency then detects the doppler shift of the reflected signal and uses it to calculate the speed of the car.  
On most newer digitaly tuned amature and comerical all mode ( A.M.,F.M,S.S.B.,C.W. ) trancevers have displays of 9 digits or more ( Icom IC-706,etc ) and tuning as precise as 1 hertz.

Peace  
t lion AKA KC5LNN  
Amature Radio Operator Since 1994

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**Author:** ![RM\_Mentock](https://avatars.discourse-cdn.com/v4/letter/r/e274bd/32.png) [@RM\_Mentock](https://boards.straightdope.com/u/RM_Mentock)\
**Post date:** [January 10, 2000, 12:20am UTC](https://boards.straightdope.com/t/radios-at-mach-i/1411/4 "2000-01-10T00:20:00Z")

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I stand corrected. There _are_ radios that are that sensitive.

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