[IRCA] coverage versus frequency over land

[Charles A Taylor <MWDXer@xxxxxxx>]

"Is it true that lower freq daytime signals travel further than higher ones 
since stations on the lower end seem to come in stronger than ones at the 
top with the same power from the same location,(disregarding signal 
direction),because the wave length is longer on the lower freqs? Also I 
thought that stations on lower channels used higher towers,while shorter 
towers were used by stations radiating shorter waves,but I see that does 
not always apply in every case. Bill in Vic. "

_______________________________________________________________________________________________________________________________________________________

Bill,

I don't think the answers you got for your question are sufficient.

Vertically polarized MW groundwave signals travel along with their lower 
end connecting with the ground. As they radiate outward, the ground absorbs 
more and more of the wavefront's energy.

The equations that predict the attenuation versus distance have a factor 
for frequency. This factor plainly predicts that an LW signal will travel 
further than a MW signal.

Also, ground conductivity has a strong influence on the 
distance-versus-attenuation behaviour of signals.

Groundwave signals above about 2 - 3 MHz attenuate so rapidly that they are 
essentially useless for broadcast purposes. What you receive at VHF and UHF 
frequencies are direct waves that don't involve the earth.

While the earth attenuates a LW or MW signal, it also provides a means for 
their propagation.

If the earth were perfectly conductive, then signals would travel much 
further than they would if they were travelling through space.

Also, horizontally-polarized signals are effectively short-circuited by 
contact with the earth. That's why horizontal antennas are not used at LW 
and MW frequencies.

Yes, horizontal antennas have been used by MW stations; but usually as a 
top-loading for a vertical member that radiates the vertically-polarized 
signals.

This all partially explains why sea paths propagate MW signals much more 
effectively than lands paths: sea wave is vastly more conductive than earth.

I hope this adds to your understanding. If you want more explanation, just 
let me know.

73 de Charlie

        -----
Charles A Taylor, WD4INP
Greenville, North Carolina 

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