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1-3 of 3 messages
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Tower detuning for AM Stations
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by jmulla on January 23, 2001
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Can anyone tell me what phenomenon causes "detuning" of AM stations from cellular towers and what measures are necessary to correct this problem?
Thanks
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RE: Tower detuning for AM Stations
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by WN3VAW on January 23, 2001
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Suggest you contact a professional broadcast engineer who can better answer the question and suggest solutions. Call your local station and ask to speak to their chief engineer (preferably not the station you have the problem with!).
I highly recommend Jack Layton W9UK, for one.
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RE: Tower detuning for AM Stations
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by VK4ZSS on February 17, 2001
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g'day
the phenomenon is quite straightforward, simply the cellular tower has become an unwanted parasitic element of the am broacast array
the mast/tower on which the cellular antennas are mounted is a significant portion of a wavelength
in the am broadcast band and hence the mutual
impedance between the cellular tower and the am
broadcast towers is also significant unless the distance of the cellular tower to any of the am broadcast towers is several wavelengths. thus the rf currents in the am broadcast towers induce rf voltage in the cellular tower.
if the cellular tower has a low reactance at the am broacast stations frequency then the induced rf voltage will lead to significant rf current at the am broadcast frequency and the cellular tower will radiate power at the am broadcast frequency and effectively becomes a parasitic element of the am broadcast array. with commonly encountered tower separations an omnidirectional am broadcast antenna (i.e. a single tower) would not be affected in terms of its radiation pattern circularity and any effect on base impedance and would be small and corrected when adjusting matching circuits in the normal course of maintenance. the problem is most frequently becomes of significance with directional arrays (i.e. multiple towers), especially those with deep nulls (or minimas to be more precise). array minima depth can be 40dB or more below maxima and since the minima is achieved by roughly equal signals close to 180 degrees out of phase a quite small parasitic contribution can lead to dramatic variations in minima depth easily putting the am broadcast array outside its approved specification.
it is rarely possible to adjust the directional am broadcast array back into specification with the presence of such a parasitic element. in any event generally the re-radiated phase and magnitude of the cellular tower is not particularly stable as it has not been designed to radiate in the am broadcast band (e.g. no or limited earth mat at am broadcast frequencies). hence even if a short term adjustment can be made to the array to correct the variation sufficiently, further adjustment would be necessary at even daily intervals.
fortunately the problem is relatively easily corrected. the cellular tower can only reradiate if its reactance at the am broadcast frequency of interest is reasonably low. this condition is only met for (1) grounded towers an electrical quarter wavelength or odd multiples thereof in height at the am broadcast frequency or (2) floating towers of an electrical half wavelength or multiples thereof in height at the am broadcast frequency.
a quarter wave cellular tower can be detuned by insulating it from ground (maybe not so easy) and a floating half wavelength can be detuned by shorting it to ground. in practice various alterations to the tower are generally made to maximise the towers reactance, that is to modify its electrical length.
i can probably dig out some reference material on specific techniques, but this has been made a fine art in north america and i am sure another respondent will soon fill you in on the details
hth
73s gd dx de
sam dellit vk4zss
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