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Old 05-17-2018, 07:23 AM
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Robert Grant Robert Grant is offline
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Location: Monroe County, MI
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Quote:
Originally Posted by NewVista View Post
Actually the lower the channel number, the further it can be picked up, giving WTMJ-4 the greatest reach.

<snip>
In terms of normal reception*, absolutely correct.
However, the effects of tropospheric refraction (an abnormal propagation mode, but not uncommon over Lake Michigan in the summer), are far more profound at UHF, and worse on low-VHF than they are on high-VHF.

On a typical summer's day (and in late Spring), warm, humid air can be cooled where it is close to the cold lake's surface, whilst the air above is still warm and dry by comparison. The speed of light (and thus of radio waves) is faster in the warm dry air than in the humid air immediately above the lake surface. The radio wave thus assumes a curved path following the transition zone where the humid air and the dry air meet.

*Without tropospheric refraction, a channel 4 signal will be usable over a greater distance than a channel 9 signal, which, in turn, would work at a greater distance than a channel 48 signal.
Keep in mind that low-VHF analog stations were generally limited to 100kW ERP, while high-VHF stations could and usually did have 316kW ERP. Analog UHF stations could use up to 5000kW ERP (1000kW before a 1958 rule change), but few used the maximum.

Normal VHF reception is of both the signal travelling directly to the antenna, and the signal travelling from the transmitting antenna, and reflecting off the earth (near the receiving antenna) back to the receiving antenna. These two paths cancel each other out to a degree, the longer the wave, the closer to 180° out of phase the reflected signal is, thus the receiving antenna needs to be high off the ground to make use of the low-VHF benefit.

Last edited by Robert Grant; 05-17-2018 at 07:45 AM.
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