Re: AM Daytime DX

[Dan Strassberg <dan.strassberg@worldnet.att.net>]

At 10:10 PM 5/28/97 +0000, you wrote:
>I have also noticed this as I drive down Route 3/Route 6 to the Cape. They
>come in like locals.
>
>----------
>> From: Paul R Hopfgarten <hopfgapr@sprynet.com>
>> To: boston-radio-interest@khavrinen.lcs.mit.edu
>> Subject: AM Daytime DX
>> Date: Wednesday, May 28, 1997 4:38 PM
>> 
>> I'm not an engineer, nor in the Radio business, so someone expain the
>> following:
>> 
>> In Boston (South End/Downtown) I can receive CHSJ/700 StJohn,NB ("Country
>> 700) VERY well, and can aslo get, although less well CHTN/720
>Charlottetown
>> PEI 
>> (Oldies Station) but once I leave the city proper heading back to NH can
>no
>> longer pick up, and cannot pickup in Derry NH. (Although Nightime DXing
>> gets WLW/Cincinnati at 700, I CAN get CHTN at 720, but only at night)
>> 
>> Thanks,
>> Paul Hopfgarten
>> Derry NH
>
CHSJ is very easy to explain at the moment. If you still get it on 700 in a
few months, it will be a true mystery, though. The station is moving to FM.

The reason that CHSJ comes in so well on the South Shore and not at all
inland in NH or even in Mass northwest of Boston is that the path from St
Johns to the South shore is entirely over salt water. Salt water is the best
conductor of medium-wave radio signals that is found abundantly in nature.
It has a conductivity of about 5 mho/m, wheras the New England soil has a
conductivity that ranges from about 2 mmho/m (around Boston) to 0.5 mmho/m
(in Litchfield County in northwest Connecticut and a few other spots). The
difference is a factor as large as 10,000:1. The best conductivity on dry
land in the 48 contiguous states is in the northern midwest (the Dakotas and
eastern Montana, for example) where the conductivity is about 40 mmho/m,
still less than 1/100 the conductivity of salt water.

Despite the highly conductive salt water, the signal is attenuated a little
more than in proportion to the distance from the TX. (If the conductor were
perfect (i.e. if the Atlantic Ocean were filled with Mercury instead of salt
water), the signal would be attenuated exactly as the reciprocal of the
distance from the TX. I think the distance to St Johns is about 500 miles.
CHSJ operates with 25 kW-D/10 kW-N DA-N. The night pattern protects WLW, so
eastern New England lies in the null and you won't get the signal at night
despite the salt water. But a 25 kW station produces an rms field from about
875 to 1200 mV/m unattenuated at one mile. Divide even the larger number by
500 and you get a middling fair signal (over 2 mV/m). But then correct for
the fact that the salt water isn't Mercury and the signal is quite a bit
less. Then correct further for attenuation caused by the curvature of the
earth and the signal is still less. As you move inland, the signal has to
travel over the low conductivity New England soil. The signal is attenuated
quite rapidly as you mive away from the water's edge. You can still pick up
CHSJ in Norwood, but it's pretty weak at that point.

Now get out your map. The path from st Johns to New Hampshire cuts across
quite a bit of land. I didn't get my map out and I can't recall which land,
but I remember that this is so.

Now, CHTN is a different story and is a bit of a mystery to me. PEI is north
of New Brunswick and there is no clear salt water path from Charlottown to
anywhere in eastern New England that I am aware of. I know that CHTN often
appears to operate with its daytime facilities at night, when it is supposed
to protect WGN, but that does not explain why you pick up CHTN during the
day on the South Shore. If the recption occurs shortly after sunrise or
shortly before sunset on PEI, what you could be picking up is daytime
skywave. You can tell when it is skywave because the signal slowly fades in
and out on occasion. Skywave reception does not begin abruptly at sunset and
end abruptly at sunrise.

- -------------------------------
Dan Strassberg (Note: Address is CASE SENSITIVE!)
ALL _LOWER_ CASE!!!--> dan.strassberg@worldnet.att.net
(617) 558-4205; Fax (617) 928-4205

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