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Re: [IRCA] KKOB Santa Fe
On Wednesday 29 October 2008 12:54, John Callarman wrote:
> Noted an interesting problem for KKOB while on our way out of Santa
> Fe Tuesday morning, where we'd spent the night with a
> brother-in-law on our way back from Arizona. A dominant cyclical
> sound from what appeared to be a subaudible heterodyne, presumably
> from the synchronized 230-watt transmitter on the west side of
> Santa Fe. (NRC log says night-time only, but FCC data base shows as
> unlimited.) I counted the cycles ... 120 in 60 seconds, indicating,
> if my fuzzy understanding of SAHes is accurate, that the Santa Fe
> transmitter and the Albuquerque transmitter are 2 Hz off. It was
> extremely annoying, and and carried as far south as milepost 277 on
> US 285, about 25 miles from the Santa Fe transmitter.
>
> I had heard a similar effect 25 to 30 years ago when I was at
> WMIX-940, weather knocked off the daytime transmitter, and we
> switched to the night-time facility. When the power went back on at
> the daytime transmitter site, the combination gave the same
> cyclical, wobbulating effect.
>
> My SAH assumption, though, may be wrong, if the effect was caused
> by the difference in distance between the two transmitters.
> Engineering knowledgeable people may set me straight on this.
>
> The bottom line, though, would seem to be that the product of the
> two transmitters would make KKOB's programming unusable within a
> 25-mile radius of the Santa Fe transmitter.
Your hypothesis sounds plausible to me, John. A difference in arrival
times (due to the difference in distance and other propagation
delays, if any) would cause nulls to appear in the frequency domain
(i.e., selective fading), which could cause some distortion, but not
the period amplitude fading that you observed. It would take a
frequency offset to do that, and I think you did the math correctly.
Barry
--
Barry McLarnon VE3JF Ottawa, ON
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